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See detailComplementarity Assessment of South Greenland Katabatic Flows and West Europe Wind Regimes
Radu, David-Constantin; Berger, Mathias; Fonteneau, Raphaël; Hardy, Simon; Fettweis, Xavier; Le Du, Marc; Panciatici, Patrick; Balea, Lucian; Ernst, Damien

in Energy (2019), 175

Current global environmental challenges require vigorous and diverse actions in the energy sector. One solution that has recently attracted interest consists in harnessing high-quality variable renewable energy resources in remote locations, while using transmission links to transport the power to end users. In this context, a comparison of western European and Greenland wind regimes is proposed. By leveraging a regional atmospheric model specifically designed to accurately capture polar phenomena, local climatic features of southern Greenland are identified to be particularly conducive to extensive renewable electricity generation from wind. A methodology to assess how connecting remote locations to major demand centres would benefit the latter from a resource availability standpoint is introduced and applied to the aforementioned Europe-Greenland case study, showing superior and complementary wind generation potential in the considered region of Greenland with respect to selected European sites.

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See detailThe Effect of Foehn‐Induced Surface Melt on Firn Evolution Over the Northeast Antarctic Peninsula
Datta, R.T.; Tedesco, M.; Fettweis, Xavier; Agosta, C.; Lhermitte, S.; Lenaerts, J.; Wever, N.

in Geophysical Research Letters (2019), 46

Surface meltwater ponding has been implicated as a major driver for recent ice shelf collapse as well as the speedup of tributary glaciers in the northeast Antarctic Peninsula. Surface melt on the NAP is impacted by the strength and frequency of westerly winds, which result in sporadic foehn flow. We estimate changes in the frequency of foehn flow and the associated impact on snow melt, density, and the percolation depth of meltwater over the period 1982–2017 using a regional climate model and passive microwave data. The first of two methods extracts spatial patterns of melt occurrence using empirical orthogonal function analysis. The second method applies the Foehn Index, introduced here to capture foehn occurrence over the full study domain. Both methods show substantial foehn‐induced melt late in the melt season since 2015, resulting in compounded densification of the near‐surface snow, with potential implications for future ice shelf stability.

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See detailAssessment of the Greenland ice sheet–atmosphere feedbacks for the next century with a regional atmospheric model coupled to an ice sheet model
Le clec’h, S.; Charbit, S.; Quiquet, A.; Fettweis, Xavier; Dumas, C.; Kageyama; Wyard, Coraline; Ritz, C.

in Cryosphere (2019), 13

In the context of global warming, growing attention is paid to the evolution of the Greenland ice sheet (GrIS) and its contribution to sea-level rise at the centennial timescale. Atmosphere–GrIS interactions, such as the temperature–elevation and the albedo feedbacks, have the potential to modify the surface energy balance and thus to impact the GrIS surface mass balance (SMB). In turn, changes in the geometrical features of the ice sheet may alter both the climate and the ice dynamics governing the ice sheet evolution. However, changes in ice sheet geometry are generally not explicitly accounted for when simulating atmospheric changes over the Greenland ice sheet in the future. To account for ice sheet–climate interactions, we developed the first two-way synchronously coupled model between a regional atmospheric model (MAR) and a 3-D ice sheet model (GRISLI). Using this novel model, we simulate the ice sheet evolution from 2000 to 2150 under a prolonged representative concentration pathway scenario, RCP8.5. Changes in surface elevation and ice sheet extent simulated by GRISLI have a direct impact on the climate simulated by MAR. They are fed to MAR from 2020 onwards, i.e. when changes in SMB produce significant topography changes in GRISLI. We further assess the importance of the atmosphere–ice sheet feedbacks through the comparison of the two-way coupled experiment with two other simulations based on simpler coupling strategies: (i) a one-way coupling with no consideration of any change in ice sheet geometry; (ii) an alternative one-way coupling in which the elevation change feedbacks are parameterized in the ice sheet model (from 2020 onwards) without taking into account the changes in ice sheet topography in the atmospheric model. The two-way coupled experiment simulates an important increase in surface melt below 2000 m of elevation, resulting in an important SMB reduction in 2150 and a shift of the equilibrium line towards elevations as high as 2500 m, despite a slight increase in SMB over the central plateau due to enhanced snowfall. In relation with these SMB changes, modifications of ice sheet geometry favour ice flux convergence towards the margins, with an increase in ice velocities in the GrIS interior due to increased surface slopes and a decrease in ice velocities at the margins due to decreasing ice thickness. This convergence counteracts the SMB signal in these areas. In the two-way coupling, the SMB is also influenced by changes in fine-scale atmospheric dynamical processes, such as the increase in katabatic winds from central to marginal regions induced by increased surface slopes. Altogether, the GrIS contribution to sea-level rise, inferred from variations in ice volume above floatation, is equal to 20.4 cm in 2150. The comparison between the coupled and the two uncoupled experiments suggests that the effect of the different feedbacks is amplified over time with the most important feedbacks being the SMB–elevation feedbacks. As a result, the experiment with parameterized SMB–elevation feedback provides a sea-level contribution from GrIS in 2150 only 2.5 % lower than the two-way coupled experiment, while the experiment with no feedback is 9.3 % lower. The change in the ablation area in the two-way coupled experiment is much larger than those provided by the two simplest methods, with an underestimation of 11.7 % (14 %) with parameterized feedbacks (no feedback). In addition, we quantify that computing the GrIS contribution to sea-level rise from SMB changes only over a fixed ice sheet mask leads to an overestimation of ice loss of at least 6 % compared to the use of a time variable ice sheet mask. Finally, our results suggest that ice-loss estimations diverge when using the different coupling strategies, with differences from the two-way method becoming significant at the end of the 21st century. In particular, even if averaged over the whole GrIS the climatic and ice sheet fields are relatively similar; at the local and regional scale there are important differences, highlighting the importance of correctly representing the interactions when interested in basin scale changes.

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See detailEstimation of the Antarctic surface mass balance using the regional climate model MAR (1979–2015) and identification of dominant processes
Agosta, Cécile; Amory, Charles; Kittel, Christoph; Orsi, Anais; Favier, Vincent; Gallée, Hubert; van den Broeke, Michiel R.; Lenearts, Jan T. M.; van Wessem, Jan Melchior; van de Berg, Willem Jan; Fettweis, Xavier

in Cryosphere (2019)

The Antarctic ice sheet mass balance is a major component of the sea level budget and results from the difference of two fluxes of a similar magnitude: ice flow discharging in the ocean and net snow accumulation on the ice sheet surface, i.e. the surface mass balance (SMB). Separately modelling ice dynamics and SMB is the only way to project future trends. In addition, mass balance studies frequently use regional climate models (RCMs) outputs as an alternative to observed fields because SMB observations are particularly scarce on the ice sheet. Here we evaluate new simulations of the polar RCM MAR forced by three reanalyses, ERA-Interim, JRA-55, and MERRA-2, for the period 1979–2015, and we compare MAR results to the last outputs of the RCM RACMO2 forced by ERA-Interim. We show that MAR and RACMO2 perform similarly well in simulating coast-to-plateau SMB gradients, and we find no significant differences in their simulated SMB when integrated over the ice sheet or its major basins. More importantly, we outline and quantify missing or underestimated processes in both RCMs. Along stake transects, we show that both models accumulate too much snow on crests, and not enough snow in valleys, as a result of drifting snow transport fluxes not included in MAR and probably underestimated in RACMO2 by a factor of 3. Our results tend to confirm that drifting snow transport and sublimation fluxes are much larger than previous model-based estimates and need to be better resolved and constrained in climate models. Sublimation of precipitating particles in low-level atmospheric layers is responsible for the significantly lower snowfall rates in MAR than in RACMO2 in katabatic channels at the ice sheet margins. Atmospheric sublimation in MAR represents 363 Gt yr−1 over the grounded ice sheet for the year 2015, which is 16 % of the simulated snowfall loaded at the ground. This estimate is consistent with a recent study based on precipitation radar observations and is more than twice as much as simulated in RACMO2 because of different time residence of precipitating particles in the atmosphere. The remaining spatial differences in snowfall between MAR and RACMO2 are attributed to differences in advection of precipitation with snowfall particles being likely advected too far inland in MAR.

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See detailSimulated Greenland Surface Mass Balance in the GISS ModelE2 GCM: Role of the Ice Sheet Surface
Alexander, P.; LeGrande, A.; Fischer, E.; Tedesco, M.; Fettweis, Xavier; Kelley, M.; Nowicki, S.; Schmidt, G.

in Journal of Geophysical Research (2019)

The rate of growth or retreat of the Greenland and Antarctic ice sheets remains a highly uncertain component of future sea level change. Here we examine the simulation of Greenland ice sheet surface mass balance (GrIS SMB) in the NASA Goddard Institute for Space Studies (GISS) ModelE2 General Circulation Model (GCM). GCMs are often limited in their ability to represent SMB compared with polar‐region Regional Climate Models (RCMs). We compare ModelE2 simulated GrIS SMB for present‐day (1996‐2005) simulations with fixed ocean conditions, at a spatial resolution of 2° latitude by 2.5° longitude (~200 km), with SMB simulated by the Modèle Atmosphérique Régionale (MAR) RCM (1996‐2005 at a 25 km resolution). ModelE2 SMB agrees well with MAR SMB on the whole, but there are distinct spatial patterns of differences and large differences in some SMB components. The impact of changes to the ModelE2 surface are tested, including a sub‐grid‐scale representation of SMB with surface elevation classes. This has a minimal effect on ice sheet‐wide SMB, but corrects local biases. Replacing fixed surface albedo with satellite‐derived values and an age‐dependent scheme has a larger impact, increasing simulated melt by 60‐100%. We also find that lower surface albedo can enhance the effects of elevation classes. Reducing ModelE2 surface roughness length to values closer to MAR reduces sublimation by ~50%. Further work is required to account for meltwater refreezing in ModelE2, and to understand how differences in atmospheric processes and model resolution influence simulated SMB.

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See detailAccelerating changes in ice mass within Greenland, and the ice sheet’s sensitivity to atmospheric forcing
Bevis, M.; Harig, C.; Khan, S.; Brown, A.; Simons, F.; Willis, M.; Fettweis, Xavier; van den Broeke, M.; Madsen, F.; Kendrick, E.; Caccamise, D.; van Dam, T.; Knudsen, P.; Nylen, T.

in Proceedings of the National Academy of Sciences of the United States of America (2019)

The recent deglaciation of Greenland is a response to both oceanic and atmospheric forcings. From 2000 to 2010, ice loss was concentrated in the southeast and northwest margins of the ice sheet, in large part due to the increasing discharge of marine-terminating outlet glaciers, emphasizing the importance of oceanic forcing. However, the largest sustained (∼10 years) acceleration detected by Gravity Recovery and Climate Experiment (GRACE) occurred in southwest Greenland, an area largely devoid of such glaciers. The sustained acceleration and the subsequent, abrupt, and even stronger deceleration were mostly driven by changes in air temperature and solar radiation. Continued atmospheric warming will lead to southwest Greenland becoming a major contributor to sea level rise.

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See detailSensitivity to Convective Schemes on Precipitation Simulated by the Regional Climate Model MAR over Belgium (1987–2017)
Doutreloup, Sébastien; Wyard, Coraline; Amory, Charles; Kittel, Christoph; Erpicum, Michel; Fettweis, Xavier

in Atmosphere (2019), 10(1), 34

The aim of this study is to assess the sensitivity of convective precipitation modelled by the regional climate model MAR (Modèle Atmosphérique Régional) over 1987–2017 to four newly implemented convective schemes: the Bechtold scheme coming from the MESO-NH regional model and the Betts-Miller-Janjić, Kain-Fritsch and modified Tiedtke schemes coming from the WRF regional model. MAR version 3.9 is used here at a resolution of 10 km over a domain covering Belgium using the ERA-Interim reanalysis as forcing. The simulated precipitation is compared against SYNOP and E-OBS gridded precipitation data. Trends in total and convective precipitation over 1987–2017 are discussed. None of the MAR experiments compares better with observations than the others and they all show the same trends in (extreme) precipitation. Over the period 1987–2017, MAR suggests a significant increase in the mean annual precipitation amount over the North Sea but a significant decrease over High Belgium.

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See detailThe SMB Model Intercomparison (SMBMIP) over Greenland: first results
Fettweis, Xavier

Conference (2018, December 14)

We will present here the first results of the SMB Model intercomparison (SMBMIP) over the Greenland Ice sheet. For the first time, all the modeled SMB estimations over the current climate (1980-2016) will be compared at the pixel scale, on the same grid (1km), on the common ice sheet mask, at the monthly timescale and by using the same forcing (ERA-Interim). The SMB components from RCMs, GCMs and PDD models will be compared with ice core measurements, the SMB data base from Machguth et al. (2016) as well as passive microwave satellite derived melt extent, in the aim of identifying poorly documented areas where model results diverge and where additional measurement campaigns will be needed.

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See detailSensitivity of the current Antarctic surface mass balance to sea surface conditions using MAR
Kittel, Christoph; Amory, Charles; Agosta, Cécile; Delhasse, Alison; Doutreloup, Sébastien; Huot, Pierre-Vincent; Wyard, Coraline; Fichefet, Thierry; Fettweis, Xavier

in Cryosphere (2018), 12

Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields prescribed by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSCs, namely sea ice concentration, SIC, and sea surface temperature, SST) over the Southern Ocean is not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5), RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM “Modèle Atmosphérique Régional” (MAR), different sensitivity experiments are performed over 1979–2015 by modifying the ERA-Interim SSCs with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (decreased) precipitation due to perturbations inducing warmer, i.e. higher SST and lower SIC (colder, i.e. lower SST and higher SIC), SSCs than ERA-Interim, significantly affecting the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e greater than the interannual variability) are found for the largest combined SST/SIC perturbations. This is notably due to moisture anomalies above the ocean, reaching sufficiently high atmospheric levels to influence accumulation rates further inland. Sensitivity experiments with warmer SSCs based on the CMIP5 biases reveal integrated SMB anomalies (+5 % to +13 %) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century.

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See detailNonlinear rise in Greenland runoff in response to post-industrial Arctic warming
Trusel, L.; Das, S.; Osman, M.; Evans, M.; Smith, B.; Fettweis, Xavier; McConnell, J.; Noël, B.; van den Broeke, M.

in Nature (2018), 564

The Greenland ice sheet (GrIS) is a growing contributor to global sea-level rise1, with recent ice mass loss dominated by surface meltwater runoff2,3. Satellite observations reveal positive trends in GrIS surface melt extent4, but melt variability, intensity and runoff remain uncertain before the satellite era. Here we present the first continuous, multi-century and observationally constrained record of GrIS surface melt intensity and runoff, revealing that the magnitude of recent GrIS melting is exceptional over at least the last 350 years. We develop this record through stratigraphic analysis of central west Greenland ice cores, and demonstrate that measurements of refrozen melt layers in percolation zone ice cores can be used to quantifiably, and reproducibly, reconstruct past melt rates. We show significant (P < 0.01) and spatially extensive correlations between these ice-core-derived melt records and modelled melt rates5,6 and satellite-derived melt duration4 across Greenland more broadly, enabling the reconstruction of past ice-sheet-scale surface melt intensity and runoff. We find that the initiation of increases in GrIS melting closely follow the onset of industrial-era Arctic warming in the mid-1800s, but that the magnitude of GrIS melting has only recently emerged beyond the range of natural variability. Owing to a nonlinear response of surface melting to increasing summer air temperatures, continued atmospheric warming will lead to rapid increases in GrIS runoff and sea-level contributions.

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See detailCritical Time Windows for Renewable Resource Complementarity Assessment
Berger, Mathias; Radu, David-Constantin; Fonteneau, Raphaël; Henry, Robin; Glavic, Mevludin; Fettweis, Xavier; Le Du, Marc; Panciatici, Patrick; Balea, Lucian; Ernst, Damien

E-print/Working paper (2018)

This paper proposes a systematic framework to assess the complementarity of renewable resources over arbitrary geographical scopes and temporal scales which is particularly well-suited to exploit very large data sets of climatological data. The concept of critical time windows is introduced, and a spatio-temporal criticality indicator is proposed, consisting in a parametrised family of scalar indicators quantifying the complementarity between renewable resources in both space and time. The criticality indicator is leveraged to devise a family of optimisation problems identifying sets of locations with maximum complementarity under arbitrary geographical deployment constraints. The applicability of the framework is shown in a case study investigating the complementarity between the wind regimes in continental western Europe and southern Greenland, and its usefulness in a power system planning context is demonstrated. Besides showing that the occurrence of low wind power production events can be significantly reduced on a regional scale by exploiting diversity in local wind patterns, results highlight the fact that aggregating wind power production sites located on different continents may result in a lower occurrence of system-wide low wind power production events and indicate potential benefits of intercontinental electrical interconnections.

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See detailThe Greenland and Antarctic ice sheets under 1.5 °C global warming
Pattyn, F.; Ritz, C.; Hanna, E.; Asay-Davis, X.; DeConto, R.; Durand, G.; Favier, L.; Fettweis, Xavier; Goelzer, H.; Golledge, N.; Kuipers Munneke, P.; Lenaerts, J.; Nowicki, S.; Payne, A.; Robinson, A.; Seroussi, H.; Trusel, L.; van den Broeke, M.

in Nature Climate Change (2018), 1758-6798

Even if anthropogenic warming were constrained to less than 2 °C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the past decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5–2.0 °C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance–elevation feedback, whereas for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening.

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See detailBrief communication: Impact of the recent atmospheric circulation change in summer on the future surface mass balance of the Greenland Ice Sheet
Delhasse, Alison; Fettweis, Xavier; Kittel, Christoph; Amory, Charles; Agosta, Cécile

in Cryosphere (2018)

Since the 2000s, a change in the atmospheric circulation over the North Atlantic resulting in more frequent blocking events has favoured warmer and sunnier weather conditions over the Greenland Ice Sheet (GrIS) in summer, enhancing the melt increase. This circulation change is not represented by general circulation models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5), which do not predict any circulation change for the next century over the North Atlantic. The goal of this study is to evaluate the impact of an atmospheric circulation change (as currently observed) on projections of the future GrIS surface mass balance (SMB). We compare GrIS SMB estimates simulated by the regional climate model MAR forced by perturbed reanalysis (ERA-Interim with a temperature correction of +1, +1.5, and +2°C at the MAR lateral boundaries) over 1980–2016 to projections of the future GrIS SMB from MAR simulations forced by three GCMs over selected periods for which a similar temperature increase of +1, +1.5, and +2°C is projected by the GCMs in comparison to 1980–1999. Mean SMB anomalies produced with perturbed reanalysis over the climatologically stable period 1980–1999 are similar to those produced with MAR forced by GCMs over future periods characterised by a similar warming over Greenland. However, over the 2 last decades (2000–2016) when an increase in the frequency of blocking events has been observed in summer, MAR forced by perturbed reanalysis suggests that the SMB decrease could be amplified by a factor of 2 if such atmospheric conditions persist compared to projections forced by GCMs for the same temperature increase but without any circulation change.

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See detailSeasonal mass variations show timing and magnitude of meltwater storage in the Greenland Ice Sheet
Ran, J.; Vizcaino, M.; Ditmar, P.; van den Broeke, M.; Moon, T.; Steger, C.; Enderlin, E.; Wouters, B.; Noël, B.; Reijmer, C.; Klees, R.; Zhong, M.; Liu, L.; Fettweis, Xavier

in Cryosphere (2018), 2981-2999

The Greenland Ice Sheet (GrIS) is currently losing ice mass. In order to accurately predict future sea level rise, the mechanisms driving the observed mass loss must be better understood. Here, we combine data from the satellite gravimetry mission Gravity Recovery and Climate Experiment (GRACE), surface mass balance (SMB) output of the Regional Atmospheric Climate Model v. 2 (RACMO2), and ice discharge estimates to analyze the mass budget of Greenland at various temporal and spatial scales. We find that the mean rate of mass variations in Greenland observed by GRACE was between −277 and −269Gtyr−1 in 2003–2012. This estimate is consistent with the sum (i.e., −304±126Gtyr−1) of individual contributions – surface mass balance (SMB, 216±122Gtyr−1) and ice discharge (520±31Gtyr−1) – and with previous studies. We further identify a seasonal mass anomaly throughout the GRACE record that peaks in July at 80–120Gt and which we interpret to be due to a combination of englacial and subglacial water storage generated by summer surface melting. The robustness of this estimate is demonstrated by using both different GRACE-based solutions and different meltwater runoff estimates (namely, RACMO2.3, SNOWPACK, and MAR3.9). Meltwater storage in the ice sheet occurs primarily due to storage in the high-accumulation regions of the southeast and northwest parts of Greenland. Analysis of seasonal variations in outlet glacier discharge shows that the contribution of ice discharge to the observed signal is minor (at the level of only a few gigatonnes) and does not explain the seasonal differences between the total mass and SMB signals. With the improved quantification of meltwater storage at the seasonal scale, we highlight its importance for understanding glacio-hydrological processes and their contributions to the ice sheet mass variability.

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See detailBrief communication: Recent changes in summer Greenland blocking captured by none of the CMIP5 models
Hanna, E.; Fettweis, Xavier; Hall, R.

in Cryosphere (2018), 12

Recent studies note a significant increase in high-pressure blocking over the Greenland region (Greenland Blocking Index, GBI) in summer since the 1990s. Such a general circulation change, indicated by a negative trend in the North Atlantic Oscillation (NAO) index, is generally highlighted as a major driver of recent surface melt records observed on the Greenland Ice Sheet (GrIS). Here we compare reanalysis-based GBI records with those from the Coupled Model Intercomparison Project 5 (CMIP5) suite of global climate models over 1950–2100. We find that the recent summer GBI increase lies well outside the range of modelled past reconstructions and future GBI projections (RCP4.5 and RCP8.5). The models consistently project a future decrease in GBI (linked to an increase in NAO), which highlights a likely key deficiency of current climate models if the recently observed circulation changes continue to persist. Given well-established connections between atmospheric pressure over the Greenland region and air temperature and precipitation extremes downstream, e.g. over northwest Europe, this brings into question the accuracy of simulated North Atlantic jet stream changes and resulting climatological anomalies over densely populated regions of northern Europe as well as of future projections of GrIS mass balance produced using global and regional climate models.

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See detailMelting over the northeast Antarctic Peninsula (1999–2009): evaluation of a high-resolution regional climate model
Datta, R.T.; Tedesco, M.; Agosta, C.; Fettweis, Xavier; Kuipers Munneke, P.; van den Broeke, M.

in Cryosphere (2018), 12

Surface melting over the Antarctic Peninsula (AP) may impact the stability of ice shelves and thus the rate at which grounded ice is discharged into the ocean. Energy and mass balance models are needed to understand how climatic change and atmospheric circulation variability drive current and future melting. In this study, we evaluate the regional climate model MAR over the AP at a 10km spatial resolution between 1999 and 2009, a period when active microwave data from the QuikSCAT mission is available. This model has been validated extensively over Greenland, has is applied here to the AP at a high resolution and for a relatively long time period (full outputs are available to 2014). We find that melting in the northeastern AP, the focus area of this study, can be initiated both by sporadic westerly föhn flow over the AP mountains and by northerly winds advecting warm air from lower latitudes. A comparison of MAR with satellite and automatic weather station (AWS) data reveals that satellite estimates show greater melt frequency, a larger melt extent, and a quicker expansion to peak melt extent than MAR in the centre and east of the Larsen C ice shelf. These differences are reduced in the north and west of the ice shelf, where the comparison with satellite data suggests that MAR is accurately capturing melt produced by warm westerly winds. MAR shows an overall warm bias and a cool bias at temperatures above 0°C as well as fewer warm, strong westerly winds than reported by AWS stations located on the eastern edge of the Larsen C ice shelf, suggesting that the underestimation of melt in this region may be the product of limited eastward flow. At higher resolutions (5km), MAR shows a further increase in wind biases and a decrease in meltwater production. We conclude that non-hydrostatic models at spatial resolutions better than 5km are needed to better-resolve the effects of föhn winds on the eastern edges of the Larsen C ice shelf.

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See detail2100 : le climat en Europe
Wyard, Coraline; Fettweis, Xavier

Article for general public (2018)

Le climat en Europe d'ici 2100 si aucune mesure d'adaptation ou d'atténuation du réchauffement climatique n'est prise d'ici la fin du siècle. Chiffres issus de publications scientifiques.

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See detailThe CORDEX.be initiative as a foundation for climate services in Belgium
Termonia, P.; Van Schaeybroeck, B.; De Cruz, L.; De Troch, R.; Caluwaerts, S.; Giot, O.; Hamdi, R.; Vannitsem, S.; Duchêne, F.; Willems, P.; Tabari, H.; Van Uytven, E.; Hosseinzadehtalaei, P.; Van Lipzig, N.; Wouters, H.; Vanden Broucke, S.; van Ypersele, J.-P.; Marbaix, P.; Villaunueva-Birriel, C.; Fettweis, Xavier; Wyard, Coraline; Scholzen, C.; Doutreloup, Sébastien; De Ridder, K.; Gobbin, A.; Lauwaet, D.; Stavrakou, T.; Bauwens, M.; Müller, J.-F.; Luyten, P.; Ponsar, S.; Van den Eynde, D.; Pottiaux, E.

in Climate Services (2018), 11

The CORDEX.be project created the foundations for Belgian climate services by producing high-resolution Belgian climate information that (a) incorporates the expertise of the different Belgian climate modeling groups and that (b) is consistent with the outcomes of the international CORDEX (“COordinated Regional Climate Downscaling Experiment”) project. The key practical tasks for the project were the coordination of activities among different Belgian climate groups, fostering the links to specific international initiatives and the creation of a stakeholder dialogue. Scientifically, the CORDEX.be project contributed to the EURO-CORDEX project, created a small ensemble of High-Resolution (H-Res) future projections over Belgium at convection-permitting resolutions and coupled these to seven Local Impact Models. Several impact studies have been carried out. The project also addressed some aspects of climate change uncertainties. The interactions and feedback from the stakeholder dialogue led to different practical applications at the Belgian national level

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See detailAtmospheric river impacts on Greenland Ice Sheet surface mass balance
Mattingly, K.S.; Mote, T.; Fettweis, Xavier

in Journal of Geophysical Research. Atmospheres (2018), online

Greenland Ice Sheet (GrIS) mass loss has accelerated since the turn of the 21st century. Several recent episodes of rapid GrIS ablation coincided with intense moisture transport over Greenland by atmospheric rivers (ARs), suggesting that these events influence the evolution of GrIS surface mass balance (SMB). ARs likely provide melt energy through several physical mechanisms, and conversely, may increase SMB through enhanced snow accumulation. In this study, we compile a long‐term (1980–2016) record of moisture transport events using a conventional AR identification algorithm as well as a self‐organizing map (SOM) classification applied to MERRA‐2 data. We then analyze AR effects on the GrIS using melt data from passive microwave satellite observations and regional climate model output. Results show that anomalously strong moisture transport by ARs clearly contributed to increased GrIS mass loss in recent years. AR activity over Greenland was above normal throughout the 2000s and early 2010s, and recent melting seasons with above‐average GrIS melt feature positive moisture transport anomalies over Greenland. Analysis of individual AR impacts shows a pronounced increase in GrIS surface melt after strong AR events. AR effects on SMB are more complex, as strong summer ARs cause sharp SMB losses in the ablation zone that exceed moderate SMB gains induced by ARs in the accumulation zone during summer and in all areas during other seasons. Our results demonstrate the influence of the strongest ARs in controlling GrIS SMB, and we conclude that projections of future GrIS SMB should accurately capture these rare ephemeral events.

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See detailEvaluation of reconstructions of snow/ice melt in Greenland by regional atmospheric climate models using laser altimetry data
Sutterley, T.; Velicogna, I.; Fettweis, Xavier; Rignot, E.; Noël, B.; van den Broeke, M.

in Geophysical Research Letters (2018), 45

he surface mass balance (SMB) of the Greenland Ice Sheet critically depend on the intensity of ice/snow melt in its ablation zone, but in‐situ data have been too limited to quantify the error of regional climate models. Here, we use 23 years of NASA satellite and airborne laser altimetry from the Airborne Topographic Mapper (ATM), Land, Vegetation and Ice Sensor (LVIS) and Ice, Cloud and land Elevation Satellite (ICESat) to generate time series of elevation change to compare with SMB products from the Regional Atmospheric Climate Model (RACMO2.3p2) and from the Modèle Atmosphérique Régional (MARv3.5.2). For 1994‐2016, the results agree at the 15‐26% level, with the largest discrepancy in north Greenland. During the cold summer 2015, the RMS discrepancy is 40% in the north, 30% in the southwest, and 18‐25% at low elevation. The difference drops to 23% in the southwest and 14% at low elevation during the 2016 warm summer.

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See detailGlobal Radiative Flux and Cloudiness Variability for the Period 1959–2010 in Belgium: A Comparison between Reanalyses and the Regional Climate Model MAR
Wyard, Coraline; Doutreloup, Sébastien; Belleflamme, Alexandre; Wild, Martin; Fettweis, Xavier

in Atmosphere (2018), 9(7), 262

The use of regional climate models (RCMs) can partly reduce the biases in global radiative flux (Eg↓) that are found in reanalysis products and global models, as they allow for a finer spatial resolution and a finer parametrisation of surface and atmospheric processes. In this study, we assess the ability of the MAR («Modèle Atmosphérique Régional») RCM to reproduce observed changes in Eg↓, and we investigate the added value of MAR with respect to reanalyses. Simulations were performed at a horizontal resolution of 5 km for the period 1959–2010 by forcing MAR with different reanalysis products: ERA40/ERA-interim, NCEP/NCAR-v1, ERA-20C, and 20CRV2C. Measurements of Eg↓ from the Global Energy Balance Archive (GEBA) and from the Royal Meteorological Institute of Belgium (RMIB), as well as cloud cover observations from Belgocontrol and RMIB, were used for the evaluation of the MAR model and the forcing reanalyses. Results show that MAR enables largely reducing the mean biases that are present in the reanalyses. The trend analysis shows that only MAR forced by ERA40/ERA-interim shows historical trends, which is probably because ERA40/ERA-interim has a better horizontal resolution and assimilates more observations than the other reanalyses that are used in this study. The results suggest that the solar brightening observed since the 1980s in Belgium has mainly been due to decreasing cloud cover.

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See detailInterest of a Regional Climate Model for doing future projections over the Greenland Ice Sheet
Fettweis, Xavier; Delhasse, Alison; agosta, C.; Amory, Charles; Lang, Charlotte; Kittel, Christoph

Conference (2018, June 22)

With the aim of evaluating the added value of a regional climate model in downscaled future projections over the Greenland Ice Sheet, we have compared the surface fields (snowfall and summer near-surface temperature) coming from the “best” CMIP5 and CMIP6 global models (GCMs) with these fields simulated by the MAR model forced by the same GCMs. These "best" GCMS were selected according to their ability to simulate the summer temperature at 700 hPa and the general circulation at 500 hPa over Greenland with respect to ERA-Interim over 1980-1999. However, despite their ability to correctly represent the free atmosphere, the selected GCMs present significant biases at the surface of the ice sheet. The comparison shows that MAR is however able to strongly reduce these GCM surface biases. We then forced the lateral boundaries of MAR with ERA-Interim to which we applied temperature corrections of +1°C and +2°C. The outputs were compared to MAR forced by GCM future projections corresponding to a climate about 1 and 2°C warmer than the current climate. The results of the different GCM-forced runs and sensitivity experiments are very similar to each other as the GCMs do not project general circulation changes. Moreover, the sensitivity experiments forced by modified ERA-Interim reveal that the projected SMB decrease is exponentially amplified if the increased occurrence of blocking events over Greenland in summer that has been observed since the 2000´s continues in the future.

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See detailModelling the impact of drifting snow on the surface mass balance
Kittel, Christoph; Amory, Charles; Agosta, Cécile; Favier, Vincent; Fettweis, Xavier

Conference (2018, June 20)

The transport of snow by the wind is an important component of the Antarctic surface mass balance (SMB) as drifting snow counts up for a large amount of snow ablation over the ice sheet. However, this process is frequently neglected in atmospheric models. Two simulations (one with drifting snow and one without) were performed at a resolution of 8 km with the regional climate model MAR forced by ERA-Interim, in order to assess the impact of drifting snow on the SMB of Adelie Land (East Antarctica) during the period 2002 - 2016. We evaluated results against field observations (including meteorological and snow skate measurements). Besides to better represent climate surface as airborne snow particles can sublimate and interact with the lowest atmospheric levels, the drifting snow simulation improves the modelled spatial distribution of the SMB and reduces the overestimation of the accumulation in comparison with MAR results without drifting snow.

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See detailImproving Greenland surface mass balance estimates through the assimilation of MODIS albedo: a case study along the K‐transect
Navari, M.; Margulis, S.; Tedesco, M.; Fettweis, Xavier; Alexander, P.

in Geophysical Research Letters (2018)

Estimating the Greenland Ice Sheet (GrIS) surface mass balance (SMB) is an important component of current and future projections of sea level rise. Given the lack of in situ information, imperfect models, and under‐utilized remote sensing data, it is critical to combine the available observations with a physically based model to better characterize the spatial and temporal variation of the GrIS SMB. This work proposes a data assimilation framework that yields SMB estimates that benefit from a state‐of‐the‐art snowpack model (Crocus) and a 16‐day albedo product. Comparison of our results against in‐situ SMB measurements from the Kangerlussuaq transect shows that assimilation of 16‐day albedo product reduces the root mean square error (RMSE) of the posterior estimates of SMB from 1240 millimeter water equivalent (mmWE/yr) to 230 mmWE/yr and reduces the bias from 1140 mmWE/yr to ‐20 mmWE/yr

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See detailMass balance of the Antarctic Ice Sheet from 1992 to 2017
Shepherd, A.; Ivins, E.; Rignot, E.; Smith, B.; van den Broeke, M.; Velicogna, I.; Whitehouse, P.; Briggs, K.; Joughin, I.; Krinner, G.; Nowicki, S.; Payne, T.; Scambos, T.; Schlegel, N.; IMBIE team; Fettweis, Xavier

in Nature (2018), 556

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5 ± 46 billion tonnes per year) being the least certain.

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See detailSeasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling
Heilig, A.; Eisen, O.; MacFerrin, M.; Tedesco, M.; Fettweis, Xavier

in Cryosphere (2018), 12

Increasing melt over the Greenland Ice Sheet (GrIS) recorded over the past several years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as a meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness have been observed in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779° N, 46.2856° W) at 2120 m a.s.l. The radar is capable of quasi-continuously monitoring changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well for both timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg m−2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.

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See detailAssessing the future evolution of climate extremes favouring floods using the regional climate model MAR over the CORDEX.be domain
Wyard, Coraline; Doutreloup, Sébastien; Fettweis, Xavier

Poster (2018, April 13)

In Belgium, most flooding events occur in winter as a result of intense precipitation events but also to the abrupt melting of the snow that covers the Ardennes summits. These conditions favourable to floods exhibit a decreasing trend over the period 1959–2010 resulting from the reduction in snow accumulation thought extreme precipitation events show a positive but non-significant signal. In this study, we investigate how these trends could evolve in a warmer climate by using future projections performed with the regional climate model MAR (for “Modèle Atmosphérique Régional”) in the framework of CORDEX.be, the Belgian EURO-CORDEX project. These future projections were obtained by nesting MAR into NorESM1-M and MIROC5 under the RCP8.5 scenario. Both these global models were selected from the CMIP5 archive after evaluation of their ability to represent the current (1976-2005) mean climate over Europe. This assessment is based on the skill score methodology. Results show that the period 2071-2100 would be marked by a decrease in snowfall amount, in snow accumulation, and consequently in conditions favourable to floods generated by snowpack melting with respect to 1976-2005. Regarding total PPN amount and extremes, the signal is less clear as both GCMs simulate different patterns and trends.

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See detailInfluence of the recent circulation change in summer on future surface mass balance of Greenland ice sheet
Delhasse, Alison; Fettweis, Xavier; Kittel, Christoph; Amory, Charles

Conference (2018, April 11)

Regional Climate Models (RCM) driven by General Circulation Models (GCM) are often used to produce future projections of the surface climate and surface mass balance (SMB) of polar ice sheets. However, GCM do not represent the recent circulation change observed in summer over the Greenland Ice Sheet (GrIS) since the 2000’s and do not predict any circulation changes for the next century. The goal of this study is to evaluate the impact of an atmospheric circulation change (as currently observed) combined with a temperature increase on the future GrIS SMB. We compare here SMB results from the RCM MAR (Modèle atmosphérique régional) forced by warmer reanalyses (ERA-Interim with a temperature correction of +1, +1,5 and +2 C at the lateral boundaries) to SMB results from MAR future simulations forced with GCM during a period where there is a temperature increase of +1, +1,5 and +2 C compared to 1980-1999. Mean SMB produced with warmer reanalyses over 1980-1999 is similar to that obtained when forcing with GCM over a period characterized by a similarly warmer climate. During last years (2000-2016) when a circulation change has been observed in summer, MAR forced with warmer reanalyses shows a significant amplified SMB decrease compared to future simulations forced by GCM for the same temperature increase.

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See detailEtude des séries temporelles : exemple de la température de l’eau
Richir, Jonathan; Borges, Alberto; Champenois, Willy; Leduc, Michèle; Binard, Marc; Fettweis, Xavier; Lejeune, Pierre; Boissery, Pierre; Gobert, Sylvie

Conference (2018, April 10)

De nombreux paramètres biologiques, environnementaux, climatologiques sont mesurés à et par STARESO depuis des décennies. Les données récoltées sont accessibles via la base de données partagée RACE de l’Université de Liège. Dans le cas de séries temporelles, les paramètres suivis sont mesurés de manière séquentielle au cours du temps. La plus représentative est sans aucun doute la série des données de température de l’eau acquise depuis près de 40 ans. La température est un paramètre important qui permet de mettre en évidence sur le long-terme des changements notamment liés au réchauffement climatique, changements qui affectent le fonctionnement des océans tant dans la physique que dans la biologie. L’analyse des séries temporelles de données nécessitent souvent un important travail préparatoire de standardisation (intervalles de mesure irréguliers, trous dans la série, évolution des méthodes d’acquisition des données …). Une fois standardisées, les séries de données peuvent être analysées avec les outils et approches statistiques propres aux séries temporelles : décomposition de la série pour en extraire la tendance générale, statistiques glissantes, calcul des anomalies, analyse des quantiles, mise en évidence d’évènements extrêmes tels les vagues de chaleurs … Tout ce travail, conséquent, doit pouvoir être partagé, vérifié, validé et permettre la mise à jour ultérieure de l’analyse. C’est le concept même de science reproductible. Cette reproductibilité est rendue notamment possible par l’utilisation du langage de programmation R. Cette communication illustre, à travers l’exemple clef de l’évolution de la température de l’eau, l’analyse des séries temporelles de données dans le cadre de STARECAPMED.

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See detailGlobal warming and natural variability
Reynaerts, Laura; Fettweis, Xavier; Nicolay, Samuel

Poster (2018, April)

In this work, we show that the lull periods of the warming episodes observed in the surface temperatures (GISS,...) can be explained through the natural variability of the climate. To do so, we use the wavelet transform to extract the physical modes in the temperature data (which can be seen as am-fm components) and show that these modes induce coolings that corresponds to the lull periods empirically observed.

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See detailExtreme temperature events on Greenland in observations and the MAR regional climate model
Leeson, A.; Eastoe, E.; Fettweis, Xavier

in Cryosphere (2018), 12

Meltwater from the Greenland Ice Sheet contributed 1.7–6.12 mm to global sea level between 1993 and 2010 and is expected to contribute 20–110 mm to future sea level rise by 2100. These estimates were produced by regional climate models (RCMs) which are known to be robust at the ice sheet scale but occasionally miss regional- and local-scale climate variability (e.g. Leeson et al., 2017; Medley et al., 2013). To date, the fidelity of these models in the context of short-period variability in time (i.e. intra-seasonal) has not been fully assessed, for example their ability to simulate extreme temperature events. We use an event identification algorithm commonly used in extreme value analysis, together with observations from the Greenland Climate Network (GC-Net), to assess the ability of the MAR (Modèle Atmosphérique Régional) RCM to reproduce observed extreme positive-temperature events at 14 sites around Greenland. We find that MAR is able to accurately simulate the frequency and duration of these events but underestimates their magnitude by more than half a degree Celsius/kelvin, although this bias is much smaller than that exhibited by coarse-scale Era-Interim reanalysis data. As a result, melt energy in MAR output is underestimated by between 16 and 41 % depending on global forcing applied. Further work is needed to precisely determine the drivers of extreme temperature events, and why the model underperforms in this area, but our findings suggest that biases are passed into MAR from boundary forcing data. This is important because these forcings are common between RCMs and their range of predictions of past and future ice sheet melting. We propose that examining extreme events should become a routine part of global and regional climate model evaluation and that addressing shortcomings in this area should be a priority for model development.

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See detailEstimation des températures au début du dernier millénaire dans l’ouest du Groenland : résultats préliminaires issus de l’application d’un modèle glaciologique de type degré‑jour sur le glacier du Lyngmarksbræen
Biette, M.; Jomelli, V.; Favier, V.; Chenet, M.; Agosta, Cécile; Fettweis, Xavier; Ho Tong Minh, D.; Ose, K.

in Géomorphologie: Relief, Processus, Environnement (2018), 24

The last millennium is defined as a “stable” climatic period with anomalies such as the Little Ice Age (LIA: ~1450 AD to 1850 AD), a period marked by low temperatures and associated with a glacier advance. Also the Medieval Climate Anomaly (MCA: ~950 AD to 1250 AD), considered as a period at least as warm as nowadays and associated with glacier retreat in the northern hemisphere. However, several studies have shown that glacial advances have occurred during the MCA period in the Baffin Land and western Greenland, in contradiction with hemispheric‑scale temperature reconstructions. In this study we propose temperature conditions for the last millennium determined from a recent study on the glacial fluctuations of the Lyngmarksbræen glacier and the application of an empirical positive degree‑day model (PDD) constrained by the outputs of the regional climate MAR atmospheric model. This simulation was conducted on the Lyngmarksbræen glacier, which shows an original succession of nested moraines dated from the last millennium. The results show that the most likely scenarios are based on air temperatures in the range of ‑1.3°C to ‑1.6°C lower during the MCA than at the end of the 20th century if we consider a variation of about ± 10% in precipitation. Sensitivity tests are then made on different parameters of the glaciological model to better constrain the uncertainty of the temperature estimations.

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See detailCyclone Activity in the Arctic From an Ensemble of Regional Climate Models (Arctic CORDEX)
Akperov, M.; Rinke, A.; Mokhov, I.; Matthes, H.; Semenov, V.; Adakudlu, M.; Cassano, J.; Christensen, J.; Dembitskaya, M.; Dethloff, K.; Fettweis, Xavier; Glisan, J.; Gutjahr, O.; Heinemann, G.; Koenigk, T.; Koldunov, N.; Laprise, R.; Mottram, R.; Nikiéma, O.; Scinocca, J.; Sein, D.; Sobolowski, S.; Winger, K.; Zhang, W.

in Journal of Geophysical Research. Atmospheres (2018), 123

The ability of state‐of‐the‐art regional climate models to simulate cyclone activity in the Arctic is assessed based on an ensemble of 13 simulations from 11 models from the Arctic‐CORDEX initiative. Some models employ large‐scale spectral nudging techniques. Cyclone characteristics simulated by the ensemble are compared with the results forced by four reanalyses (ERA‐Interim, National Centers for Environmental Prediction‐Climate Forecast System Reanalysis, National Aeronautics and Space Administration‐Modern‐Era Retrospective analysis for Research and Applications Version 2, and Japan Meteorological Agency‐Japanese 55‐year reanalysis) in winter and summer for 1981–2010 period. In addition, we compare cyclone statistics between ERA‐Interim and the Arctic System Reanalysis reanalyses for 2000–2010. Biases in cyclone frequency, intensity, and size over the Arctic are also quantified. Variations in cyclone frequency across the models are partly attributed to the differences in cyclone frequency over land. The variations across the models are largest for small and shallow cyclones for both seasons. A connection between biases in the zonal wind at 200 hPa and cyclone characteristics is found for both seasons. Most models underestimate zonal wind speed in both seasons, which likely leads to underestimation of cyclone mean depth and deep cyclone frequency in the Arctic. In general, the regional climate models are able to represent the spatial distribution of cyclone characteristics in the Arctic but models that employ large‐scale spectral nudging show a better agreement with ERA‐Interim reanalysis than the rest of the models. Trends also exhibit the benefits of nudging. Models with spectral nudging are able to reproduce the cyclone trends, whereas most of the nonnudged models fail to do so. However, the cyclone characteristics and trends are sensitive to the choice of nudged variables.

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See detailCombining regional downscaling expertise in Belgium: CORDEX and beyond
Termonia, Piet; Van Schaeybroeck, Bert; De Cruz, Lesley; De Troch, Rozemien; Giot, Olivier; Hamdi, Rafiq; Vannitsem, Stéphane; Duchêne, François; Willems, Patrick; Tabari, Hossein; Van Uytven, Els; Hosseinzadehtalaei, Parisa; Van Lipzig, Nicole; Wouters, Hendrik; Vanden Broucke, Sam; Demuzere, Matthias; van Ypersele, Jean-Pascal; Marbaix, Philippe; Villanueva-Birriel, Cecille; Fettweis, Xavier; Wyard, Coraline; Scholzen, Chloé; Doutreloup, Sébastien; De Ridder, Koen; Lauwaet, Dirk; Gobin, Anne; Stavrakou, Trissevgeni; Bauwens, Maite; Müller, Jean-François; Luyten, Patrick; Ponsar, Stéphanie; Van den Eynde, Dries; Pottiaux, Eric

Report (2018)

The main objectives of the CORDEX.be project were: 1. Contribute to the international climate community by participating to EURO-CORDEX by performing regional climate simulations over Europe. 2. Provide an ensemble of High-Resolution (H-Res) climate simulations over Belgium i.e. to create a small ensemble of high-resolution future projections over Belgium at convectionpermitting resolutions. 3. Couple these model simulations to seven local-impact models for impact studies. 4. Present an overview of the ongoing climate modeling activities in Belgium. 5. Provide coherent climate information for Belgium targeted to end-users, backed by: (i) a unified framework for the H-Res climate runs and (ii) uncertainty estimations on the climate change signal; 6. Provide and present a climate-impact report for stakeholders and the general public that highlight the most important results of the project.

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See detailObservations and modelling od drifting snow occurrences in coastal East Antarctica
Amory, Charles; Kittel, Christoph; Fettweis, Xavier

Conference (2017, December 15)

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See detailSensitivity of the Antarctic surface mass balance to oceanic perturbations
Kittel, Christoph; Amory, Charles; Agosta, Cécile; Fettweis, Xavier

Poster (2017, December 15)

Regional climate models (RCMs) are suitable numerical tools to study the surface mass balance (SMB) of the wide polar ice sheets due to their high spatial resolution and polar-adapted physics. Nonetheless, RCMs are driven at their boundaries and over the ocean by reanalysis or global climate model (GCM) products and are thus influenced by potential biases in these large-scale fields. These biases can be significant for both the atmosphere and the sea surface conditions (i.e. sea ice concentration and sea surface temperature). With the RCM MAR, a set of sensitivity experiments has been realized to assess the direct response of the SMB of the Antarctic ice sheet to oceanic perturbations. MAR is forced by ERA-Interim and anomalies based on mean GCM biases are introduced in sea surface conditions. Results show significant increases (decreases) of liquid and solid precipitation due to biases related to warm (cold) oceans. As precipitation is mainly caused by low-pressure systems that intrude into the continent and do not penetrate far inland, coastal areas are more sensitive than inland regions. Furthermore, warm ocean representative biases lead to anomalies as large as anomalies simulated by other RCMs or GCMs for the end of the 21st century.

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See detailInterests of using the RCM MAR to downscale CMIP6 outputs
Amory, Charles; Kittel, Christoph; Delhasse, Alison; Agosta, Cécile; Fettweis, Xavier

Conference (2017, December 10)

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See detailWinter season changes in Belgium: the MAR model contribution to the CORDEX.be project
Wyard, Coraline; Doutreloup, Sébastien; Scholzen, Chloé; Fettweis, Xavier

Conference (2017, November 17)

In the framework of the CORDEX.be project funded by Belspo, most universities and research institutes of Belgium have worked together in order to gather existing and ongoing Belgian research activities in the domain of climate modelling to create a coherent scientific basis for future climate services in Belgium. The Laboratory of Climatology of the University of Liège has performed climate simulations using the regional climate model MAR (“Modèle Atmosphérique Régional” in French) at a resolution of 5 km over the period 1959-2014. This research aims to study the evolution of several variables computed by MAR during the winters of the last 50 years. Except in snow accumulation, results show no statistically significant trend in winter temperature or precipitation in Belgium. This results from the strong influence of natural large-scale/low-frequency oscillations in the atmospheric circulation in winter such as the North Atlantic Oscillation.

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See detailGreenland Ice Sheet Surface Mass Loss: Recent Developments in Observation and Modeling
van den Broeke, M.; Box, J.; Fettweis, Xavier; Hanna, E.; Noël, B.; Tedesco, M.; van As, D.; van de Berg, W.J.; van Kampenhout, L.

in Current Climate Change Reports (2017)

Surface processes currently dominate Greenland ice sheet (GrIS) mass loss. We review recent developments in the observation and modeling of GrIS surface mass balance (SMB), published after the July 2012 deadline for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). Since IPCC AR5, our understanding of GrIS SMB has further improved, but new observational and model studies have also revealed that temporal and spatial variability of many processes are still poorly quantified and understood, e.g., bio-albedo, the formation of ice lenses and their impact on lateral meltwater transport, heterogeneous vertical meltwater transport (‘piping’), the impact of atmospheric-circulation changes and mixed-phase clouds on the surface energy balance, and the magnitude of turbulent heat exchange over rough ice surfaces. As a result, these processes are only schematically or not at all included in models that are currently used to assess and predict future GrIS surface mass loss.

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See detailDark ice dynamics of the south-west Greenland Ice Sheet
Tedstone, A.; Bamber, J.; Cook, J.; Williamson, C.; Fettweis, Xavier; Hodson, A.; Tranter, M.

in Cryosphere (2017), 11

Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to changes in atmospheric circulation and atmospheric warming. Albedo reductions resulting from these changes have amplified surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of dark ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large interannual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare ice sectors of the GrIS will evolve in the future, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades. Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years but so too does its duration (from 0 to > 80 % of June–July–August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant interannual synoptic controls. We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics. We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from ablating ice, these particulates alone do not drive dark ice dynamics. Instead, they may enable the growth of pigmented ice algal assemblages which cause visible surface darkening, but only when the climatological prerequisites of liquid meltwater presence and sufficient photosynthetically active radiation fluxes are met. Further field studies are required to fully constrain the processes by which ice algae growth proceeds and the apparent dependency of algae growth on melt-out particulates.

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See detailThe MAR model : CORDEX.be and EURO-CORDEX results
Wyard, Coraline; Doutreloup, Sébastien; Fettweis, Xavier

Scientific conference (2017, September 14)

This research discusses the results obtained by running the MAR model over the CORDEX.be and EURO-CORDEX domains. The MAR results depend on its horizontal resolution (5 - 10 - 20 km), its version (v3.6 vs v3.7), and on the reanalysis used as forcing.

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See detailSurface solar radiation modelling over 1900-2014: comparison between the regional climate model MAR and reanalyses
Wyard, Coraline; Doutreloup, Sébastien; Belleflamme, Alexandre; Fettweis, Xavier

Poster (2017, September 04)

Many studies show that the surface solar radiation has underwent large variations over the second half of the 20th century as a result of variations in cloud cover and aerosol loading in the atmosphere. However, it is difficult to build strong conclusions before the 1950s because of the observations scarcity. The evolution of the surface solar radiation has been reconstructed over 1900-2014 using the regional model MAR (« Modèle Atmosphérique Régional ») which has recently been chosen to be part of the EURO-CORDEX project, thanks to the CORDEX.be project. Simulations were performed at a horizontal resolution of 5 km over a domain of 600 x 550 km² covering Belgium. Boundary conditions were provided by four reanalysis products: ERA-interim (1979-2014) completed by the ERA40 (1958-1978), NCEP/NCAR-v1 (1948-2014), ERA-20C (1900-2010) and 20CRV2C (1900-2010). Surface solar radiation measurements from the Global Energy Balance Archive and cloud cover observations from Belgocontrol covering 1966-2007 were used for the evaluation of the MAR model and the forcing reanalyses. Results show that MAR produces much better results than the reanalyses. The driving reanalyses can generate divergent trends while they assimilate observations and are supposed to represent the same climate.

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See detailApplication of a two-step approach for mapping ice thickness to various glacier types on Svalbard
Fürst, J.; Gillet-Chaulet, F.; Benham, T.; Dowdeswell, J.; Grabiec, M.; Navarro, F.; Pettersson, R.; Moholdt, G.; Nuth, C.; Sass, B.; Aas, K.; Fettweis, Xavier; Lang, Charlotte; Seehaus, T.; Braun, M.

in Cryosphere (2017)

The basal topography is largely unknown beneath most glaciers and ice caps, and many attempts have been made to estimate a thickness field from other more accessible information at the surface. Here, we present a two-step reconstruction approach for ice thickness that solves mass conservation over single or several connected drainage basins. The approach is applied to a variety of test geometries with abundant thickness measurements including marine- and land-terminating glaciers as well as a 2400 km2 ice cap on Svalbard. The input requirements are kept to a minimum for the first step. In this step, a geometrically controlled, non-local flux solution is converted into thickness values relying on the shallow ice approximation (SIA). In a second step, the thickness field is updated along fast-flowing glacier trunks on the basis of velocity observations. Both steps account for available thickness measurements. Each thickness field is presented together with an error-estimate map based on a formal propagation of input uncertainties. These error estimates point out that the thickness field is least constrained near ice divides or in other stagnant areas. Withholding a share of the thickness measurements, error estimates tend to overestimate mismatch values in a median sense. We also have to accept an aggregate uncertainty of at least 25 % in the reconstructed thickness field for glaciers with very sparse or no observations. For Vestfonna ice cap (VIC), a previous ice volume estimate based on the same measurement record as used here has to be corrected upward by 22 %. We also find that a 13 % area fraction of the ice cap is in fact grounded below sea level. The former 5 % estimate from a direct measurement interpolation exceeds an aggregate maximum range of 6–23 % as inferred from the error estimates here.

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See detailNoël 2010 en Belgique : neige en Flandre et pluie en Haute-Ardenne
Fettweis, Xavier; Wyard, Coraline; Doutreloup, Sébastien; Belleflamme, Alexandre

in Bulletin de la Société Géographique de Liège (2017), 68

On December 2010, several snow events allowed an exceptional snow cover over Belgium. 27 days with snow cover were observed at Uccle and snow depths of 20, 30 and 70 cm were measured on Christmas 2010 respectively at Uccle, Bierset and Mont Rigi in the Hautes-Fagnes. On December 20, while the entire Belgium was covered by a thick blanket of snow, warmer air invaded the country on December 21. This air was quickly replaced by polar air in Lower and Central Belgium (including Bierset). Heavy snowfalls were observed on December 22 and 23, except in the Upper Ardennes where rainfalls occurred under positive temperature which then dropped to -5°C. This event was due to a strong thermal inversion in the lower layers with warm air at 850 hPa above the Ardennes only. This paper aims to explain this atypical extreme event using the regional climate model MAR developed at the University of Liège

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See detailEvaluating Model Simulations of Twentieth-Century Sea Level Rise. Part 2: Regional Sea-Level Changes
Meyssignac, B.; Slangen, A.; Melet, A.; Church, J.; Fettweis, Xavier; Marzeion, B.; Agosta, Cécile; Ligtenberg, S.; Spada, G.; Richter, K.; Palmer, M.; Roberts, C.; Champollion, N.

in Journal of Climate (2017), in press

Twentieth century regional sea-level changes are estimated from 12 climate models from the 5th phase of the Climate Model Intercomparison Project (CMIP5). The output of the CMIP5 climate model simulations were used to calculate the global and regional sea-level changes associated with dynamic sea level, atmospheric loading, glacier mass changes and ice sheet surface mass balance contributions. The contribution from groundwater depletion, reservoir storage and dynamic ice sheet mass changes are estimated from observations as they are not simulated by climate models. All contributions are summed, including the GIA contribution, and compared to observational estimates from 27 tide gauge records over the twentieth century (1900-2015). We find a general agreement between the simulated sea level and tide gauge records in terms of inter-annual to multi-decadal variability over 1900-2015. But climate models tend to systematically underestimate the observed sea-level trends, particularly in the first half of the 20th century. The corrections based on attributable biases between observations and models that have been identified in the part-1-paper, result in an improved explanation of the spatial variability in observed sea-level trends by climate models. Climate models show that the spatial variability in sea-level trends observed by tide-gauge records is dominated by the GIA contribution and the steric contribution over 1900-2015. Climate models also show that it is important to include all contributions to sea-level changes as they cause significant local deviations; for example, the groundwater depletion around India which is responsible for the low 20th century sea-level rise in the region.

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See detailEvaluating model simulations of 20th century sea-level rise. Part 1: Global mean sea-level change
Slangen, A.; Meyssignac, B.; Agosta, Cécile; Champollion, N.; Church, J.; Fettweis, Xavier; Ligtenberg, S.; Marzeion, B.; Melet, A.; Palmer, M.; Richter, K.; Roberts, C.; Spada, G.

in Journal of Climate (2017)

Sea-level change is one of the major consequences of climate change and is projected to affect coastal communities around the world. Here, we compare Global Mean Sea-Level (GMSL) change estimated by 12 climate models from the 5th phase of the World Climate Research Programme’s Climate Model Intercomparison Project (CMIP5) to observational estimates for the period 1900-2015. We analyse observed and simulated individual contributions to GMSL change (thermal expansion, glacier mass change, ice sheet mass change, landwater storage change) and compare the summed simulated contributions to observed GMSL change over the period 1900-2007 using tide gauge reconstructions, and over the period 1993-2015 using satellite altimetry estimates. The model-simulated contributions allow us to explain 50 ± 30% (uncertainties 1.65σ unless indicated otherwise) of the mean observed change from 1901-1920 to 1988-2007. Based on attributable biases between observations and models, we propose to add a number of corrections, which result in an improved explanation of 75 ± 38% of the observed change. For the satellite era (1993-1997 to 2011-2015) we find an improved budget closure of 102 ± 33% (105 ± 35% when including the proposed bias corrections). Simulated decadal trends over the 20th century increase, both in the thermal expansion and the combined mass contributions (glaciers, ice sheets and landwater storage). The mass components explain the majority of sea-level rise over the 20th century, but the thermal expansion has increasingly contributed to sea-level rise, starting from 1910 onwards and in 2015 accounting for 46% of the total simulated sea-level change.

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See detailRECONSTRUCTION DE L'ÉVOLUTION DU RAYONNEMENT SOLAIRE REÇU EN SURFACE EN EUROPE OCCIDENTALE SUR LA PÉRIODE 1900-2014 À L'AIDE DU MODÈLE ATMOSPHÉRIQUE RÉGIONAL MAR
Wyard, Coraline; Fettweis, Xavier; Belleflamme, Alexandre; Doutreloup, Sébastien; Erpicum, Michel

in Dahech, Salem; Charfi, Sami (Eds.) Actes du XXXe colloque de l'Association Internationale de Climatologie : Climat, ville et environnement (2017, July)

Many studies show that the surface solar radiation has underwent large variations over the second half of the 20th century as a result of variations in cloud cover and aerosol loading in the atmosphere. However, it is difficult to build strong conclusions before the 1950' because of the observations scarcity. The evolution of the surface solar radiation has been reconstructed over 1900-2014 using the regional model MAR (« Modèle Atmosphérique Régional ») in Belgium. Boundary conditions were provided by four reanalysis products : the ERA-interim (1979-2014) completed by the ERA40 (1958-1978), the NCEP/NCAR-v1 (1948-2014), the ERA-20C (1900-2010) and the 20CRV2C (1900-2010). Results show that the reanalyses can generate divergent trends while they assimilate observations and are supposed to represent the same climate.

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See detailÉvolution de l'enneigement moyen dans les Alpes au cours du 20e siècle à l'aide du modèle atmosphérique régional MAR
Belleflamme, Alexandre; Wyard, Coraline; Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

in Dahech, Salem; Charfi, Sami (Eds.) Actes du XXXème colloque de l'Association Internationale de Climatologie - Climat, ville et environnement (2017, July)

The evolution of the snow height over the Alps can strongly impact tourism, but also the water availability of the region. In this study, we have reproduced the evolution of the climate in the Alps over the 20th century with the help of the regional atmospheric model MAR forced by three reanalyses (ERA-20C, NCEP/NCAR, and ERA-Interim). MAR shows that the snow height has increased since the beginning of the 20th century, first only at higher altitudes, then also at lower levels, before knowing a strong and abrupt decrease between 1985 and 1990. This evolution, which is consistent with observations given in the literature, is directly linked with the trends of NAO and AO. In fact, the atmospheric circulation changes highlighted by NAO and AO induce temperature and precipitation changes that directly determine the snow height in the Alps.

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See detailÉvaluation de la capacité du Modèle Atmosphérique Régional (MAR) à simuler la saison des pluies en Afrique Intertropicale
Doutreloup, Sébastien; Wyard, Coraline; Belleflamme, Alexandre; François, Louis; Fettweis, Xavier; Erpicum, Michel

in Dahech, Salem; Charfi, Sami (Eds.) Actes du XXXe colloque de l'Association Internationale de Climatologie : CLIMAT, VILLE ET ENVIRONNEMENT (2017, July)

In Intertropical Africa, climate is essentially characterized by the amount of precipitation and its annual regime. These precipitations and their evolution during the period 1970-1999 are simulated thanks to the Regional Atmospheric Model (MAR), developed at the ULg, and forced by the NCEP1 reanalyses and by the outputs of three global models (GCM) of the CMIP5 database. These MAR simulations are compared to the gridded data of the Climate Research Unit (CRU). It is clear from our investigations that the simulation of the MAR model forced by the NCEP1 reanalyses is better reproducing the quantities as well as the annual rainfall regime in the semi-arid regions than in equatorial regions. On the other hand, simulations of the MAR forced by the outputs of the GCMs are globally unsatisfactory throughout the intertropical domain in terms of quantities as well as the seasonality of precipitation.

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See detailDecreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet
Hofer, S.; Tedstone, A.; Fettweis, Xavier; Bamber, J.

in Science (2017), 3(6),

The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate since the mid-1990s. This has been due to both increased ice discharge into the ocean and melting at the surface, with the latter being the dominant contribution. This change in state has been attributed to rising temperatures and a decrease in surface albedo. We show, using satellite data and climate model output, that the abrupt reduction in surface mass balance since about 1995 can be attributed largely to a coincident trend of decreasing summer cloud cover enhancing the melt-albedo feedback. Satellite observations show that, from 1995 to 2009, summer cloud cover decreased by 0.9 ± 0.3% per year. Model output indicates that the GrIS summer melt increases by 27 ± 13 gigatons (Gt) per percent reduction in summer cloud cover, principally because of the impact of increased shortwave radiation over the low albedo ablation zone. The observed reduction in cloud cover is strongly correlated with a state shift in the North Atlantic Oscillation promoting anticyclonic conditions in summer and suggests that the enhanced surface mass loss from the GrIS is driven by synoptic-scale changes in Arctic-wide atmospheric circulation.

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See detailReconstructions of the 1900–2015 Greenland ice sheet surface mass balance using the regional climate MAR model
Fettweis, Xavier; Box, Jason; Agosta, Cécile; Amory, Charles; Kittel, Christoph; Lang, Charlotte; van As, Dirk; Machguth, Horst; Gallée, Hubert

in Cryosphere (2017), 11

With the aim of studying the recent Greenland ice sheet (GrIS) surface mass balance (SMB) decrease relative to the last century, we have forced the regional climate MAR (Modèle Atmosphérique Régional; version 3.5.2) model with the ERA-Interim (ECMWF Interim Re-Analysis; 1979–2015), ERA-40 (1958–2001), NCEP–NCARv1 (National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis version 1; 1948–2015), NCEP–NCARv2 (1979–2015), JRA-55 (Japanese 55-year Reanalysis; 1958–2014), 20CRv2(c) (Twentieth Century Reanalysis version 2; 1900–2014) and ERA-20C (1900–2010) reanalyses. While all these forcing products are reanalyses that are assumed to represent the same climate, they produce significant differences in the MAR-simulated SMB over their common period. A temperature adjustment of +1 °C (respectively −1 °C) was, for example, needed at the MAR boundaries with ERA-20C (20CRv2) reanalysis, given that ERA-20C (20CRv2) is ∼ 1 °C colder (warmer) than ERA-Interim over Greenland during the period 1980–2010. Comparisons with daily PROMICE (Programme for Monitoring of the Greenland Ice Sheet) near-surface observations support these adjustments. Comparisons with SMB measurements, ice cores and satellite-derived melt extent reveal the most accurate forcing datasets for the simulation of the GrIS SMB to be ERA-Interim and NCEP–NCARv1. However, some biases remain in MAR, suggesting that some improvements are still needed in its cloudiness and radiative schemes as well as in the representation of the bare ice albedo. Results from all MAR simulations indicate that (i) the period 1961–1990, commonly chosen as a stable reference period for Greenland SMB and ice dynamics, is actually a period of anomalously positive SMB (∼ +40 Gt yr−1) compared to 1900–2010; (ii) SMB has decreased significantly after this reference period due to increasing and unprecedented melt reaching the highest rates in the 120-year common period; (iii) before 1960, both ERA-20C and 20CRv2-forced MAR simulations suggest a significant precipitation increase over 1900–1950, but this increase could be the result of an artefact in the reanalyses that are not well-enough constrained by observations during this period and (iv) since the 1980s, snowfall is quite stable after having reached a maximum in the 1970s. These MAR-based SMB and accumulation reconstructions are, however, quite similar to those from Box (2013) after 1930 and confirm that SMB was quite stable from the 1940s to the 1990s. Finally, only the ERA-20C-forced simulation suggests that SMB during the 1920–1930 warm period over Greenland was comparable to the SMB of the 2000s, due to both higher melt and lower precipitation than normal.

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See detailThe recent warming trend in North Greenland
Orsi, A.; Kawamura, K.; Masson-Delmotte, V.; Fettweis, Xavier; Box, J.; Dahl-Jensen, D.; Clow, G.; Landais, A.; Severinghaus, J.

in Geophysical Research Letters (2017)

The Arctic is among the fastest warming regions on Earth, but it is also one with limited spatial coverage of multi-decadal instrumental surface air temperature measurements. Consequently, atmospheric reanalyses are relatively unconstrained in this region, resulting in a large spread of estimated 30-year recent warming trends, which limits their use to investigate the mechanisms responsible for this trend. Here, we present a surface temperature reconstruction over 1982-2011 at NEEM (51∘ W, 77∘ N), in North Greenland, based on the inversion of borehole temperature and inert gas isotope data. We find that NEEM has warmed by 2.7±0.33∘C over the past 30 years, from the long-term 1900-1970 average of -28.55±0.29∘C. The warming trend is principally caused by an increase in downward longwave heat flux. Atmospheric reanalyses underestimate this trend by 17%, underlining the need for more in situ observations to validate reanalyses.

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See detailDecrease in climatic conditions favouring floods in the south-east of Belgium over 1959-2010 using the regional climate model MAR
Wyard, Coraline; Scholzen, Chloé; Fettweis, Xavier; Van Campenhout, Jean; François, Louis

in International Journal of Climatology (2017), 37(5), 27822796

The Ourthe River, in the south-east of Belgium, has a catchment area of 3,500 km2 and is one of the main tributaries of the Meuse River. In the Ourthe, most of the flood events occur during winter and about 50% of them are due to heavy rainfall events combined to an abrupt melting of the snowpack covering the Ardennes massif during winter. This study aims to determine whether trends in extreme hydroclimatic events generating floods can be detected over the last century in Belgium, where a global warming signal can be observed. Hydroclimatic conditions favourable to floods were reconstructed over 1959- 2010 using the regional climate model MAR (“Modèle Atmosphérique Régional”) forced by the ERA-Interim/ERA-40, the ERA-20C and the NCEP/NCAR-v1 reanalyses. Extreme run-off events, which could potentially generate floods, were detected using run-off caused by precipitation events and snowpack melting from the MAR model. In the validation process, the MAR-driven temperature, precipitation and snow depth were successfully compared to daily weather data over the period 2008-2014 for 20 stations in Belgium. MAR also showed its ability to detect up to 90% of the hydroclimatic conditions which effectively generated observed floods in the Ourthe River over the period 1974- 2010. Conditions favourable to floods in the Ourthe River catchment present a negative trend over the period 1959-2010 as a result of a decrease in snow accumulation and a shortening of the snow season. This trend is expected to accelerate in a warmer climate. However, regarding the impact of the extreme precipitation events evolution on conditions favouring floods, the signal is less clear since the trends depend on the reanalysis used to force the MAR model.

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See detailChangement climatique : conséquences en Ardenne
Wyard, Coraline; Fettweis, Xavier

Article for general public (2017)

Le GIEC (Groupe d'experts Intergouvernemental sur l'évolution du climat) prévoit pour le futur plus de précipitations hivernales et donc à priori un risque accru d'inondations en Belgique. En Ardenne, la majorité des débordements de rivières, telles que l'Ourthe, l'Amblève ou encore la Vesdre, survient en hiver et près de la moitié d'entre eux est due à la combinaison de fortes pluies à une fonte rapide du manteau neigeux. Une reconstitution de l'évolution des précipitations et de l'enneigement en Belgique à l'aide d'un modèle du climat, développé au Laboratoire de Climatologie de l'Université de Liège, montre cependant que les conditions climatiques favorisant les inondations hivernales ont diminué en Ardenne au cours de ces cinquante dernières années.

See detailModelling past and present distributions of tropical African biomes and species using a dynamic vegetation model.
Dury, Marie; Doutreloup, Sébastien; Hardy, Olivier; Fayolle, Adeline; Fettweis, Xavier; Hambuckers, Alain; Gallee, Hubert; François, Louis

Poster (2017, February 08)

In the framework of the AFRIFORD project (Genetic and paleoecological signatures of African rainforest dynamics: pre-adapted to change?, http://www.ulb.ac.be/facs/sciences/afriford/), we used the CARAIB dynamic vegetation model to simulate past and present distributions of tropical African vegetation at the biome and species levels to better project and understand future dynamics. We studied individual species (e.g., Afzelia africana, Pericopsis elata, etc) for which we determined climatic requirements and gathered specific traits. To perform palaeovegetation reconstructions, we used outputs of snapshot climate simulations (e.g., CNRM-CM5, FGOALS-g2 and MRI-CGCM5 global climatic models) from the PaleoModelling Intercomparison Project (PMIP3, https://pmip3.lsce.ipsl.fr/) for the mid-Holocene (6 ka) and the Last Glacial Maximum (LGM, 21 ka). These global climatic outputs were downscaled at a 0.45° spatial resolution over Equatorial Africa using the MAR regional climate model (RCM). For current conditions, the RCM was nested in different historical climate datasets. We compared modelled species distributions with species occurrences from different databases for present and with palaeorecords for past periods. MAR regional climate simulations notably allow CARAIB to reproduce the Dahomey Gap separating the Upper and Lower Guinean forests in West Africa in present biome distribution. The vegetation model also simulates LGM rainforest distribution in agreement with hypothetical glacial rainforest refuge areas inferred from palaeorecords.

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See detailMontée des eaux : Un nouveau modèle de simulation climatique
Wyard, Coraline; Fettweis, Xavier

Article for general public (2017)

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See detailRegional sea level changes for the 20th and the 21st century induced by the regional variability in Greenland ice sheet surface mass loss
Meyssignac, B.; Fettweis, Xavier; Chevrier, R.; pada, G.

in Journal of Climate (2017), online

Surface mass balance (SMB) variations of the Greenland ice sheet (GrIS) has been identified as an important contributor to contemporary and projected global mean sea level variations but their impact on the regional sea level change pattern is still poorly known. This study provides for the first time, consistent estimates (i.e. computed with the same models over the past -1900-2015- and over the future -2015-2100-) of GrIS SMB over 1900-2100 based on the output of 32 atmospheric-ocean General Circulation Models and Earth system models involved in the Climate Model Intercomparison Project phase 5 (CMIP5). It is based on a downscaling technique calibrated against the MAR regional climate model in order to calculate an ensemble of 32 Greenland SMB estimates at regional scale. Because the GrIS SMB does not respond uniformly to greenhouse gases (GHG) emissions. the southern part of the GrIS is more sensitive to climate warming. This study shows that it should be in imbalance in the 21st century sooner that the northern part. This regional variability affects significantly the associated relative sea level pattern over the entire ocean and particularly along the eastern coast of US and the northern coast of Europe. This highlights the necessity of taking into account GrIS regional SMB changes to evaluate accurately relative sea level changes in future projections.

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See detailRefining plant traits in vegetation models using forest inventory and LAI measurements. An application to the modelling of Cedrus atlantica in the Rif Mountains with the CARAIB model
Henrot, Alexandra-Jane; Hambuckers, Alain; François, Louis; Cheddadi, Rachid; Trolliet, Franck; Fettweis, Xavier; Dury, Marie; Porteman, Kristof

Conference (2017)

It appears today established that climate change will alter biodiversity, since the migration speed of many species, especially plants, are presumably too small to follow climate change. Mountain ecosystem floras of Mediterranean regions are particularly vulnerable to the climatic threat, because they combine high ecosystem diversity and large proportion of endemic species, with the risk of reaching the summits of the mountains which would limit their migration. Moreover, these environments are often strongly impacted by man. Being able to identify and predict the areas favourable to the species – microrefugia - becomes crucial in view of the fragmentation of the space devoted to their conservation. Dynamic vegetation models (DVMs) are well-designed tools for performing such projections, since they incorporate the physiological effects of CO2. However, they are usually run at the plant functional type level (PFT), whereas conservation studies require specific projections for each individual species. Thus, some efforts focus now on applying DVMs at species level, refining the definition of morphophysiological parameters from initial PFT traits to specific traits collected in the field or found in trait databases. Here we simulated the modern distribution of Cedrus atlantica, an endangered species of the north Africa mountains with the CARAIB DVM (Dury et al., iForest - Biogeosciences and Forestry, 4:82-99, 2011), over the Rif Mountains. Model results in terms of biomass and NPP are evaluated against data coming from forest inventory and LAI measurements. Morphological traits of C. atlantica derived from plant material collected in situ (such as specific leaf area, C:N ratio of leaves, etc) are adapted in the model simulation. CARAIB is run at high resolution using either climatic inputs derived from the Climate Research Unit climate dataset combined with WorldClim climatology at 30 arc sec or the ouputs of a 5 km resolution simulation of the regional climate model MAR (Fettweis et al., The Cryosphere, 7 :469-489, 2013) over the focal area.

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See detailHigh resolution (1 km) positive degree-day modelling of Greenland ice sheet surface mass balance, 1870–2012 using reanalysis data
Wilton, D.; Jowett, A.; Hanna, E.; Bigg, G.; van den Broeke, M.; Fettweis, Xavier; Huybrechts, P.

in Journal of Glaciology (2017), 63

We show results from a positive degree-day (PDD) model of Greenland ice sheet (GrIS) surface mass balance (SMB), 1870–2012, forced with reanalysis data. The model includes an improved daily temperature parameterization as compared with a previous version and is run at 1 km rather than 5 km resolution. The improvements lead overall to higher SMB with the same forcing data. We also compare our model with results from two regional climate models (RCMs). While there is good qualitative agreement between our PDD model and the RCMs, it usually results in lower precipitation and lower runoff but approximately equivalent SMB: mean 1979–2012 SMB (± standard deviation), in Gt a−1, is 382 ± 78 in the PDD model, compared with 379 ± 101 and 425 ± 90 for the RCMs. Comparison with in situ SMB observations suggests that the RCMs may be more accurate than PDD at local level, in some areas, although the latter generally compares well. Dividing the GrIS into seven drainage basins we show that SMB has decreased sharply in all regions since 2000. Finally we show correlation between runoff close to two calving glaciers and either calving front retreat or calving flux, this being most noticeable from the mid-1990s.

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See detailA daily, 1 km resolution data set of downscaled Greenland ice sheet surface mass balance (1958–2015)
Noel, B.; van de Berg, W. J.; Machguth, H.; Lhermitte, S.; Howat, I.; Fettweis, Xavier; van den Broeke, M.

in Cryosphere (2016), 10

This study presents a data set of daily, 1 km resolution Greenland ice sheet (GrIS) surface mass balance (SMB) covering the period 1958–2015. Applying corrections for elevation, bare ice albedo and accumulation bias, the high-resolution product is statistically downscaled from the native daily output of the polar regional climate model RACMO2.3 at 11 km. The data set includes all individual SMB components projected to a down-sampled version of the Greenland Ice Mapping Project (GIMP) digital elevation model and ice mask. The 1 km mask better resolves narrow ablation zones, valley glaciers, fjords and disconnected ice caps. Relative to the 11 km product, the more detailed representation of isolated glaciated areas leads to increased precipitation over the southeastern GrIS. In addition, the downscaled product shows a significant increase in runoff owing to better resolved low-lying marginal glaciated regions. The combined corrections for elevation and bare ice albedo markedly improve model agreement with a newly compiled data set of ablation measurements.

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See detailApplication of GRACE to the assessment of model-based estimates of monthly Greenland Ice Sheet mass balance (2003–2012)
Schlegel, N.; Wiese, D.; Larour, E.; Watkins, M.; Box, J.; Fettweis, Xavier; van den Broeke, M.

in Cryosphere (2016), 10

Quantifying the Greenland Ice Sheet's future contribution to sea level rise is a challenging task that requires accurate estimates of ice sheet sensitivity to climate change. Forward ice sheet models are promising tools for estimating future ice sheet behavior, yet confidence is low because evaluation of historical simulations is challenging due to the scarcity of continental-wide data for model evaluation. Recent advancements in processing of Gravity Recovery and Climate Experiment (GRACE) data using Bayesian-constrained mass concentration ("mascon") functions have led to improvements in spatial resolution and noise reduction of monthly global gravity fields. Specifically, the Jet Propulsion Laboratory's JPL RL05M GRACE mascon solution (GRACE_JPL) offers an opportunity for the assessment of model-based estimates of ice sheet mass balance (MB) at ∼ 300 km spatial scales. Here, we quantify the differences between Greenland monthly observed MB (GRACE_JPL) and that estimated by state-of-the-art, high-resolution models, with respect to GRACE_JPL and model uncertainties. To simulate the years 2003–2012, we force the Ice Sheet System Model (ISSM) with anomalies from three different surface mass balance (SMB) products derived from regional climate models. Resulting MB is compared against GRACE_JPL within individual mascons. Overall, we find agreement in the northeast and southwest where MB is assumed to be primarily controlled by SMB. In the interior, we find a discrepancy in trend, which we presume to be related to millennial-scale dynamic thickening not considered by our model. In the northwest, seasonal amplitudes agree, but modeled mass trends are muted relative to GRACE_JPL. Here, discrepancies are likely controlled by temporal variability in ice discharge and other related processes not represented by our model simulations, i.e., hydrological processes and ice–ocean interaction. In the southeast, GRACE_JPL exhibits larger seasonal amplitude than predicted by the models while simultaneously having more pronounced trends; thus, discrepancies are likely controlled by a combination of missing processes and errors in both the SMB products and ISSM. At the margins, we find evidence of consistent intra-annual variations in regional MB that deviate distinctively from the SMB annual cycle. Ultimately, these monthly-scale variations, likely associated with hydrology or ice–ocean interaction, contribute to steeper negative mass trends observed by GRACE_JPL. Thus, models should consider such processes at relatively high (monthly-to-seasonal) temporal resolutions to achieve accurate estimates of Greenland MB.

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See detailHow reliable are the models to study recent climate change ? A study of heat/cold waves and radiative fluxes trends over 1900-2010 using the model MAR in Belgium
Wyard, Coraline; Fettweis, Xavier; Scholzen, Chloé

Poster (2016, August 29)

This research aims to assess the ability of the regional climate model MAR ("Modèle Atmosphérique Régional") to reconstruct the observed twentieth century climatology of extreme events and solar radiation in Belgium, as a necessary condition for reliable future projections. Simulations were performed by forcing MAR with several reanalyses: the ERA40/ERA-Interim, the ERA-20C and the NCEP/NCAR-v1. The results suggests that increasing air temperature would have generated decreasing relative humidity which would have lead to a decrease in cloudiness and an increase in solar downward radiation. This research illustrates the dependency between RCMs and their forcings. The forcing reanalyses can generate divergent trends while contrary to Global Climate Models (GCM), the reanalyses assimilate observations and are supposed to represent the same climate.

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See detailAnnual Greenland accumulation rates (2009–2012) from airborne snow radar
Koenig, L.; Ivanoff, A.; Alexander, P.; MacGregor, J.; Fettweis, Xavier; Panzer, B.; Paden, J.; Forster, R.; Das, I.; McConnell, J.; Tedesco, M.; Leuschen, C.; Gogineni, P.

in Cryosphere (2016), 10

Contemporary climate warming over the Arctic is accelerating mass loss from the Greenland Ice Sheet through increasing surface melt, emphasizing the need to closely monitor its surface mass balance in order to improve sea-level rise predictions. Snow accumulation is the largest component of the ice sheet's surface mass balance, but in situ observations thereof are inherently sparse and models are difficult to evaluate at large scales. Here, we quantify recent Greenland accumulation rates using ultra-wideband (2–6.5 GHz) airborne snow radar data collected as part of NASA's Operation IceBridge between 2009 and 2012. We use a semiautomated method to trace the observed radiostratigraphy and then derive annual net accumulation rates for 2009–2012. The uncertainty in these radar-derived accumulation rates is on average 14 %. A comparison of the radar-derived accumulation rates and contemporaneous ice cores shows that snow radar captures both the annual and long-term mean accumulation rate accurately. A comparison with outputs from a regional climate model (MAR) shows that this model matches radar-derived accumulation rates in the ice sheet interior but produces higher values over southeastern Greenland. Our results demonstrate that snow radar can efficiently and accurately map patterns of snow accumulation across an ice sheet and that it is valuable for evaluating the accuracy of surface mass balance models.

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See detailGreenland Ice sheet [in "State of the Climate in 2015"]
Tedesco, M.; Box, J.; Cappelen, J.; Fettweis, Xavier; Hansen, K.; Mote, T.; Smeets, P.; van As, D.; van de Wal, R.; Velicogna, I.; Wahr, J.

in Bulletin of the American Meteorological Society (2016), 97(8),

The Greenland Ice Sheet, with the capacity to contribute ~7 m to sea level rise, experienced melting over more than 50% of its surface for the first time since the record melt of 2012.

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See detailGreenland Ice Sheet seasonal and spatial mass variability from model simulations and GRACE (2003–2012)
Alexander, P.; Tedesco, M.; Schlegel, N-J; Luthcke, S.; Fettweis, Xavier; Larour, E.

in Cryosphere (2016), 10

Improving the ability of regional climate models (RCMs) and ice sheet models (ISMs) to simulate spatiotemporal variations in the mass of the Greenland Ice Sheet (GrIS) is crucial for prediction of future sea level rise. While several studies have examined recent trends in GrIS mass loss, studies focusing on mass variations at sub-annual and sub-basin-wide scales are still lacking. At these scales, processes responsible for mass change are less well understood and modeled, and could potentially play an important role in future GrIS mass change. Here, we examine spatiotemporal variations in mass over the GrIS derived from the Gravity Recovery and Climate Experiment (GRACE) satellites for the January 2003–December 2012 period using a "mascon" approach, with a nominal spatial resolution of 100 km, and a temporal resolution of 10 days. We compare GRACE-estimated mass variations against those simulated by the Modèle Atmosphérique Régionale (MAR) RCM and the Ice Sheet System Model (ISSM). In order to properly compare spatial and temporal variations in GrIS mass from GRACE with model outputs, we find it necessary to spatially and temporally filter model results to reproduce leakage of mass inherent in the GRACE solution. Both modeled and satellite-derived results point to a decline (of −178.9 ± 4.4 and −239.4 ± 7.7 Gt yr−1 respectively) in GrIS mass over the period examined, but the models appear to underestimate the rate of mass loss, especially in areas below 2000 m in elevation, where the majority of recent GrIS mass loss is occurring. On an ice-sheet-wide scale, the timing of the modeled seasonal cycle of cumulative mass (driven by summer mass loss) agrees with the GRACE-derived seasonal cycle, within limits of uncertainty from the GRACE solution. However, on sub-ice-sheet-wide scales, some areas exhibit significant differences in the timing of peaks in the annual cycle of mass change. At these scales, model biases, or processes not accounted for by models related to ice dynamics or hydrology, may lead to the observed differences. This highlights the need for further evaluation of modeled processes at regional and seasonal scales, and further study of ice sheet processes not accounted for, such as the role of subglacial hydrology in variations in glacial flow.

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See detailArctic cut-off high drives the poleward shift of a new Greenland melting record
Tedesco, M.; Mote, T.; Fettweis, Xavier; Hanna, E.; Jeyaratnam, J.; Booth, J. F.; Datta, R.; Briggs, K.

in Nature Communications (2016), 7(11723),

Large-scale atmospheric circulation controls the mass and energy balance of the Greenland ice sheet through its impact on radiative budget, runoff and accumulation. Here, using reanalysis data and the outputs of a regional climate model, we show that the persistence of an exceptional atmospheric ridge, centred over the Arctic Ocean, was responsible for a poleward shift of runoff, albedo and surface temperature records over the Greenland during the summer of 2015. New records of monthly mean zonal winds at 500 hPa and of the maximum latitude of ridge peaks of the 5,700±50 m isohypse over the Arctic were associated with the formation and persistency of a cutoff high. The unprecedented (1948–2015) and sustained atmospheric conditions promoted enhanced runoff, increased the surface temperatures and decreased the albedo in northern Greenland, while inhibiting melting in the south, where new melting records were set over the past decade.

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See detailThe MAR model performances over Belgium
Wyard, Coraline; Fettweis, Xavier

Conference (2016, June 08)

This presentation deals with the set-up of the regional climate model MAR over Belgium. It also presents the performances of MAR to simulate the present-day climate following three reanalysis used as forcing of the model (ERA-Interim, ERA-20C and NCEP/NCAR-v1).

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See detailReconstruction of the Greenland ice sheet surface mass balance over 1900-2015 with the help of the regional climate MARv3.6 model
Fettweis, Xavier; Agosta, Cécile; Hubert, Gallée

Speech/Talk (2016)

With the aim of studying the recent Greenland ice sheet Surface Mass Balance (SMB) decrease with respect to the last century, we have forced the regional climate MAR model (version 3.6) with the ERA-Interim (1979-2015), ERA-40 (1958-2001), NCEP1 (1948-2015), NCEP2 (1979-2015), JRA-55 (1958-2015), 20CRv2(c) (1880-2012) and ERA-20C (1900-2010) reanalysis. While all of these forcing products are reanalyses, MAR simulates differences in SMB over the common period. A temperature correction of +1°C (resp. -1°C) had notably to be applied to the MAR boundary conditions given that ERA-20C (resp. 20CRv2) is ~1° colder (resp. warmer) over Greenland than ERA-Interim data over 1980-2010. Comparisons with PROMICE daily temperature measurements valid these corrections. In most of regions, the SMB discrepancies between the different simulations are not significant except in the South-East where the maximum of precipitation occurs and where SMB measurements are missing. This suggests that uncertainties in the current SMB reconstruction remain and that observations are still needed. Comparisons with SMB measurements from the PROMICE data set, ice cores and satellite derived melt extent allows to select the best reanalysis forced data set. All of these simulations show that i) the period 1961-1990 usually chosen as reference for SMB and ice dynamics (stable ice sheet) over Greenland is a period when the SMB was abnormally high in respect to the last 120 years; ii) SMB has been significantly decreasing after this reference period due to increasing melt. Both ERA-20C and 20CRv2 forced simulations suggest a precipitation increase since the beginning of the last century and the ERA-20C forced simulation only suggests that SMB during the 1920-1930 warm period over Greenland was comparable with the SMB of the 2000's. Finally, the sensitivity of switching on the erosion of the snow by the wind in MARv3.6 will be discussed.

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See detailVery high resolution surface mass balance over Greenland modeled by the regional climate model MAR with a downscaling technique
Kittel, Christoph; Lang, Charlotte; Agosta, Cécile; Prignon, Maxime; Fettweis, Xavier; Erpicum, Michel

Poster (2016, April 20)

This study presents surface mass balance (SMB) results at 10 km resolution with the regional climate MAR model over the Greenland ice sheet. Here, we use the last MAR version (v3.6) where the land-ice module (SISVAT) using a high resolution grid (10km) for surface variables is fully coupled while the MAR atmospheric module running at a lower resolution of 20km. This online downscaling technique enables to correct near-surface temperature and humidity from MAR by a gradient based on elevation before forcing SISVAT. The 20 km precipitations are not corrected. Corrections are stronger over the ablation zone where topography presents more variations. The model has been force by ERA-Interim between 1979 and 2014. We will show the advantages of using an online SMB downscaling technique in respect to an offline downscaling extrapolation based on local SMB vertical gradients. Results at 10 km show a better agreement with the PROMICE surface mass balance data base than the extrapolated 20 km MAR SMB results.

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See detailDecrease in hydroclimatic conditions generating floods in the southeast of Belgium over the last 50 years
Wyard, Coraline; Fettweis, Xavier

Conference (2016, April 19)

As a consequence of climate change, several studies concluded that winter flood occurrence could increase in the future in many rivers of northern and western Europe in response to an increase in extreme precipitation events. This study aims to determine if trends in extreme hydroclimatic events generating floods can already be detected over the last century. In particular, we focus on the Ourthe River (southeast of Belgium) which is one of the main tributaries of the Meuse River with a catchment area of 3500 km² . In this river, most of the floods occur during winter and about 50% of them are due to rainfall events associated with the melting of the snow which covers the Ardennes during winter. In this study, hydroclimatic conditions favourable to floods were reconstructed over the period 1959-2010 using the regional climate model MAR (“Modèle Atmosphérique Régional”) forced by the following reanalyses: the ERA-20C, the ERA-Interim and the NCEP/NCAR-v1. The use of the MAR model allows to compute precipitation, snow depth and run-off resulting from precipitation events and snow melting in any part of the Ourthe river catchment area. Therefore, extreme hydroclimatic events, namely extreme run-off events, which could potentially generate floods, can be reconstructed using the MAR model. As validation, the MAR results were compared to weather station-based data. A trend analysis was then performed in order to study the evolution of conditions favourable to flooding in the Ourthe River catchment. The results show that the MAR model allows the detection of about 90% of the hydroclimatic conditions which effectively generated observed floods in the Ourthe River over the period 1974-2010. Whatever the reanalysis used to force the MAR model, the conditions favourable to floods due to snowpack melting combined with rainfall events present a significant negative trend over the last 50 years as a result of a decrease in snow accumulation. However, regarding the conditions favourable to floods due to rainfall events alone, the signal of the trend depends on the reanalysis used to force the model.

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See detailAnthropogenic forcing dominates global mean sea-level rise since 1970
Slagen, A.; Church, J.; Agosta, Cécile; Fettweis, Xavier; Marzeion, B.; Richter, K.

in Nature Climate Change (2016)

Sea-level change is an important consequence of anthropogenic climate change, as higher sea levels increase the frequency of sea-level extremes and the impact of coastal flooding and erosion on the coastal environment, infrastructure and coastal communities1, 2. Although individual attribution studies have been done for ocean thermal expansion3, 4 and glacier mass loss5, two of the largest contributors to twentieth-century sea-level rise, this has not been done for the other contributors or total global mean sea-level change (GMSLC). Here, we evaluate the influence of greenhouse gases (GHGs), anthropogenic aerosols, natural radiative forcings and internal climate variability on sea-level contributions of ocean thermal expansion, glaciers, ice-sheet surface mass balance and total GMSLC. For each contribution, dedicated models are forced with results from the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model archive6. The sum of all included contributions explains 74 ± 22% (±2σ) of the observed GMSLC over the period 1900–2005. The natural radiative forcing makes essentially zero contribution over the twentieth century (2 ± 15% over the period 1900–2005), but combined with the response to past climatic variations explains 67 ± 23% of the observed rise before 1950 and only 9 ± 18% after 1970 (38 ± 12% over the period 1900–2005). In contrast, the anthropogenic forcing (primarily a balance between a positive sea-level contribution from GHGs and a partially offsetting component from anthropogenic aerosols) explains only 15 ± 55% of the observations before 1950, but increases to become the dominant contribution to sea-level rise after 1970 (69 ± 31%), reaching 72 ± 39% in 2000 (37 ± 38% over the period 1900–2005).

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See detailFirst steps of the regional climate model MAR over the Euro-CORDEX domain
Scholzen, Chloé; Fettweis, Xavier

Poster (2016, April)

In the framework of the Euro-CORDEX initiative, the Laboratory of Climatology of the University of Liège, Belgium, is currently using the regional climate model MAR (for “Modèle Atmosphérique Régional”) to simulate the past, present and future climate over Europe. Simulations are to be performed for both available resolutions over the Euro-CORDEX domain, namely 0.11 deg. (12.5 km) and 0.44 deg. (50 km). Historical and present-day runs (1979-2015) will use the ERA-Interim and the NCEP/NCAR-v1 reanalyses as boundary conditions, whereas future projections will be driven by two selected GCMs from the CMIP5 database: NorESM1-M and MIROC5. All CMIP5-GCMs were previously compared against ERA-Interim reanalysis data in terms of their ability to represent the current mean climate over Europe. The GCMs also underwent a statistical classification based on the calculation of skill-scores evaluating for instance 850 hPa temperature and 500 hPa geopotential height. Several settings and parameters were tested in order to calibrate the regional climate model MAR over the Euro-CORDEX domain. MAR is to be validated against observations from the European Climate Assessment & Dataset (ECA&D). The final aim of this study is to assess the performance of MAR in comparing its results to other RCMs used within the Euro-CORDEX initiative.

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See detailThe darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)
Tedesco, M.; Doherty, S.; Fettweis, Xavier; Alexander, P.; Jeyaratnam, J.; Stroeve, J.

in Cryosphere (2016), 10

The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade−1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate ( ∼ −0.01 decade−1) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the dry-snow zone showing no trend. The period 1981–1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.

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See detailFeasibility of improving a priori regional climate model estimates of Greenland ice sheet surface mass loss through assimilation of measured ice surface temperatures
Navari, M.; Margulis, S.A.; Bateni, S.M.; Tedesco, M.; Alexander, P.; Fettweis, Xavier

in Cryosphere (2016), 10

The Greenland ice sheet (GrIS) has been the focus of climate studies due to its considerable impact on sea level rise. Accurate estimates of surface mass fluxes would contribute to understanding the cause of its recent changes and would help to better estimate the past, current and future contribution of the GrIS to sea level rise. Though the estimates of the GrIS surface mass fluxes have improved significantly over the last decade, there is still considerable disparity between the results from different methodologies (e.g., Rae et al., 2012; Vernon et al., 2013). The data assimilation approach can merge information from different methodologies in a consistent way to improve the GrIS surface mass fluxes. In this study, an ensemble batch smoother data assimilation approach was developed to assess the feasibility of generating a reanalysis estimate of the GrIS surface mass fluxes via integrating remotely sensed ice surface temperature measurements with a regional climate model (a priori) estimate. The performance of the proposed methodology for generating an improved posterior estimate was investigated within an observing system simulation experiment (OSSE) framework using synthetically generated ice surface temperature measurements. The results showed that assimilation of ice surface temperature time series were able to overcome uncertainties in near-surface meteorological forcing variables that drive the GrIS surface processes. Our findings show that the proposed methodology is able to generate posterior reanalysis estimates of the surface mass fluxes that are in good agreement with the synthetic true estimates. The results also showed that the proposed data assimilation framework improves the root-mean-square error of the posterior estimates of runoff, sublimation/evaporation, surface condensation, and surface mass loss fluxes by 61, 64, 76, and 62 %, respectively, over the nominal a priori climate model estimates.

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See detailImpact of cyclonic and anticyclonic activity on Greenland ice sheet surface mass balance variation during 1980–2013
Chen, L.; Fettweis, Xavier; Knudsen, E.; Johannessen, O.

in International Journal of Climatology (2016)

The Greenland ice sheet (GrIS) has experienced dramatic ice loss during recent decades, but the drivers of the surface mass balance (SMB) variation remain unclear. From a dynamical perspective, extratropical cyclones and anticyclones are the major systems influencing Greenland weather conditions. Seasonal cyclonic and anticyclonic activities have been quantified for the area of 50°–90°N, 80°W–10°E during 1980–2013. Based on a singular value decomposition (SVD) analysis, we investigated the role of synoptic scale cyclonic and anticyclonic activities in determining snow accumulation (SA) and surface air temperature (SAT). Thus, the SA-driven and melt-driven SMB variability has been determined. SA-related synoptic patterns identified from the leading and the second SVD modes explain up to 80% of the inter-annual SMB variance, especially in southern and northwestern Greenland, where we found the largest and second largest amount of annual SA. SAT-related patterns account for up to 80% of the inter-annual SMB variation along the west and northwest coast of Greenland, where significant surface mass loss has been observed over the last decades. It should be noted that the negative phase of the SA-related pattern derived from the first SVD mode in June-July-August and the positive phase of the SAT-related (anti)cyclonic patterns have occurred more frequently since 2005, meaning that the phase change of these patterns has made a major contribution to the accelerated GrIS surface ice loss during recent years.

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See detailSnow cover evolution and its impact on flooding in the Ourthe River catchment (southeast of Belgium) over the period 1958­-2014 using the MAR model
Wyard, Coraline; Fettweis, Xavier; Erpicum, Michel

Conference (2015, November 13)

The Ourthe River (southeast of Belgium) is one of the main tributaries of the Meuse River with a catchment area of 3500 km². About 50 % of the floods which occur in the Ourthe River catchment are due to rainfall events associated with the melting of the snow which covers the Ardennes in winter. In this study, hydroclimatic conditions favourable to flooding were reconstructed over the period 1958-2014 using the regional climate model MAR (« Modèle Atmosphérique Régional ») forced by the ERA-interim reanalysis and by the NCEP1 reanalysis. As validation, the MAR results were compared to weather station-based data. A trends analysis was then performed in order to study the evolution of conditions favourable to flooding in the Ourthe River catchment. When the MAR model is forced by the NCEP1 reanalysis, results show a significant decrease in hydroclimatic conditions favourable to flooding because of a decrease in snow accumulation as well as a decrease in the frequency of extreme precipitation events in winter. When MAR is forced by the ERA-interim reanalysis, non-significant trends are found, which could be explained by an underestimation of the precipitation amount computed by the ERA-40 reanalysis before 1979. Further studies are needed to explain the decreasing trends in snow accumulation and extreme precipitation events. Moreover, an hydrological model could also be forced by the MAR outputs in order to improve flood detection.

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See detailWhat Darkens the Greenland Ice Sheet?
Tedesco, M; Doherty, S.; Warren, S.; Tranter, M.; Stroeve, J.; Fettweis, Xavier; Alexander, P.

in EOS (2015)

Most of the massive ice sheet that covers roughly four fifths of Greenland melts at the surface in summer. As long as the ice sheet regains its mass in the winter, this is not catastrophic. However, if the ice sheet melted entirely, sea levels would rise by more than 7 meters, with obvious and severe consequences for human civilization. Not surprisingly, scientists are working hard to determine if and when the ice sheet will transition (or if it has already transitioned) from a stable state to a net mass loss state. The impact of increasing greenhouse gas levels on the Greenland ice sheet (GrIS) depends on many complex and interacting factors. One is the ice sheet’s albedo—the fraction of incoming solar radiation that is reflected from the surface of the ice sheet. Indeed, scientists have determined that net solar radiation reaching the ice is the largest contributor to the energy balance driving melting [e.g., van den Broeke et al., 2011]. Despite the crucial role of albedo in energy balance, we have yet to quantify the role of the different processes driving it. Such an understanding is crucial to determining the past behavior of the GrIS and projecting its future contribution to sea level rise. Scientists seeking to quantify how much various factors contribute to ice sheet albedo face numerous challenges. These include intrinsic limitations in current observational capabilities (e.g., spatial and radiometric resolution of currently available spaceborne sensors) and limitations on how accurately surface energy balance models handle ice sheet albedo. Moreover, the sparseness in space and time of in situ observations of quantities such as impurity concentrations, biological processes, and grain growth impedes our ability to separate their respective contributions to broadband albedo (integrated over the entire spectrum).

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See detailGreenland ice sheet [in "State of the Climate in 2014"]
Tedesco, M.; Box, J.; Cappelen, J.; Fettweis, Xavier; Mote, T.; van de Wal, R.; van den Broeke, M.; Smeets, P.; Wahr, J.

in Bulletin of the American Meteorological Society (2015), 96(7),

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See detailRecent changes in north-west Greenland climate documented by NEEM shallow ice core data and simulations, and implications for past-temperature reconstructions
Masson-Delmotte, V.; Steen-Larsen, H.; Ortega, P.; Swingedouw, D.; Popp, T.; Vinther, B.; Oerter, H.; Sveinbjornsdottir, A.; Gudlaugsdottir, H.; Box, J.; Falourd, S.; Fettweis, Xavier; Gallée, H.; Garnier, E.; Gkinis, V.; Jouzel, J.; Landais, A.; Minster, B.; Paradis, N.; Orsi, A.; Risi, C.; Werner, M.; White, J.

in Cryosphere (2015), 9

Combined records of snow accumulation rate, δ18O and deuterium excess were produced from several shallow ice cores and snow pits at NEEM (North Greenland Eemian Ice Drilling), covering the period from 1724 to 2007. They are used to investigate recent climate variability and characterise the isotope–temperature relationship. We find that NEEM records are only weakly affected by inter-annual changes in the North Atlantic Oscillation. Decadal δ18O and accumulation variability is related to North Atlantic sea surface temperature and is enhanced at the beginning of the 19th century. No long-term trend is observed in the accumulation record. By contrast, NEEM δ18O shows multidecadal increasing trends in the late 19th century and since the 1980s. The strongest annual positive δ18O values are recorded at NEEM in 1928 and 2010, while maximum accumulation occurs in 1933. The last decade is the most enriched in δ18O (warmest), while the 11-year periods with the strongest depletion (coldest) are depicted at NEEM in 1815–1825 and 1836–1846, which are also the driest 11-year periods. The NEEM accumulation and δ18O records are strongly correlated with outputs from atmospheric models, nudged to atmospheric reanalyses. Best performance is observed for ERA reanalyses. Gridded temperature reconstructions, instrumental data and model outputs at NEEM are used to estimate the multidecadal accumulation–temperature and δ18O–temperature relationships for the strong warming period in 1979–2007. The accumulation sensitivity to temperature is estimated at 11 ± 2 % °C−1 and the δ18O–temperature slope at 1.1 ± 0.2 ‰ °C−1, about twice as large as previously used to estimate last interglacial temperature change from the bottom part of the NEEM deep ice core.

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See detailContribution du bilan de masse de surface antarctique à l’évolution du niveau des mers avec le modèle atmosphérique régional MAR
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

in Actes du 28e colloque de l’Association Internationale de Climatologie (2015, July 02)

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See detailEtude de l'évolution de l'enneigement dans les Hautes Fagnes (Belgique) au cours des cinquante dernières années à l'aide du modèle climatique régional MAR
Wyard, Coraline; Fettweis, Xavier; Erpicum, Michel

in Erpicum, Michel (Ed.) Actes du XXVIIIe colloque annuel de l’Association Internationale de Climatologie : Modélisations et variabilités (2015, July)

The “Modèle Atmosphérique Régionale” MAR is a regional climate model originally developed to study the polar ice sheets. In this study, the MAR model has been adapted to Belgium in order to study the snow cover evolution of the High Fens (east of Belgium), a region covered by snow on average one to two months per year. As validation, we have sucessfully compared MAR based daily snow heights with snowcam-based and/or laser sensor-based observations over the period 2008-2013. Then, the model has been forced by ERA-Interim since 1958 to study the snow cover evolution during the last fifty years at the summit of Belgium. The results show no significant trend despite global warming.

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See detailHybrid inventory, gravimetry and altimetry (HIGA) mass balance product for Greenland and the Canadian Arctic
Colgan, W.; Abdalati, W.; Citterio, M.; Csatho, B.; Fettweis, Xavier; Luthcke, S.; Moholdt, G.; Simonsen, S.; Stober, M.

in Remote Sensing of Environment (2015), 168

We present a novel inversion algorithm that generates a mass balance field that is simultaneously consistent with independent observations of glacier inventory derived from optical imagery, cryosphere-attributed mass trends derived from satellite gravimetry, and ice surface elevation trends derived from airborne and satellite altimetry. We use this algorithm to assess mass balance across Greenland and the Canadian Arctic over the Sep-2003 to Oct- 2009 period at 26 km resolution. We evaluate local algorithm-inferred mass balance against forty in situ point observations. This evaluation yields an RMSE of 0.15 mWE/a, and highlights a paucity of in situ observations from regions of high dynamic mass loss and peripheral glaciers. We assess mass losses of 212 ± 67 Gt/a to the Greenland ice sheet proper, 38 ± 11 Gt/a to peripheral glaciers in Greenland, and 42 ± 11 Gt/a to glaciers in the Canadian Arctic. These magnitudes of mass loss are dependent on the gravimetry-derived spherical harmonic mass trend we invert. We spatially partition the transient glacier continuity equation by differencing algorithm-inferred mass balance from modeled surface mass balance, in order to solve the horizontal divergence of ice flux as a residual. This residual ice dynamic field infers flux divergence (or submergent flow) in the ice sheet accumu- lation area and at tidewater margins, and flux convergence (or emergent flow) in land-terminating ablation areas, which is consistent with continuum mechanics theory.

See detailEvaluation of CMIP5 models toward regional modelling of the Antarctic surface mass balance
Agosta, Cécile; Fettweis, Xavier

Conference (2015, June 25)

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See detailA forecasting method using a wavelet-based mode decomposition and application to the ENSO index
Deliège, Adrien; Nicolay, Samuel; Fettweis, Xavier

Conference (2015, June)

This work consists of a presentation and applications of a forecasting methodology based on a mode decomposition performed through a continuous wavelet transform. The idea is comparable to the Fourier series decomposition but where the amplitudes of the components are not constant anymore: the signal is written as a sum of periodic components with smooth time-varying amplitudes. This leads to a drastic decrease in the number of terms needed to decompose and rebuild the original signal without loss of precision. Once the decomposition is performed, the components are separately extrapolated, which leads to an extrapolation of the reconstructed signal that stands for a forecast of the original one. The quality of the forecast is assessed through a hindcast procedure (running retroactive probing forecasts) and Pearson correlations and root mean square errors are computed as functions of the lead time. This technique is first illustrated in details with a toy example, then with the El Niño Southern Oscillation (ENSO) time series. This signal consists of monthly-sampled sea surface temperature (SST) anomalies in the Eastern Pacific Ocean and is well-known to be one of the most influential climate patterns on the planet, inducing many consequences worldwide (hurricanes, droughts, flooding,…) and affecting human activities. Therefore, short-term predictions are of first importance in order to plan actions before the occurrence of these phenomena. As far as the ENSO time series is concerned, the wavelet-based mode decomposition leads to four components corresponding to periods of about 20, 31, 43 and 61 months respectively and the reconstruction recovers 97% of the El Niño/La Niña events (anomalous warming/cooling of the SST) of the last 65 years. Also, it turns out that more than 78% of these extreme events can be retrieved up to three years in advance. Finally, a forecast of the ENSO index is issued: the next La Niña event should start early in 2018 and should be followed soon after by a strong El Niño event in the second semester of 2019.

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See detailFuture climate and surface mass balance of Svalbard glaciers in an RCP8.5 climate scenario: a study with the regional climate model MAR forced by MIROC5
Lang, Charlotte; Fettweis, Xavier; Erpicum, Michel

in Cryosphere (2015), 9

We simulated the 21st century Svalbard SMB with the regional model MAR (RCP8.5 scenario). Melt is projected to increase gently up to 2050 and then dramatically increase, with a larger increase in the south of the archipelago. This difference is due to larger ice albedo decrease in the south causing larger increase of absorbed solar radiation. The ablation area is projected to disappear over the entire Svalbard by 2085. The SMB decrease compared to present is projected to contribute 7mm to SLR.

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See detailGlobal, diffuse and direct irradiances modelling over northwestern Europe using regional climate model MAR : validation and construction of a 30-year climatology
Beaumet, Julien; Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Poster (2015, April 17)

Incoming solar global irradiances are modelled using MAR regional climate model forced by ERA-Interim reanalysis. Global irradiances are decomposed into direct and diffuse using sigmoid model from Ruiz-Arias et al. (2010). Results are validated using data from the European Solar Radiation Atlas for Uccle and Braunschweig weather stations. A 30-year climatology has been built and trends and variability have been analyzed.

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See detailSnow cover evolution during the last fifty years in the Hautes Fagnes (Belgium) using the regional climate MAR model
Wyard, Coraline; Fettweis, Xavier; Erpicum, Michel

Conference (2015, April 16)

The MAR model is a regional climate model originally developped for the polar regions to study the surface mass balance. In this study, the MAR model has been adapted to Belgium in order to study the snow cover evolution of the Hautes Fagnes (south-east of Belgium), a region covered by snow one to two months per year. As validation, we have sucessfully compared MAR based daily snow heights with snowcam-based observations. Then, the model has been forced by ERA-Interim since 1958 to study the snow cover evolution during the last fifty years at the summit of Belgium. The results show non-significant trend.

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See detailFuture projections of the Greenland ice sheet mass balance using the regional climate MAR model coupled with the GRISLI ice sheet model
Wyard, Coraline; Fettweis, Xavier; Ritz, Catherine

Poster (2015, April 14)

During the two last decades, the Greenland ice sheet (GrIS) contribution to the global mean sea level rise has significantly increased. But, difficulties remain to assess GrIS future contribution because of large uncertainties linked to the feedback between the surface mass balance (SMB) and GrIS topography changes. The regional climate MAR model has been coupled with the GRISLI ice sheet model, in order to account of this feedback in the future projections. The aim of this study is to assess the pertinence of the MAR-GRISLI coupling which requires long computation time. In order to identify GRISLI sensitivity to MAR forcing, GRISLI has been forced with various non-coupled (i.e. using a fixed topography), coupled and modified non-coupled MAR outputs. To adapt the non-coupled MAR outputs to the GRISLI topography changes, we use an interpolation technique based on SMB vs elevation vertical gradient. These experiences evaluate the performances/limits of this interpolation technique used to avoid a RCM-ice sheet model coupling.

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See detailEGU2015 - ENSO forecast using a wavelet-based mode decomposition
Deliège, Adrien; Nicolay, Samuel; Fettweis, Xavier

Conference (2015, April 13)

The aim of this work is to introduce a new method for forecasting major El Niño/ La Niña events with the use of a wavelet-based mode decomposition. These major events are related to sea surface temperature anomalies in the tropical Pacific Ocean: anomalous warmings are known as El Niño events, while excessive coolings are referred as La Niña episodes. These climatological phenomena are of primary importance since they are involved in many teleconnections ; predicting them long before they occur is therefore a crucial concern. First, we perform a wavelet transform (WT) of the monthly sampled El Niño Southern Oscillation 3.4 index (from 1950 to present) and compute the associated scale spectrum, which can be seen as the energy carried in the WT as a function of the scale. It can be observed that the spectrum reaches five peaks, corresponding to time scales of about 7, 20, 31, 43 and 61 months respectively. Therefore, the Niño 3.4 signal can be decomposed into five dominant oscillating components with time-varying amplitudes, these latter being given by the modulus of the WT at the associated pseudo-periods. The reconstruction of the index based on these five components is accurate since more than 93% of the El Niño/ La Niña events of the last 60 years are recovered and no major event is erroneously predicted. Then, the components are smoothly extrapolated using polynomials and added together, giving so several years forecasts of the Niño 3.4 index. In order to increase the reliability of the forecasts, we perform several months hindcasts (i.e. retroactive probing forecasts) which can be validated with the existing data. It turns out that most of the major events can be accurately predicted up to three years in advance, which makes our methodology competitive for such forecasts. Finally, we discuss the El Niño conditions currently undergone and give indications about the next La Niña event.

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See detailGreenland high-elevation mass balance: inference and implication of reference period (1961–90) imbalance
Colgan, W.; Box, J.; Andersen, M.; Fettweis, Xavier; Csatho, B.; Fausto, R.; van As, D.; Wahr, J.

in Annals of Glaciology (2015), 56(70), 105117

We revisit the input–output mass budget of the high-elevation region of the Greenland ice sheet evaluated by the Program for Arctic Regional Climate Assessment (PARCA). Our revised reference period (1961–90) mass balance of 54 48 Gt a–1 is substantially greater than the 0 21 Gt a–1 assessed by PARCA, but consistent with a recent, fully independent, input–output estimate of high-elevation mass balance (41 61 Gt a–1). Together these estimates infer a reference period high-elevation specific mass balance of 4.8 5.4 cm w.e. a–1. The probability density function (PDF) associated with this combined input–output estimate infers an 81% likelihood of high-elevation specific mass balance being positive (>0 cm w.e. a–1) during the reference period, and a 70% likelihood that specific balance was >2 cm w.e. a–1. Given that reference period accumulation is characteristic of centurial and millennial means, and that in situ mass-balance observations exhibit a dependence on surface slope rather than surface mass balance, we suggest that millennial-scale ice dynamics are the primary driver of subtle reference period high-elevation mass gain. Failure to acknowledge subtle reference period dynamic mass gain can result in underestimating recent dynamic mass loss by 17%, and recent total Greenland mass loss by 7%.

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See detailThe summer 2012 Greenland heat wave: in situ and remote sensing observations of water vapour isotopic composition during an atmospheric river event†
Bonne, JL; Steen-Larsen, HC; Risi, C.; Werner, M.; Sodemann, H.; Lacour, J.L.; Fettweis, Xavier; Cesana, G.; Delmotte, M.; Cattani, O.; Vallelonga, P.; Kjær, H.E.; Clerbaux, C.; Sveinbjörnsdóttirffilmark, A.E.; Masson-Delmotte, V.

in Journal of Geophysical Research. Atmospheres (2015)

During July 7-12, 2012, extreme moist and warm conditions occurred over Greenland, leading to widespread surface melt. To investigate the physical processes during the atmospheric moisture transport of this event, we study the water vapour isotopic composition using surface in situ observations in Bermuda Island, South Greenland coast (Ivittuut) and Northwest Greenland ice sheet (NEEM), as well as remote sensing observations (IASI instrument on-board MetOp-A), depicting propagation of similar surface and mid-tropospheric humidity and δD signals. Simulations using Lagrangian moisture source diagnostic and water tagging in a regional model showed that Greenland was affected by an atmospheric river transporting moisture from the western subtropical North Atlantic Ocean, which is coherent with observations of snow pit impurities deposited at NEEM. At Ivittuut, surface air temperature, humidity and δD increases are observed. At NEEM, similar temperature increase is associated with a large and long-lasting ~100 δD enrichment and ~15 deuterium excess decrease, thereby reaching Ivittuut level. We assess the simulation of this event in two isotope-enabled atmospheric general circulation models (LMDz-iso and ECHAM5-wiso). LMDz-iso correctly captures the timing of propagation for this event identified in IASI data but depict too gradual variations when compared to surface data. Both models reproduce the surface meteorological and isotopic values during the event but underestimate the background deuterium excess at NEEM. Cloud liquid water content parametrization in LMDz-iso poorly impacts the vapour isotopic composition. Our data demonstrate that during this atmospheric river event the deuterium excess signal is conserved from the moisture source to Northwest Greenland.

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See detailRecent summer Arctic atmospheric circulation anomalies in a historical perspective
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

in Cryosphere (2015), 9

A significant increase in the summertime occurrence of a high pressure area over the Beaufort Sea, the Canadian Arctic Archipelago, and Greenland has been observed since the beginning of the 2000s, and particularly between 2007 and 2012. These circulation anomalies are likely partly responsible for the enhanced Greenland ice sheet melt as well as the Arctic sea ice loss observed since 2007. Therefore, it is interesting to analyse whether similar conditions might have happened since the late 19th century over the Arctic region. We have used an atmospheric circulation type classification based on daily mean sea level pressure and 500 hPa geopotential height data from five reanalysis data sets (ERA-Interim, ERA-40, NCEP/NCAR, ERA-20C, and 20CRv2) to put the recent circulation anomalies in perspective with the atmospheric circulation variability since 1871. We found that circulation conditions similar to 2007–2012 have occurred in the past, despite a higher uncertainty of the reconstructed circulation before 1940. For example, only ERA-20C shows circulation anomalies that could explain the 1920–1930 summertime Greenland warming, in contrast to 20CRv2. While the recent anomalies exceed by a factor of 2 the interannual variability of the atmospheric circulation of the Arctic region, their origin (natural variability or global warming) remains debatable.

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See detailStable climate and surface mass balance in Svalbard over 1979–2013 despite the Arctic warming
Lang, Charlotte; Fettweis, Xavier; Erpicum, Michel

in Cryosphere (2015), 9

With the help of the regional climate model MAR (Modèle Atmosphérique Régional) forced by the ERA-Interim reanalysis (MARERA) and the MIROC5 (Model for Interdisciplinary Research on Climate) global model (MARMIROC5) from the CMIP5 (Coupled Model Intercomparison Project) database, we have modelled the climate and surface mass balance of Svalbard at a 10 km resolution over 1979–2013. The integrated total surface mass balance (SMB) over Svalbard modelled by MARERA is negative (−1.6 Gt yr−1) with a large interannual variability (7.1 Gt) but, unlike over Greenland, there has been no acceleration of the surface melt over the past 35 years because of the recent change in atmospheric circulation bringing northwesterly flows in summer over Svalbard, contrasting the recent observed Arctic warming. However, in 2013, the atmospheric circulation changed to a south–southwesterly flow over Svalbard causing record melt, SMB (−20.4 Gt yr−1) and summer temperature. MIROC5 is significantly colder than ERA-Interim over 1980–2005 but MARMIROC5 is able to improve the near-surface MIROC5 results by simulating not significant SMB differences with MARERA over 1980–2005. On the other hand, MIROC5 does not represent the recent atmospheric circulation shift in summer and induces in MARMIROC5 a significant trend of decreasing SMB (−0.6 Gt yr−2) over 1980–2005.

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See detailSupraglacial lakes on the Greenland ice sheet advance inland under warming climate
Leeson, A.; Shepherd, A.; Briggs, K.; Howat, I.; Fettweis, Xavier; Morlighem, M.; Rignot, E.

in Nature Climate Change (2015), 5

Supraglacial lakes (SGLs) form annually on the Greenland ice sheet and, when they drain, their discharge enhances ice-sheet flow by lubricating the base and potentially by warming the ice. Today, SGLs tend to form within the ablation zone, where enhanced lubrication is offset by efficient subglacial drainage. However, it is not clear what impact a warming climate will have on this arrangement. Here, we use an SGL initiation and growth model to show that lakes form at higher altitudes as temperatures rise, consistent with satellite observations. Our simulations show that in southwest Greenland, SGLs spread 103 and 110 km further inland by the year 2060 under moderate (RCP 4.5) and extreme (RCP 8.5) climate change scenarios, respectively, leading to an estimated 48–53% increase in the area over which they are distributed across the ice sheet as a whole. Up to half of these new lakes may be large enough to drain, potentially delivering water and heat to the ice-sheet base in regions where subglacial drainage is inefficient. In such places, ice flow responds positively to increases in surface water delivered to the bed through enhanced basal lubrication and warming of the ice, and so the inland advance of SGLs should be considered in projections of ice-sheet change.

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See detailBasin-scale partitioning of Greenland ice sheet mass balance components (2007–2011)
Andersen, M.L.; Stenseng, L.; Skourup, H.; Colgan, W.; Khan, S.A.; Kristensen, S.S.; Andersen, S.B.; Box, J.E.; Ahlstrøm, A.P.; Fettweis, Xavier; Forsberg, R.

in Earth and Planetary Science Letters (2015), 409

The current deficit in Greenland ice sheet mass balance is due to both a decrease in surface mass balance (SMB ) input and an increase in ice discharge (D ) output. While SMB processes are beginning to be well captured by observationally-constrained climate modeling, insight into D is relatively limited. We use InSAR-derived velocities, in combination with ice thickness observations, to quantify the mass flux (F ) across a flux perimeter around the ice sheet at ∼1700 m elevation. To quantify D , we correct F for SMB , as well as changes in volume due to ice dynamics, in the area downstream of the gate. Using a 1961–1990 reference climatology SMB field from the MAR regional climate model, we quantify ice sheet mass balance within eighteen basins. We find a 2007–2011 mean D of View the MathML source. We find a 2007–2011 mean total mass balance of View the MathML source, which is equal to a 0.73 mm yr−1 global sea level rise contribution. This mass loss is dominated by SMB, which accounts for 61% of mass loss in the basins where partitioning is possible.

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See detailVery high resolution surface mass balance of Svalbard with the regional climate model MAR
Lang, Charlotte; Fettweis, Xavier

Poster (2015)

We present climate and surface mass balance (SMB) results over Svalbard simulated by a new version of the regional climate MAR model allowing to reach ∼km resolution without highly time-consuming runs. Spitsbergen, the largest island of the Svalbard archipelago, has a very hilly topography and, as the SMB strongly depends on the local topography and ice distribution, we need a high spatial resolution to accurately represent the SMB of Svalbard and its complex spatial distribution. However, higher resolution simulations are also very time consuming. That is why we have developed a new version of the MAR model in which the atmospheric module runs at a resolution of 7.5 km and the snow/ice module runs at a resolution of 3.75 km. Simulations over 1960-2014 forced by ERA show better agreement with SMB observations compared to our recent 10 km resolution recently published results.

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See detailEvaluation of the CMIP5 models in the aim of regional modelling of the Antarctic surface mass balance
Agosta, Cécile; Fettweis, Xavier; Datta, Rajashree

in Cryosphere (2015), 9

The surface mass balance (SMB) of the Antarctic Ice Sheet cannot be reliably deduced from global climate models (GCMs), both because their spatial resolution is insufficient and because their physics are not adapted for cold and snow-covered regions. By contrast, regional climate models (RCMs) adapted for polar regions can physically and dynamically downscale SMB components over the ice sheet using large-scale forcing at their boundaries. Polar-oriented RCMs require appropriate GCM fields for forcing because the response of the cryosphere to a warming climate is dependent on its initial state and is not linear with respect to temperature increase. In this context, we evaluate the current climate in 41 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) data set over Antarctica by focusing on forcing fields which may have the greatest impact on SMB components simulated by RCMs. Our inter-comparison includes six reanalyses, among which ERA-Interim reanalysis is chosen as a reference over 1979–2014. Model efficiency is assessed taking into account the multi-decadal variability of the fields over the 1850–1980 period. We show that fewer than 10 CMIP5 models show reasonable biases compared to ERA-Interim, among which ACCESS1-3 is the most pertinent choice for forcing RCMs over Antarctica, followed by ACCESS1-0, CESM1-BGC, CESM1-CAM5, NorESM1-M, CCSM4 and EC-EARTH. Finally, climate change over the Southern Ocean in CMIP5 is less sensitive to the global warming signal than it is to the present-day simulated sea-ice extent and to the feedback between sea-ice decrease and air temperature increase around Antarctica.

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See detailHausse du niveau des mers : pourquoi est-elle inégale sur le globe et à quoi peut-on s'attendre d'ici la fin du siècle? Un arrêt du Gulf Stream pourrait-il plonger l'Europe occidentale dans un nouvel âge glaciaire?
Wyard, Coraline; Lang, Charlotte; Fettweis, Xavier

Learning material (2015)

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See detailRapid dynamic activation of a marine-based Arctic ice cap
McMillan, M; Shepherd, A; Gourmelen, N; Dehecq, A; Leeson, A; Ridout, A; Flament, T; Hogg, A; Gilbert, L; Benham, T; van den Broeke, M; Dowdeswell, J; Fettweis, Xavier; Noël, B; Strozzi, T

in Geophysical Research Letters (2014), 41(24), 89028909

We use satellite observations to document rapid acceleration and ice loss from a formerly slow-flowing, marine-based sector of Austfonna, the largest ice cap in the Eurasian Arctic. During the past two decades, the sector ice discharge has increased 45-fold, the velocity regime has switched from predominantly slow (~ 101 m/yr) to fast (~ 103 m/yr) flow, and rates of ice thinning have exceeded 25 m/yr. At the time of widespread dynamic activation, parts of the terminus may have been near floatation. Subsequently, the imbalance has propagated 50 km inland to within 8 km of the ice cap summit. Our observations demonstrate the ability of slow-flowing ice to mobilize and quickly transmit the dynamic imbalance inland; a process that we show has initiated rapid ice loss to the ocean and redistribution of ice mass to locations more susceptible to melt, yet which remains poorly understood.

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See detailAssessing spatio-temporal variability and trends in modelled and measured Greenland Ice Sheet albedo (2000–2013)
Alexander, P.; Tedesco, M.; Fettweis, Xavier; van de Wal, R.; Smeets, P.; van den Broeke, M.

in Cryosphere (2014), 8

Accurate measurements and simulations of Greenland Ice Sheet (GrIS) surface albedo are essential, given the role of surface albedo in modulating the amount of absorbed solar radiation and meltwater production. In this study, we assess the spatio-temporal variability of GrIS albedo during June, July, and August (JJA) for the period 2000–2013. We use two remote sensing products derived from data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS), as well as outputs from the Modèle Atmosphérique Régionale (MAR) regional climate model (RCM) and data from in situ automatic weather stations. Our results point to an overall consistency in spatio-temporal variability between remote sensing and RCM albedo, but reveal a difference in mean albedo of up to ~0.08 between the two remote sensing products north of 70° N. At low elevations, albedo values simulated by the RCM are positively biased with respect to remote sensing products by up to ~0.1 and exhibit low variability compared with observations. We infer that these differences are the result of a positive bias in simulated bare ice albedo. MODIS albedo, RCM outputs, and in situ observations consistently indicate a decrease in albedo of −0.03 to −0.06 per decade over the period 2003–2013 for the GrIS ablation area. Nevertheless, satellite products show a decline in JJA albedo of −0.03 to −0.04 per decade for regions within the accumulation area that is not confirmed by either the model or in situ observations. These findings appear to contradict a previous study that found an agreement between in situ and MODIS trends for individual months. The results indicate a need for further evaluation of high elevation albedo trends, a reconciliation of MODIS mean albedo at high latitudes, and the importance of accurately simulating bare ice albedo in RCMs.

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See detailENSO forecast using a wavelet-based mode decomposition
Deliège, Adrien; Nicolay, Samuel; Fettweis, Xavier

Poster (2014, December)

We introduce a new method for forecasting major El Niño/ La Niña events based on a wavelet mode decomposition. This methodology allows us to approximate the ENSO time series with a superposition of three periodic signals corresponding to periods of about 31, 43 and 61 months respectively with time-varying amplitudes. This pseudo-periodic approximation is then extrapolated to give forecasts. While this last one only resolves the large variations in the ENSO time series, three years hindcast as retroactive prediction allows to recover most of the El Niño/ La Niña events of the last 60 years.

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See detailSensitivity of Greenland Ice Sheet surface mass balance to perturbations in sea surface temperature and sea ice cover: a study with the regional climate model MAR
Noel, Brice; Fettweis, Xavier; van de Berg, W.J.; van den Broeke, M.R.; Erpicum, Michel

in Cryosphere (2014), 8

During recent summers (2007–2012), several surface melt records were broken over the Greenland Ice Sheet (GrIS). The extreme summer melt resulted in part from a persistent negative phase of the North Atlantic Oscillation (NAO), favoring warmer atmospheric conditions than normal over the GrIS. Simultaneously, large anomalies in sea ice cover (SIC) and sea surface temperature (SST) were observed in the North Atlantic, suggesting a possible connection. To assess the direct impact of 2007–2012 SIC and SST anomalies on GrIS surface mass balance (SMB), a set of sensitivity experiments was carried out with the regional climate model MAR forced by ERA-Interim. These simulations suggest that perturbations in SST and SIC in the seas surrounding Greenland do not considerably impact GrIS SMB, as a result of the katabatic wind blocking effect. These offshore-directed winds prevent oceanic near-surface air, influenced by SIC and SST anomalies, from penetrating far inland. Therefore, the ice sheet SMB response is restricted to coastal regions, where katabatic winds cease. A topic for further investigation is how anomalies in SIC and SST might have indirectly affected the surface melt by changing the general circulation in the North Atlantic region, hence favoring more frequent warm air advection towards the GrIS.

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See detailSolar irradiance modelling over Belgium using Regional Climate Models within the frame of a day-ahead photovoltaic production forecasting system
Beaumet, Julien; Doutreloup, Sébastien; Fettweis, Xavier; Hermans, Aline; Erpicum, Michel

Poster (2014, October 06)

WRF-ARW and MAR climate models performances for the modelling of solar irradiances over Belgium are evaluated using in-situ measurements at Sart-Tilman and Daussoulx. Different WRF-ARW settings are tested. Sigmoid model proposed by Ruis-Ariaz etal. (2010) is used to decompose solar irradiance into direct and diffuse fraction. The performance of this model using measured and modelled global irradiances is also evaluated.

See detailEstimating Antarctic surface mass balance contribution to sea level rise using the regional atmospheric climate model MAR.
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

Conference (2014, August 27)

See detailSensitivity Study of the antarctic surface mass balance to snow erosion by the wind.
Gallée, Hubert; Agosta, Cécile; Amory, Charles; Trouvilliez, A.; Fettweis, Xavier

Conference (2014, August 26)

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See detailGreenland Ice sheet [in "State of the Climate in 2013"]
Tedesco, M.; Box, J.; Cappelen, J.; Fettweis, Xavier; Jensen, T.; Mote, T.; Rennermalm, A.; Smith, L.; W. van de Wal, R.; Wahr, J.

in Bulletin of the American Meteorological Society (2014), 95(7),

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See detailComparaison entre le profil vertical de la vitesse du vent observé dans les basses couches de la troposphère et celui simulé par le modèle WRF en Belgique
Doutreloup, Sébastien; Fettweis, Xavier; Beaumet, Julien; Erpicum, Michel

in Camberlin, Pierre; Richard, Yves (Eds.) Actes du XXVIIe Colloque de l'Association Internationale de Climatologie : CLIMAT : SYSTÈME & INTERACTIONS (2014, July 02)

In the framework of FLEXIPAC project funded by the "RELIABLE" program of Walloon Region (Belgium), the Laboratory of Climatology Topoclimatology (LCT) of the University of Liège (Belgium) aims to adjust the WRF regional model (v.3.4.) forced by the ERA-Interim reanalysis for Belgium. Our analysis shows that wind speeds at 100m simulated by WRF are systematically overestimated compared to wind speeds extracted from wind productions of two wind farms. In order to solve this problem, four ways are considered in this contribution. The first way is to compare the WRF model with the reanalysis data. The second way is to test the influence of the spatial resolution by running WRF with a finer resolution. The third way is to smooth WRF outputs, where in order to analyze the variability created by the model. And finally, the fourth way is to compare the WRF model with the MAR (v3.3.) regional model. This last way seems to confirm that the MAR model better simulates wind speeds at 10m and at 100m than the WRF model.

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See detailMICROPHYSIQUE DES NUAGES ET RAYONNEMENT SOLAIRE : COMPARAISON DES MESURES IN SITU AU MONT RIGI EN HAUTE BELGIQUE ET DES DONNÉES CLOUD PHYSICAL PROPERTIES (CPP) OBTENUES À PARTIR DES IMAGES METEOSAT-9
Beaumet, Julien; Clerbaux, Nicolas; Cornet, Yves; Fettweis, Xavier; Erpicum, Michel

in Camberlin, Pierre; Richard, Yves (Eds.) Actes du XXVIIe Colloque de l'Association Internationale de Climatologie : CLIMAT : SYSTÈME & INTERACTIONS (2014, July 02)

Le rayonnement solaire global mesuré au mont Rigi a été comparé à l'épaisseur optique des nuages (COT) estimée à l'aide des données SEVIRI. Une relation logarithmique avec un coefficient de détermination d'environ 0,5 a été trouvée. Ce résultat plutôt faible peut en grande partie s'expliquer par un nombre limité de cas où subsistent des erreurs de positionnement ou par des interactions plus complexes entre nébulosité et rayonnement. De plus, l'incertitude sur l'estimation de l'épaisseur optique des nuages à l'aide des données SEVIRI pour les nuages optiquement plus épais n'est pas négligeable.

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See detailSolar irradiance modelling over Belgium using WRF-ARW : A sensitivity analysis of Mellor-Yamada-Nakanishi-Niino (BYNN) boundary layer scheme parameters
Beaumet, Julien; Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Conference (2014, June 06)

Global solar irradiances at ground level are modelled over Belgium using latest version of WRF-ARW regional climate model (RCM). The model set-up used has a resolution of 5 kilometres. The boundary layer scheme chosen is the Mellor-Yamada-Nakanishi-Niino (MYNN) 2.5 scheme with the Turbulent Kinetic Energy (TKE) closure proposed by Canuto et al., (2008) and Kitamura (2010). In this scheme, the modification of some parameters allows to change the determinant mixing length (surface layer, planet boundary layer, top of boundary layer/entrainment) which then modifies heat and moistures fluxes produced by turbulent mixing. Such modifications have significant influences on modelled cloudiness and therefore on modelled global solar irradiance incoming at the surface. The present study proposes a sensitivity analysis of the different parameters that influence the mixing length ('alp1' to 'alp5') and the TKE diffusion ('Sqfac') in order to find the most suitable constant values of these parameters for the modelling of cloudiness over Belgium. Results of different simulations are compared with global solar irradiance measurements performed by the Centre Spatial de Liège at Sart-Tilman in 2013 and 2014. Firsts results show that the dry bias frequently found when using WRF-ARW with standard set-ups can be greatly reduced thanks to an increased modelled cloudiness. The quantitative and qualitative effects of these modifications over cloudiness are also analysed by displaying 2D representation of modelled clouds over Sart-Tilman and confronting them with on-site observations.

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See detailComparison between wind speed observed at 100m height and wind speeds simulated by the WRF and MAR models
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel; Breuer, Christophe; Beaumet, Julien

Poster (2014, June 06)

In the context of FLEXIPAC project funded by "RELIABLE" program of Walloon Region (Belgium), the Laboratory of Climatology and Topoclimatology (LCT) of the University of Liège (Belgium) aims to adjust the WRF regional model (v.3.4.) forced by ERA-Interim model. Our analysis shows that wind speeds at 100m height simulated by WRF are systematically overestimated compared to wind speeds extracted from wind productions of two wind farms. In order to identify this problem, four comparisons were performed in this contribution. Firstly, we compare WRF model with reanalysis based forcing model. Secondly, we compare two WRF simulations, where one of them has a more precise spatial resolution. Thirdly, we smooth WRF outputs in time (6-hr running mean) in order to study the accuracy of the 30-min variability generated by WRF model. Finally, we comp compare the WRF model with the MAR (v3.3.) regional model using the same forcing at its lateral boundaries. This last one seems to suggest that the MAR model better simulates wind speeds at 10m and at 100m than WRF model and then that wind speed underestimation by WRF is well linked to the WRF physics itself.

See detailAntarctic surface mass balance contribution to sea level rise using the regional atmospheric climate model MAR
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

Conference (2014, June)

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See detailÉtude des changements de circulation au-dessus de l'océan Austral en été
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

in Camberlin, Pierre; Richard, Yves (Eds.) Actes du XXVIIème colloque de l'Association de Climatologie - Climat : système & interactions (2014, June)

Over the five last decades, the reanalyses (ERA and NCEP/NCAR) show a strengthening of the pressure gradient between the southern hemisphere subtropical anticyclone belt and the southern circumpolar lows during summer. With the help of an automatic circulation type classification, we show that the strengthening of the pressure gradient is generalised to all circulation types and, paradoxically, it does not cause circulation changes. It is probably implied by the strengthening of the temperature gradient between the tropics and the South Pole, without consequences on the general circulation. Our classification also allows a successful comparison between the two reanalyses in a region where the observation data are rare.

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See detailEstimating Antarctic ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

Conference (2014, May 26)

The Antarctic ice-sheet surface mass balance (SMB) is a significant contribution to sea level changes which may mitigate the rise in sea level in a warmer climate, but this term is still poorly known. The Antarctic SMB cannot be directly deduced from global climate models (GCMs) because of their too low resolution (~100 km) and their unadapted physic over cold and snow-covered areas. That is why the use of a regional climate models (RCM) specifically developed for polar regions is particularly relevant. We present here new estimations of the Antarctic SMB changes for the 20th and the 21st century at 40 km of resolution with the MAR (Modèle Atmosphérique Régional) RCM. Recent studies showed that large scale forcing from GCMs was the main source of uncertainty for RCM-deduced SMB, thus we first present a carefully analysis of the CMIP5 GCMs (used in the AR5 IPCC report) compared to the ERA-Interim reanalysis over the Antarctic region, from which we could select the less biased large scale forcing for MAR. We thus show the Antarctic SMB evolution as modeled with MAR forced by ACCESS1-3 for RCP 4.5 and 8.5 greenhouse gaz scenarios. We evaluate our outputs by comparing MAR forced by ACCESS1-3 and ERA-Interim for the 1980-2000 period to more than 2700 quality-controlled observations and to surface meteorological data from the READER database. We then give SMB changes estimations for the 21st century together with an analysis of uncertainties coming from the MAR model, the GCM forcing and the greenhouse gaz scenarios.

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See detailIncreasing meltwater discharge from the Nuuk region of the Greenland ice sheet and implications for mass balance (1960-2012)
Van As, D; Andersen, ML; Petersen, D; Fettweis, Xavier; Van Angelen, JH; Lenaerts, JTM; Van den Broeke, MR; Lea, JM; Bøggild, CE; Ahlstrøm, AP; Steffen, K

in Journal of Glaciology (2014), 60(220), 314-322

We assess the runoff and surface mass balance (SMB) of the Greenland ice sheet in the Nuuk region (southwest) using output of two regional climate models (RCMs) evaluated by observations. The region encompasses six glaciers that drain into Godtha ̊bsfjord. RCM data (1960–2012) are resampled to a high spatial resolution to include the narrow (relative to the native grid spacing) glacier trunks in the ice mask. Comparing RCM gridded results with automatic weather station (AWS) point measurements reveals that locally models can underestimate ablation and overestimate accumulation by up to tens of per cent. However, comparison with lake discharge indicates that modelled regional runoff totals are more accurate. Model results show that melt and runoff in the Nuuk region have doubled over the past two decades. Regional SMB attained negative values in recent high-melt years. Taking into account frontal ablation of the marine-terminating glaciers, the region lost 10–20 km3 w.e. a–1 in 2010–12. If 2010 melting prevails during the remainder of this century, a low-end estimate of sea-level rise of 5 mm is expected by 2100 from this relatively small section (2.6%) of the ice sheet alone.

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See detailCalottes glaciaires et niveau marin
Fettweis, Xavier

Conference given outside the academic context (2014)

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See detailCoastal flood damage and adaptation costs under 21st century sea-level rise
Hinkel, J.; Lincke, D.; Vafeidis, A. T.; Perrette, M.; Nicholls, R. J.; Tol, R.; Marzeion, B.; Fettweis, Xavier; Ionescu, C.; Levermann, A.

in Proceedings of the National Academy of Sciences of the United States of America (2014), 111(9), 32923297

Coastal flood damage and adaptation costs under 21st century sea-level rise are assessed on a global scale taking into account a wide range of uncertainties in continental topography data, population data, protection strategies, socioeconomic development and sea-level rise. Uncertainty in global mean and regional sea level was derived from four different climate models from the Coupled Model Intercomparison Project Phase 5, each combined with three land-ice scenarios based on the published range of contributions from ice sheets and glaciers. Without adaptation, 0.2–4.6% of global population is expected to be flooded annually in 2100 under 25–123 cm of global mean sea-level rise, with expected annual losses of 0.3–9.3% of global gross domestic product. Damages of this magnitude are very unlikely to be tolerated by society and adaptation will be widespread. The global costs of protecting the coast with dikes are significant with annual investment and maintenance costs of US$ 12–71 billion in 2100, but much smaller than the global cost of avoided damages even without accounting for indirect costs of damage to regional production supply. Flood damages by the end of this century are much more sensitive to the applied protection strategy than to variations in climate and socioeconomic scenarios as well as in physical data sources (topography and climate model). Our results emphasize the central role of long-term coastal adaptation strategies. These should also take into account that protecting large parts of the developed coast increases the risk of catastrophic consequences in the case of defense failure.

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See detailEffect of uncertainty in surface mass balance–elevation feedback on projections of the future sea level contribution of the Greenland ice sheet
Edwards, T.; Fettweis, Xavier; Gagliardini, O.; Gillet-Chaulet, F.; Goelzer, H.; Gregory, J.; Hoffman, M.; Huybrechts, P.; Payne, A.; Perego, M.; Price, S.; Quiquet, A.; Ritz, C.

in Cryosphere (2014), 8

We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2014) to projections of future climate change using five ice sheet models (ISMs). The MAR (Modèle Atmosphérique Régional: Fettweis, 2007) climate projections are for 2000–2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario. The additional sea level contribution due to the SMB–elevation feedback averaged over five ISM projections for ECHAM5 and three for HadCM3 is 4.3% (best estimate; 95% credibility interval 1.8–6.9%) at 2100, and 9.6% (best estimate; 95% credibility interval 3.6–16.0%) at 2200. In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the "no feedback" case for all ISMs and GCMs. Our method is novel in sea level projections because we propagate three types of modelling uncertainty – GCM and ISM structural uncertainties, and elevation feedback parameterisation uncertainty – along the causal chain, from SRES scenario to sea level, within a coherent experimental design and statistical framework. The relative contributions to uncertainty depend on the timescale of interest. At 2100, the GCM uncertainty is largest, but by 2200 both the ISM and parameterisation uncertainties are larger. We also perform a perturbed parameter ensemble with one ISM to estimate the shape of the projected sea level probability distribution; our results indicate that the probability density is slightly skewed towards higher sea level contributions.

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See detailProbabilistic parameterisation of the surface mass balance–elevation feedback in regional climate model simulations of the Greenland ice sheet
Edwards, T.; Fettweis, Xavier; Gagliardini, O.; Gillet-Chaulet, F.; Goelzer, H.; Gregory, J.; Hoffman, M.; Huybrechts, Ph.; Payne, A.; Perego, M.; Price, S.; Quiquet, A.; Ritz, C.

in Cryosphere (2014), 8

We present a new parameterisation that relates surface mass balance (SMB: the sum of surface accumulation and surface ablation) to changes in surface elevation of the Greenland ice sheet (GrIS) for the MAR (Modèle Atmosphérique Régional: Fettweis, 2007) regional climate model. The motivation is to dynamically adjust SMB as the GrIS evolves, allowing us to force ice sheet models with SMB simulated by MAR while incorporating the SMB–elevation feedback, without the substantial technical challenges of coupling ice sheet and climate models. This also allows us to assess the effect of elevation feedback uncertainty on the GrIS contribution to sea level, using multiple global climate and ice sheet models, without the need for additional, expensive MAR simulations. We estimate this relationship separately below and above the equilibrium line altitude (ELA, separating negative and positive SMB) and for regions north and south of 77° N, from a set of MAR simulations in which we alter the ice sheet surface elevation. These give four "SMB lapse rates", gradients that relate SMB changes to elevation changes. We assess uncertainties within a Bayesian framework, estimating probability distributions for each gradient from which we present best estimates and credibility intervals (CI) that bound 95% of the probability. Below the ELA our gradient estimates are mostly positive, because SMB usually increases with elevation: 0.56 (95% CI: −0.22 to 1.33) kg m−3 a−1 for the north, and 1.91 (1.03 to 2.61) kg m−3 a−1 for the south. Above the ELA, the gradients are much smaller in magnitude: 0.09 (−0.03 to 0.23) kg m−3 a−1 in the north, and 0.07 (−0.07 to 0.59) kg m−3 a−1 in the south, because SMB can either increase or decrease in response to increased elevation.

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See detailDo global warming-induced circulation pattern changes affect temperature and precipitation over Europe during summer?
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

in International Journal of Climatology (2014)

Future climate change projections are not limited to a simple warming, but changes in precipitation and sea level pressure (SLP) are also projected. The SLP changes and the associated atmospheric circulation changes could directly mitigate or enhance potential projected changes in temperature and precipitation associated with rising temperatures. With the aim of analysing the projected circulation changes and their possible impacts on temperature and precipitation over Europe in summer [June–July–August (JJA)], we apply an automatic circulation type classification method, based on daily SLP, on general circulation model (GCM) outputs from the Coupled Model Intercomparison Project phase 5 (CMIP5) database over the historical period (1951–2005) and for climate under two future scenarios (2006–2100). We focus on summer as it is the season when changes in temperature and precipitation have the highest impact on human health and agriculture. Over the historical observed reference period (1960–1999), our results show that most of the GCMs have significant biases over Europe when compared to reanalysis data sets, both for simulating the observed circulation types and their frequencies, as well as for reproducing the intraclass means of the studied variables. The future projections suggest a decrease of circulation types favouring a low centred over the British Isles for the benefit of more anticyclonic conditions. These circulation changes mitigate the projected precipitation increase over north-western Europe in summer, but they do not significantly affect the projected temperature increase and the precipitation decrease over the Mediterranean region and eastern Europe. However, the circulation changes and the associated precipitation changes are tarnished by a high uncertainty among the GCM projections.

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See detailMass loss from an ice-sheet drainage basin in West Greenland
Andersen,, M.; Andersen,, S.; Stenseng,, L.; Skourup,, H.; Colgan,, W.; Kristensen,, S.; erryman Boncori, J.; Ahlstrøm; Fettweis, Xavier; Forsberg, R.; Citterio, M.; Box, J.; van As, D.; Fausto, F.

in Geological Survey of Denmark and Greenland Bulletin (2014), 31

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See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries
Agosta, Cécile; Favier, Vincent; Krinner, Gerhard; Gallée, Hubert; Fettweis, Xavier; Genthon, Christophe

in Climate Dynamics (2013), 41(11-12), 3247-3260

About 75% of the Antarctic surface mass gain occurs over areas below 2000 m asl, which cover 40% of the grounded ice-sheet. As the topography is complex in many of these regions, SMB modelling is highly dependent on resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a low time consuming, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess SMB variation in the 21st and the 22nd centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the 20th century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL compare the observed values equally well. Nevertheless, field data below 2000 m asl are too scarce to efficiency show the interest of SMHiL and measuring the SMB in these undocumented areas should be then a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15km) may give a future increase in SMB in Antarctica about 30% higher than by using its standard resolution (60 km) due to higher increase in precipitation in the coastal areas at 15 km. However, a part (~ 15%) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and then likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in the coastal areas.

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See detailA comparison of supraglacial lake observations derived from MODIS imagery at the western margin of the Greenland ice sheet
Leeson, A.; Shepherd, A.; Sundal, A.; Johansson; Selmes, N.; Briggs, K.; Hogg, A.; Fettweis, Xavier

in Journal of Glaciology (2013), 59(208), 1179-1188

Supraglacial lakes (SGLs) affect the dynamics of the Greenland ice sheet by storing runoff and draining episodically. We investigate the evolution of SGLs as reported in three datasets, each based on automated classification of satellite imagery. Although the datasets span the period 2001–10, there are differences in temporal sampling, and only the years 2005–07 are common. By subsampling the most populous dataset, we recommend a sampling frequency of one image per 6.5 days in order to minimize uncertainty associated with poor temporal sampling. When compared with manual classification of satellite imagery, all three datasets are found to omit a sizeable (29, 48 and 41%) fraction of lakes and are estimated to document the average size of SGLs to within 0.78, 0.48 and 0.95 km2 . We combine the datasets using a hierarchical scheme, producing a single, optimized, dataset. This combined record reports up to 67% more lakes than a single dataset. During 2005–07, the rate of SGL growth tends to follow the rate at which runoff increases in each year. In 2007, lakes drain earlier than in 2005 and 2006 and remain absent despite continued runoff. This suggests that lakes continue to act as open surface–bed conduits following drainage.

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See detailModelling of the present and future climate and surface mass balance of Svalbard with the regional climate model MAR
Lang, Charlotte; Fettweis, Xavier; Erpicum, Michel

Poster (2013, November 01)

See detailContribution future du bilan de masse de surface Antarctique au niveau des mers par la modélisation régionale
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

Scientific conference (2013, October 15)

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu’on sache qu’il contribue de façon significative à l’évolution actuelle du niveau des mers et que sa contribution soit supposée s’intensifier au cours des prochains siècles. Outre son effet direct sur le niveau des mers, le BMS est également un champs de forçage primordial pour les modèles de calotte. Enfin, alors qu’il existe des mesures directes de l’écoulement de la glace vers l’océan et des variations de masse totales (surface+écoulement) de la calotte, il n’existe pas de mesure directe du bilan de masse de surface à l’échelle du continent. La climatologie actuelle du BMS Antarctique est donc estimée principalement à partir de résultats de modélisation. Par ailleurs, le BMS est le résultat de processus complexes. Afin de le modéliser correctement, il est nécessaire de bien représenter la circulation atmosphérique et les processus physiques spécifiques aux régions polaires. Or les modèles de circulation générale présentent une résolution trop grossière et une physique peu adaptée pour modéliser correctement ces processus. Nous présentons ici des résultats de simulations réalisées le modèle atmosphérique régional MAR, qui fait référence pour la modélisation de l’atmosphère et des processus de surface en région polaire, à une résolution de 50 km pour la fin du 20ème et du 21ème siècle. Nous connaissons la qualité du modèle MAR, cependant, comme tout modèle atmosphérique régional, ses performances sont fortement liées à la qualité des forçages aux limites provenant des Modèles de Circulation Générale (MCG). Nous avons donc sélectionné le MCG le plus apte à simuler le climat présent parmi la nouvelle génération des MCGs provenant de la base de données CMIP5 (http://cmip- pcmdi.llnl.gov/cmip5/), qui seront utilisés dans le prochain rapport du GIEC. Cela est une étape cruciale car les MCGs ne représentant pas correctement le climat présent ne pourront pas donner de résultats probants pour les simulations futures. Nous nous penchons enfin sur l’épineux problème de l’évaluation du BMS modélisé à partir de données de terrain. En effet, un effort important a été réalisé pour répertorier les données de BMS de qualité en Antarctique, cependant nous montrons que ces données ne permettent pas d’évaluer les performances des modèles de façon suffisamment contraignante. L’utilisation d’autres types de données, satellites ou aéroportées par exemple, semble nécessaire, ce qui constitue un volet important de mes recherches en cours.

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See detailAnalyse du « Dossier méthodologique relatif à l'élaboration d'une carte positive de référence traduisant le cadre actualisé, associée à un productible minimal par lot permettant de développer le grand éolien à concurrence d'un objectif de 3.800 GWh à l'horizon 2020 »
Canters, Frank; Cornet, Yves; De Keersmaecker, Marie-Laurence; De Maeyer, Philippe; Donnay, Jean-Paul; Erpicum, Michel; Fettweis, Xavier; Houbrechts, Geoffrey; Marion, Jean-Marc; Poty, Edouard; Thomas, Isabelle

Report (2013)

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See detailEnhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise
Shannon, S.; Payne, A.; Bartholomew, I.; van den Broeke, M.; Edwards, T.; Fettweis, Xavier; Gagliardini, O.; Gillet-Chaulet, F.; Goelzer, H.; Hoffman, M.; Huybrechts, Ph.; Mair, D.; Nienow, P.; Perego, M.; Price, S.; Smeets, P.; Sole, A.; van de Wa, R.; Zwinger, T.

in Proceedings of the National Academy of Sciences of the United States of America (2013), 110(49), 19719-19724

We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A wide range of observations suggest that water generated by melt at the surface of the ice sheet reaches its bed by both fracture and drainage through moulins. Once at the bed, this water is likely to affect lubrication, although current observations are insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow. An uncertainty analysis based on our best-fit parameterization admits both possibilities: continuously increasing or bounded lubrication. We apply the parameterization to four higher-order ice-sheet models in a series of experiments forced by changes in both lubrication and surface mass budget and determine the additional mass loss brought about by lubrication in comparison with experiments forced only by changes in surface mass balance. We use forcing from a regional climate model, itself forced by output from the European Centre Hamburg Model (ECHAM5) global climate model run under scenario A1B. Although changes in lubrication generate widespread effects on the flow and form of the ice sheet, they do not affect substantial net mass loss; increase in the ice sheet’s contribution to sea-level rise from basal lubrication is projected by all models to be no more than 5% of the contribution from surface mass budget forcing alone.

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See detailSensitivity of Greenland ice sheet projections to model formulations
Goelzer, H.; Huybrechts, P.; Furst, J.; Nick, F.; Andersen, M.; Eswards, T.; Fettweis, Xavier; Payne, A.; Shannon, S.

in Journal of Glaciology (2013), 59(216), 733749

Physically based projections of the Greenland ice sheet contribution to future sea-level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself. Here a higher-order, three-dimensional thermomechanical ice flow model is used, initialized to the present-day geometry. The forcing comes from a high-resolution regional climate model and from a flowline model applied to four individual marine-terminated glaciers, and results are subsequently extended to the entire ice sheet. The experiments span the next 200 years and consider climate scenario SRES A1B. The surface mass-balance (SMB) scheme is taken either from a regional climate model or from a positive-degree-day (PDD) model using temperature and precipitation anomalies from the underlying climate models. Our model results show that outlet glacier dynamics only account for 6–18% of the sea-level contribution after 200 years, confirming earlier findings that stress the dominant effect of SMB changes. Furthermore, interaction between SMB and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing. Forcing from the regional climate model produces a 14–31% higher sea-level contribution compared to a PDD model run with the same parameters as for IPCC AR4.

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See detailAtmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012
Hanna, E.; Fettweis, Xavier; Mernild, S.; Cappelen, J.; Ribergaard, M.; Shuman, C.; Steffen, K.; Wood, L.; Mote, T.

in International Journal of Climatology (2013), 34(4), 10221037

The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a ‘heat dome’ over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994–2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future ‘average’ summers projected this century.

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See detailIce-sheet mass balance and climate change
Hanna, E.; Navarro, F.; Pattyn, F.; Domingues, C.; Fettweis, Xavier; Ivins, E.; Nicholls, R.; Ritz, C.; Smith, B.; Tulaczyk, S.; Whitehouse, P.; Zwally, H.

in Nature (2013), 498

Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain.

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See detailRapid loss of firn pore space accelerates 21st century Greenland mass loss
van Angelen, J.; Lenaerts, J.; van den Broeke, M.; Fettweis, Xavier; van Meijgaard, E.

in Geophysical Research Letters (2013), 40

Mass loss from the two major ice sheets and their contribution to global sea level rise is accelerating. In Antarctica, mass loss is dominated by increased flow velocities of outlet glaciers, following the thinning or disintegration of coastal ice shelves into which they flow. In contrast, ∼55% of post‒1992 Greenland ice sheet (GrIS) mass loss is accounted for by surface processes, notably increased meltwater runoff. A subtle process in the surface mass balance of the GrIS is the retention and refreezing of meltwater, currently preventing ∼40% of the meltwater to reach the ocean. Here we force a high‒resolution atmosphere/snow model with a mid‒range warming scenario (RCP4.5, 1970–2100), to show that rapid loss of firn pore space, by >50% at the end of the 21st century, quickly reduces this refreezing buffer. As a result, GrIS surface mass loss accelerates throughout the 21st century and its contribution to global sea level rise increases to 1.7 ±0.5 mm yr−1, more than four times the current value.

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See detailModelling of the surface mass balance of Svalbard with the regional climate model MAR
Lang, Charlotte; Fettweis, Xavier; Erpicum, Michel

Poster (2013, April 11)

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See detailThe future sea-level rise contribution of Greenland's glaciers and ice caps
Machguth, H.; Rastner, P.; Bolch, T.; Mölg, N.; Sandberg Sørensen, L.; Aðalgeirsdottir, G.; van Angelen, J.; van den Broeke, M.; Fettweis, Xavier

in Environmental Research Letters (2013), 8(025005), 14

We calculate the future sea-level rise contribution from the surface mass balance of all of Greenland's glaciers and ice caps (GICs, ~90 000 km2) using a simplified energy balance model which is driven by three future climate scenarios from the regional climate models HIRHAM5, RACMO2 and MAR. Glacier extent and surface elevation are modified during the mass balance model runs according to a glacier retreat parameterization. Mass balance and glacier surface change are both calculated on a 250 m resolution digital elevation model yielding a high level of detail and ensuring that important feedback mechanisms are considered. The mass loss of all GICs by 2098 is calculated to be 2016 ± 129 Gt (HIRHAM5 forcing), 2584 ± 109 Gt (RACMO2) and 3907 ± 108 Gt (MAR). This corresponds to a total contribution to sea-level rise of 5.8 ± 0.4, 7.4 ± 0.3 and 11.2 ± 0.3 mm, respectively. Sensitivity experiments suggest that mass loss could be higher by 20–30% if a strong lowering of the surface albedo were to take place in the future. It is shown that the sea-level rise contribution from the north-easterly regions of Greenland is reduced by increasing precipitation while mass loss in the southern half of Greenland is dominated by steadily decreasing summer mass balances. In addition we observe glaciers in the north-eastern part of Greenland changing their characteristics towards greater activity and mass turnover.

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See detailEstimation of the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional climate model MAR
Fettweis, Xavier; Gallée, H.; van den Broeke, M.; Tedesco, M.; van Angelen, J.; Lenearts, J.; Erpicum, Michel

Conference (2013, April 10)

With the aim of estimating the sea level rise (SLR) coming from Surface Mass Balance (SMB) changes over the Greenland ice sheet (GrIS), we report future projections obtained with the regional climate model MAR, forced by outputs of three CMIP5 General Circulation Models (GCMs). Our results indicate that in warmer climates, the mass gained due to increased winter snowfall over GrIS does not compensate the mass lost through increased meltwater run-off in summer. All the MAR projections shows similar non-linear melt increases with rising temperatures as a result of the positive surface albedo feedback, because no change is projected in the general atmospheric circulation over Greenland. Nevertheless, MAR exhibits a large range in its future projections. By coarsely estimating the GrIS SMB changes from CMIP5 GCMs outputs, we show that the uncertainty coming from the GCM-based forcing represents about half of projected SMB changes. In 2100, the CMIP5 ensemble mean projects a SLR, resulting from a GrIS SMB decrease, estimated to be 4 2 cm and 9 4 cm for the RCP 4.5 and RCP 8.5 scenarios, respectively. However, these future projections do not consider the positive melt-elevation feedback. Sensitivity MAR experiments using perturbed ice sheet topographies consistent with the projected SMB changes highlight the importance of coupling climate models to an ice sheet model. Such a coupling will allow to consider the future response of both surface processes and ice-dynamic changes, and their mutual feedbacks to rising temperatures.

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See detailEvidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data
Tedesco, M.; Fettweis, Xavier; Mote, T.; Wahr, J.; Alexander, P.; Box, J.; Wouters, B.

in Cryosphere (2013), 7

A combined analysis of remote sensing observations, regional climate model (RCM) outputs and reanalysis data over the Greenland ice sheet provides evidence that multiple records were set during summer 2012. Melt extent was the largest in the satellite era (extending up to ∼97% of the ice sheet) and melting lasted up to ∼2 months longer than the 1979–2011 mean. Model results indicate that near surface temperature was ∼3 standard deviations (σ) above the 1958–2011 mean, while surface mass balance (SMB) was ∼3σ below the mean and runoff was 3.9σ above the mean over the same period. Albedo, exposure of bare ice and surface mass balance also set new records, as did the total mass balance with summer and annual mass changes of, respectively, −627 Gt and −574 Gt, 2σ below the 2003–2012 mean. We identify persistent anticyclonic conditions over Greenland associated with anomalies in the North Atlantic Oscillation (NAO), changes in surface conditions (e.g., albedo, surface temperature) and preconditioning of surface properties from recent extreme melting as major driving mechanisms for the 2012 records. Less positive if not increasingly negative SMB will likely occur should these characteristics persist.

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See detailSurface mass balance model intercomparison for the Greenland ice sheet
Vernon, C.L.; Bamber, J.L.; Box, J.E.; van den Broeke, M.R.; Fettweis, Xavier; Hanna, E.; Huybrechts, Ph.

in Cryosphere (2013), 7

A number of high resolution reconstructions of the surface mass balance (SMB) of the Greenland ice sheet (GrIS) have been produced using global re-analyses data extending back to 1958. These reconstructions have been used in a variety of applications but little is known about their consistency with each other and the impact of the downscaling method on the result. Here, we compare four reconstructions for the period 1960–2008 to assess the consistency in regional, seasonal and integrated SMB components. Total SMB estimates for the GrIS are in agreement within 34% of the four model average when a common ice sheet mask is used. When models' native land/ice/sea masks are used this spread increases to 57%. Variation in the spread of components of SMB from their mean: runoff 42% (29% native masks), precipitation 20% (24% native masks), melt 38% (74% native masks), refreeze 83% (142% native masks) show, with the exception of refreeze, a similar level of agreement once a common mask is used. Previously noted differences in the models' estimates are partially explained by ice sheet mask differences. Regionally there is less agreement, suggesting spatially compensating errors improve the integrated estimates. Modelled SMB estimates are compared with in situ observations from the accumulation and ablation areas. Agreement is higher in the accumulation area than the ablation area suggesting relatively high uncertainty in the estimation of ablation processes. Since the mid-1990s each model estimates a decreasing annual SMB. A similar period of decreasing SMB is also estimated for the period 1960–1972. The earlier decrease is due to reduced precipitation with runoff remaining unchanged, however, the recent decrease is associated with increased precipitation, now more than compensated for by increased melt driven runoff. Additionally, in three of the four models the equilibrium line altitude has risen since the mid-1990s, reducing the accumulation area at a rate of approximately 60 000 km2 per decade due to increased melting. Improving process representation requires further study but the use of a single accurate ice sheet mask is a logical way to reduce uncertainty among models.

See detailEstimating Antarctic ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
Agosta, Cécile; Fettweis, Xavier; Gallée, Hubert

Poster (2013, April)

We report future projections of Surface Mass Balance (SMB) over the Antarctic ice sheet obtained with the regional climate model MAR, for different warming scenarios. MAR forcing is carefully selected among the CMIP5 GCMs panel according to its ability to simulate the current climate over Antarctica. MAR includes blowing snow modeling, an important process in Antarctica.

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See detailModeling of the present surface mass balance over the Ellesmere Island using the regional climate model MAR
Watelet, Sylvain; Noel, Brice; Fettweis, Xavier; Erpicum, Michel

Conference (2013, April)

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See detailHow does the ocean surrounding the Greenland ice sheet impact its surface mass balance ?
Noel, Brice; Watelet, Sylvain; Fettweis, Xavier; Erpicum, Michel

Poster (2013, April)

See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the 20th, 21st and 22nd centuries
Agosta, Cécile; Favier, Vincent; Krinner, Gerhard; Gallée, Hubert; Fettweis, Xavier; Genthon, Christophe

Poster (2013, April)

Although areas below 2000 m above sea level (a.s.l.) cover 40% of the Antarctic grounded ice-sheet, they represent about 75% of the surface mass balance (SMB) of the continent. Because the topography is complex in many of these regions, SMB modelling is highly dependent on resolution, and studying the impact of Antarctica on the fu- ture rise in sea level requires high resolution physical approaches. We have developed a new, low time consuming, physical downscaling model for high-resolution (15 km) long-term SMB projections. Here, we present results of our SMHiL (surface mass balance high-resolution downscaling) model, which was forced with the LMDZ4 atmo- spheric general circulation model to assess SMB variation in the 21st and the 22nd centuries under two different scenarios. The higher resolution of SMHiL reproduces the geographical patterns of SMB better and induces a significantly higher averaged SMB over the grounded ice-sheet for the end of the 20th century. Our comparison of more than 2700 quality-controlled field data showed that LMDZ4 and SMHiL fit the observed values equally well. Never- theless, field data below 2000 m a.s.l. are too scarce to settle SMHiL efficiency. Measuring the SMB in these undocumented areas is a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution may give a future increase in SMB in Antarctica between 15% to 30% higher than its standard resolution. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and runoff. For this reason, developing a downscaling model was crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas.

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See detailEstimating the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
Fettweis, Xavier; Franco, Bruno; Tedesco, M.; van Angelen, J.; Lenaerts, J.; van den Broeke, M.; Gallée, H.

in Cryosphere (2013), 7

To estimate the sea level rise (SLR) originating from changes in surface mass balance (SMB) of the Greenland ice sheet (GrIS), we present 21st century climate projections obtained with the regional climate model MAR (Modèle Atmosphérique Régional), forced by output of three CMIP5 (Coupled Model Intercomparison Project Phase 5) general circulation models (GCMs). Our results indicate that in a warmer climate, mass gain from increased winter snowfall over the GrIS does not compensate mass loss through increased meltwater run-off in summer. Despite the large spread in the projected near-surface warming, all the MAR projections show similar non-linear increase of GrIS surface melt volume because no change is projected in the general atmospheric circulation over Greenland. By coarsely estimating the GrIS SMB changes from GCM output, we show that the uncertainty from the GCM-based forcing represents about half of the projected SMB changes. In 2100, the CMIP5 ensemble mean projects a GrIS SMB decrease equivalent to a mean SLR of +4 ± 2 cm and +9 ± 4 cm for the RCP (Representative Concentration Pathways) 4.5 and RCP 8.5 scenarios respectively. These estimates do not consider the positive melt–elevation feedback, although sensitivity experiments using perturbed ice sheet topographies consistent with the projected SMB changes demonstrate that this is a significant feedback, and highlight the importance of coupling regional climate models to an ice sheet model. Such a coupling will allow the assessment of future response of both surface processes and ice-dynamic changes to rising temperatures, as well as their mutual feedbacks.

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See detailImportant role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet
Fettweis, Xavier; Hanna, Edward; Lang, Charlotte; Belleflamme, Alexandre; Erpicum, Michel; Gallée, Hubert

in Cryosphere (2013), 7

Since 2007, there has been a series of surface melt records over the Greenland ice sheet (GrIS), continuing the trend towards increased melt observed since the end of the 1990's. The last two decades are characterized by an increase of negative phases of the North Atlantic Oscillation (NAO) favouring warmer and drier summers than normal over GrIS. In this context, we use a circulation type classification based on daily 500 hPa geopotential height to evaluate the role of atmospheric dynamics in this surface melt acceleration for the last two decades. Due to the lack of direct observations, the interannual melt variability is gauged here by the summer (June–July–August) mean temperature from reanalyses at 700 hPa over Greenland; analogous atmospheric circulations in the past show that ~70% of the 1993–2012 warming at 700 hPa over Greenland has been driven by changes in the atmospheric flow frequencies. Indeed, the occurrence of anticyclones centred over the GrIS at the surface and at 500 hPa has doubled since the end of 1990's, which induces more frequent southerly warm air advection along the western Greenland coast and over the neighbouring Canadian Arctic Archipelago (CAA). These changes in the NAO modes explain also why no significant warming has been observed these last summers over Svalbard, where northerly atmospheric flows are twice as frequent as before. Therefore, the recent warmer summers over GrIS and CAA cannot be considered as a long-term climate warming but are more a consequence of NAO variability affecting atmospheric heat transport. Although no global model from the CMIP5 database projects subsequent significant changes in NAO through this century, we cannot exclude the possibility that the observed NAO changes are due to global warming.

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See detailGreenland Ice Sheet [in Arctic Report Card 2012]
Box, J.; Cappelen, J.; Chen, C.; Decker, D.; Fettweis, Xavier; Mote, T.; Tedesco, M.; van de Wal, R.; Wahr, J.

Report (2013)

- The duration of melting at the surface of the ice sheet in summer 2012 was the longest since satellite observations began in 1979, and a rare, near-ice sheet-wide surface melt event was recorded by satellites for the first time. - The lowest surface albedo observed in 13 years of satellite observations (2000-2012) was a consequence of a persistent and compounding feedback of enhanced surface melting and below normal summer snowfall. - Field measurements along a transect (the K-Transect) on the western slope of the ice sheet revealed record-setting mass losses at high elevations. - A persistent and strong negative North Atlantic Oscillation (NAO) index caused southerly air flow into western Greenland, anomalously warm weather and the spatially and temporally extensive melting, low albedo and mass losses observed in summer 2012.

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See detailFuture projections of the Greenland ice sheet energy balance driving the surface melt
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

in Cryosphere (2013), 7

In this study, simulations at 25 km resolution are performed over the Greenland ice sheet (GrIS) throughout the 20th and 21st centuries, using the regional climate model MAR forced by four RCP scenarios from three CMIP5 global circulation models (GCMs), in order to investigate the projected changes of the surface energy balance (SEB) components driving the surface melt. Analysis of 2000–2100 melt anomalies compared to melt results over 1980–1999 reveals an exponential relationship of the GrIS surface melt rate simulated by MAR to the near-surface air temperature (TAS) anomalies, mainly due to the surface albedo positive feedback associated with the extension of bare ice areas in summer. On the GrIS margins, the future melt anomalies are preferentially driven by stronger sensible heat fluxes, induced by enhanced warm air advection over the ice sheet. Over the central dry snow zone, the surface albedo positive feedback induced by the increase in summer melt exceeds the negative feedback of heavier snowfall for TAS anomalies higher than 4 °C. In addition to the incoming longwave flux increase associated with the atmosphere warming, GCM-forced MAR simulations project an increase of the cloud cover decreasing the ratio of the incoming shortwave versus longwave radiation and dampening the albedo feedback. However, it should be noted that this trend in the cloud cover is contrary to that simulated by ERA-Interim–forced MAR for recent climate conditions, where the observed melt increase since the 1990s seems mainly to be a consequence of more anticyclonic atmospheric conditions. Finally, no significant change is projected in the length of the melt season, which highlights the importance of solar radiation absorbed by the ice sheet surface in the melt SEB.

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See detailModélisation du bilan de masse de surface Antarctique : quelle stratégie et quelle validation ?
Agosta, Cécile; Favier, Vincent; Fettweis, Xavier; Genthon, Christophe; Gallée, Hubert; Krinner, Gerhard

Conference (2013, January)

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu'on sache qu'il contribue de façon significative à l'évolution actuelle du niveau des mers et que sa contribution soit supposée s'intensifier au cours des prochains siècles. Outre son effet direct sur le niveau des mers, le BMS est également un champs de forçage primordial pour les modèles de calotte. Enfin, alors qu'il existe des mesures directes de l'écoulement de la glace vers l'océan et des variations de masse totales (surface+écoulement) de la calotte, il n'existe pas de mesure directe du bilan de masse de surface à l'échelle du continent. La climatologie actuelle du BMS Antarctique est donc estimée principalement à partir de résultats de modélisation. Il est donc crucial de modéliser correctement le bilan de masse de surface Antarctique. Or cette modélisation n'est pas aisée, car il existe peu de modèles de climat, globaux ou régionaux, dont la physique soit appropriée pour modéliser l'atmosphère sur des surfaces englacées. De plus, la résolution a une influence importante sur la représentation du BMS, ce qui oblige à faire des compromis entre résolution et complexité des modèles pour conserver des coûts de calcul raisonnables. Nous présentons la méthodologie que nous avons adoptée pour modéliser le BMS Antarctique sur plusieurs siècles et à haute résolution. Elle s'appuie sur une cascade de modèles adaptés aux conditions polaires à différentes échelles. Nous nous penchons également sur l'épineux problème de l'évaluation du BMS modélisé à partir de données de terrain. En effet, un effort important a été réalisé pour répertorier les données de BMS de qualité en Antarctique, mais ces données restent éparses et échantillonnent mal le continent. L'utilisation d'autres types de données, satellites ou aéroportées par exemple, semble nécessaire et nous ferons un état des lieux des limitations qui restent à dépasser pour y parvenir.

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See detailCurrent and future atmospheric circulation at 500 hPa over Greenland simulated by the CMIP3 and CMIP5 global models
Belleflamme, Alexandre; Fettweis, Xavier; Lang, Charlotte; Erpicum, Michel

in Climate Dynamics (2013), 41(7-8),

The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere.

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See detailRainfall retrieval using Spinning Enhanced Visible and Infrared Imager (SEVIRI-MSG) and Cloud Physical Properties (CPP) algorithm : validation over Belgium and applications
Beaumet, Julien; Clerbaux, Nicolas; Fettweis, Xavier; Erpicum, Michel

in Bulletin de la Société Géographique de Liège (2013), 61(2013-2), 5-14

Precipitation is the main variable of the water cycle and the water resources availability. Despite numerous available methods, precipitation measurements are still insufficient to quantify with certainty ongoing changes and to provide data for numerical models validation. Roebeling & Holleman (2009) presented the Cloud Physical Properties algorithm using data from the SEVIRI instrument on board Meteosat Second Generation. The goal of present study is to extend previous validations and verify the algorithm performances throughout yearly and daily cycles in order to identify possible use and applications. A seven-years data set of parallax-shift corrected clouds and precipitation data over Western Europe have therefore been processed using CPP algorithm. Results are encouraging for both precipitation areas delimitation and rain rates assessment. However, rain rates estimation are strongly affected by sun zenith angle with increasing overestimation for sza above 60°. Systematic errors also affect the retrieval of cloud properties for very thick clouds with an overestimation of extreme precipitation events.

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See detailFuture surface mass balance contribution of the Antarctic ice-sheet to sea level rise
Agosta, Cécile; Fettweis, Xavier; Krinner, Gerhard; Gallée, Hubert; Genthon, Christophe

Scientific conference (2012, December 04)

Most of the IPCC-AR4 global circulation models predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. High-resolution modeling is necessary to adequately capture the Antarctic SMB, that is why we present here a downscaling method leading to 15-km SMB resolution for century time-scales over Antarctica. Our first results show that a higher resolution induce at the same time more run-off but a significantly higher mitigation of sea level rise for the next centuries.

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See detailGreenland ice sheet surface mass balance: evaluating simulations and making projections with regional climate models
Rae, J.; Aðalgeirsdóttir, G.; Edwards, T.; Fettweis, Xavier; Gregory, G.; Hewitt, H.; Lowe, J.; Lucas-Picher, P.; Mottram, R.; Payne, A.; Ridley, J.; Shannon, S.; van de Berg, W.J.; van de Wal, R.; van den Broeke, M.

in Cryosphere (2012), 6

Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its contribution to sea level rise, with greater accuracy than is possible from coarser-resolution general circulation models (GCMs). This is the first time an intercomparison has been carried out of RCM results for Greenland climate and SMB. Output from RCM simulations for the recent past with the four RCMs is evaluated against available observations. The evaluation highlights the importance of using a detailed snow physics scheme, especially regarding the representations of albedo and meltwater refreezing. Simulations with three of the RCMs for the 21st century using SRES scenario A1B from two GCMs produce trends of between −5.5 and −1.1 Gt yr−2 in SMB (equivalent to +0.015 and +0.003 mm sea level equivalent yr−2), with trends of smaller magnitude for scenario E1, in which emissions are mitigated. Results from one of the RCMs whose present-day simulation is most realistic indicate that an annual mean near-surface air temperature increase over Greenland of ~ 2°C would be required for the mass loss to increase such that it exceeds accumulation, thereby causing the SMB to become negative, which has been suggested as a threshold beyond which the ice sheet would eventually be eliminated.

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See detail21st century projections of surface mass balance changes for major drainage systems of the Greenland ice sheet
Tedesco, M.; Fettweis, Xavier

in Environmental Research Letters (2012), 7

Outputs from the regional climate model Modèle Atmosphérique Régionale at a spatial resolution of 25 km are used to study 21st century projected surface mass balance (SMB) over six major drainage basins of the Greenland ice sheet (GrIS). The regional model is forced with the outputs of three different Earth System Models (CanESM2, NorESM1 and MIROC5) obtained when considering two greenhouse gas future scenarios with levels of CO2 equivalent of, respectively, 850 and >1370 ppm by 2100. Results indicate that the increase in runoff due to warming will exceed the increased precipitation deriving from the increase in evaporation for all basins, with the amount of net loss of mass at the surface varying spatially. Basins along the southwest and north coast are projected to have the highest sensitivity of SMB to increasing temperatures. For these basins, the global temperature anomaly corresponding to a decrease of the SMB below the 1980–99 average (when the ice sheet was near the equilibrium) ranges between +0.60 and +2.16 °C. For the basins along the northwest and northeast, these values range between +1.50 and +3.40 °C. Our results are conservative as they do not account for ice dynamics and changes in the ice sheet topography.

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See detailGreenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn
Harper, J.; Humphrey, N.; Pfeffer, W.; Brown, J.; Fettweis, Xavier

in Nature (2012), 491

Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much of the increased surface melt is occurring in the percolation zone, a region of the accumulation area that is perennially covered by snow and firn (partly compacted snow). The fate of melt water in the percolation zone is poorly constrained: some may travel away from its point of origin and eventually influence the ice sheet’s flow dynamics and mass balance and the global sea level, whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects. Here we quantify the existing water storage capacity of the percolation zone of the Greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage. We collected in situ observations of firn structure and meltwater retention along a roughly 85-kilometre-long transect of the melting accumulation area. Our data show that repeated infiltration events in which melt water penetrates deeply (more than 10 metres) eventually fill all pore space with water. As future surface melt intensifies under Arctic warming, a fraction of melt water that would otherwise contribute to sea-level rise will fill existing pore space of the percolation zone. We estimate the lower and upper bounds of this storage sink to be 322 ± 44 gigatonnes and 1.289 gigatonnes, respectively. Furthermore, we find that decades are required to fill this pore space under a range of plausible future climate conditions. Hence, routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea-level rise, although once the pore space is filled it cannot quickly be regenerated.

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See detailSensitivity of Greenland Ice Sheet surface mass balance to surface albedo parameterization: a study with a regional climate model
van Angelen, J.; Lenaerts, J.; Lhermitte, S.; Fettweis, Xavier; Kuipers Munneke, P.; van den Broeke, M.; van Meijgaard, E.

in Cryosphere (2012), 6

We present a sensitivity study of the surface mass balance (SMB) of the Greenland Ice Sheet, as modeled using a regional atmospheric climate model, to various parameter settings in the albedo scheme. The snow albedo scheme uses grain size as a prognostic variable and further depends on cloud cover, solar zenith angle and black carbon concentration. For the control experiment the overestimation of absorbed shortwave radiation (+6%) at the K-transect (west Greenland) for the period 2004–2009 is considerably reduced compared to the previous density-dependent albedo scheme (+22%). To simulate realistic snow albedo values, a small concentration of black carbon is needed, which has strongest impact on melt in the accumulation area. A background ice albedo field derived from MODIS imagery improves the agreement between the modeled and observed SMB gradient along the K-transect. The effect of enhanced meltwater retention and refreezing is a decrease of the albedo due to an increase in snow grain size. As a secondary effect of refreezing the snowpack is heated, enhancing melt and further lowering the albedo. Especially in a warmer climate this process is important, since it reduces the refreezing potential of the firn layer that covers the Greenland Ice Sheet.

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See detailSimulating the growth of supra-glacial lakes at the western margin of the Greenland ice sheet
Leeson, A.; Shepherd, A.; Palmer, S.; Sundal, A.; Fettweis, Xavier

in Cryosphere (2012), 6

We present a new method of modelling the growth of supraglacial lakes at the western margin of the Greenland ice sheet, based on routing runoff estimated by a regional climate model across a digital elevation model (DEM) of the ice sheet surface. Using data acquired during the 2003 melt season, we demonstrate that the model is 19 times more likely to correctly predict the presence (or absence) of lakes than it is to make incorrect predictions, within an elevation range of 1100 to 1700 metres above sea level (m a.s.l.), when compared with MODIS satellite imagery. Of the 66% of observed lake locations which the model correctly reproduces, the simulated lake onset day is found to be correlated with that observed with a Pearson correlation coefficient of 0.76. Our model accurately simulates maximum cumulative lake area with only a 1.5% overestimate. However, because our model does not simulate processes leading to lake stagnation or decay, such as refreezing or drainage, at present we do not simulate absolute daily lake area. We find that the maximum potential lake-covered ice sheet area is limited by topography to 6.4%. We estimate that this corresponds to a volume of 1.49 km3, 12% of the runoff produced in 2003. This can be taken as an upper bound given uncertainty in the DEM. This study has proved a good first step towards capturing the variability of supraglacial lake evolution with a numerical model. These initial results are promising and suggest that the model is a useful tool for use in analysing the behaviour of supraglacial lakes on the Greenland ice sheet in the present day and potentially beyond.

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See detailSensitivity of a Greenland ice sheet model to atmospheric forcing fields
Quiquet, A.; Punge, H.; Ritz, C.; Fettweis, Xavier; Kageyama, M.; Krinner, G.; Salas y Mélia, D.; Sjolte, J.

in Cryosphere (2012), 6

Predicting the climate for the future and how it will impact ice sheet evolution requires coupling ice sheet models with climate models. However, before we attempt to develop a realistic coupled setup, we propose, in this study, to first analyse the impact of a model simulated climate on an ice sheet. We undertake this exercise for a set of regional and global climate models. Modelled near surface air temperature and precipitation are provided as upper boundary conditions to the GRISLI (GRenoble Ice Shelf and Land Ice model) hybrid ice sheet model (ISM) in its Greenland configuration. After 20 kyrs of simulation, the resulting ice sheets highlight the differences between the climate models. While modelled ice sheet sizes are generally comparable to the observed one, there are considerable deviations among the ice sheets on regional scales. These deviations can be explained by biases in temperature and precipitation near the coast. This is especially true in the case of global models. But the deviations between the climate models are also due to the differences in the atmospheric general circulation. To account for these differences in the context of coupling ice sheet models with climate models, we conclude that appropriate downscaling methods will be needed. In some cases, systematic corrections of the climatic variables at the interface may be required to obtain realistic results for the Greenland ice sheet (GIS).

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See detailOn the possibility to develop a rainfall data set over Belgium and Europe for climate monitoring using SEVIRI data : Validation and application of Cloud Physical Properties algorithm from the KNMI
Beaumet, Julien; Erpicum, Michel; Fettweis, Xavier; Clerbaux, Nicolas

Conference (2012, September 13)

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See detailFuture projections of the Greenland ice sheet surface mass balance simulated by the regional climate model MAR forced by three CMIP5 global models.
Fettweis, Xavier; Franco, Bruno; Lang, Charlotte; Erpicum, Michel

Conference (2012, September 11)

As part of the ICE2SEA project, the regional climate model MAR was forced by the global models HadCM3 and ECHAM5 for making future projections of the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) over 1980-2099 at a resolution of 25km. However, the comparison with MAR forced by the ERA-40 reanalysis over 1980-1999 shows that MAR forced by these GCMs is not able to represent reliably the current SMB due to biases in the general circulation and in the free atmosphere summer temperature modelled by these GCMs around the GrIS. That is why, we present here first results of MAR forced by the next generation of GCMs from the CMIP5 data base (CanESM2, NorESM1 and MIROC5 here). The comparison with the ERA-INTERIM forced MAR simulations over current climate is a lot of better, which increases the reliability and the interest of these new MAR projections. In addition, the new scenarios (RCP 2.6, 4.5, 6.0 and 8.5) of the next IPCC Assessment Report (AR5) are used here. These new simulations show notably that the response of SMB to rising temperature is not a linear function of the temperature anomalies due to the positive albedo feedback which enhances the surface melt. For 2100, in case of extreme rising temperature (RCP 8.5 scenario), MAR simulates a surface GrIS mass loss corresponding to a cumulated sea level rise (SLR) of about 15 cm since 2000! Mainly the changes in SMB and in surface energy balance will be discussed here and estimations of the GrIS surface melt contribution to the SLR using all the CMIP5 outputs will be given.

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See detailSeveral years El Niño forecast using a wavelet-based mode decomposition
Nicolay, Samuel; Fettweis, Xavier

Conference (2012, September 10)

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See detailRECOURS À UNE CAMPAGNE DE MESURES TOPOCLIMATIQUES APPLIQUÉE POUR UNE ÉTUDE COMPARATIVE DE VARIABLES MÉTÉOROLOGIQUES CONCOMITANTES PROVENANT DES STATIONS MÉTÉOROLOGIQUES DE BRUGGE ET DU MONT RIGI EN BELGIQUE
Pirard, Xavier; Jorion, Nicolas; Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Poster (2012, September 07)

Une station météorologique automatique munie d’un mât de 10 mètres a été installée à l’Ouest de l’agglomération urbaine de Brugge (Belgique) par le Laboratoire de Climatologie et Topoclimatologie de l’ULg. Elle a été équipée afin de confirmer la bonne qualité des prévisions météorologiques du modèle WRF établies dans le cadre du projet européen TWENTIES. Les données récoltées de minute en minute par cette station météorologique offrent également l’opportunité de mettre en évidence le détail de situations météorologiques bien particulières comme celles correspondant aux passages de fronts.

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See detailEvaluation des modèles climatiques régionaux MAR et WRF sur le Svalbard
Lang, Charlotte; Fettweis, Xavier; Erpicum, Michel

Poster (2012, September)

: Il est bien connu que les zones de hautes latitudes sont très sensibles aux changements climatiques. A cause du réchauffement global, la fonte des calottes a augmenté, ce qui à son tour a une influence sur le climat via des modifications de la circulation thermohaline, la rétroaction de l’albédo de la glace, l’augmentation du niveau des mers… Nous avons comparé le climat du Svalbard modélisé par deux modèles régionaux (MAR et WRF) à une résolution de 10 km sur la période 2000-2010 à des mesures provenant de plusieurs stations météorologiques localisées dans différentes régions de l’archipel afin d'évaluer lequel de ces modèles pouvait représenter au mieux le climat du Svalbard.

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See detailLes modèles globaux projettent-ils plus de blocages anticycloniques en Europe pour le futur ?
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

in Bigot, Sylvain; Rome, Sandra (Eds.) XXVème colloque de l'Association Internationale de Climatologie - Les climats régionaux : observation et modélisation (2012, September)

The IPCC projects more frequent and longer heat waves and droughts during summer for future over Western Europe. These extreme events occur during anticyclonic blocking events. We use atmospheric circulation type classifications to determine if the models project an increase of the number and the persistence of these anticyclonic blockings. For recent climate, the number of blocking events depends on the ability of the models to reproduce the observed general circulation. The future projections do not show any systematic evolution of the number of anticyclonic blockings over Western Europe. Nevertheless, other changes like an increase of the temperature will lead to more frequent heat waves and droughts.

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See detailÉVOLUTION DU COMPORTEMENT DU VENT ET DE SON POTENTIEL POUR LA PRODUCTION D'ÉNERGIE ÉOLIENNE DURANT LES 30 DERNIÈRES ANNÉES : LE CAS DE LA BELGIQUE
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

in Bigot, Sylvain; Rome, Sandra (Eds.) Les climats régionaux : observation et modélisation. (Actes du colloque organisé à Grenoble du mercredi 5 au samedi 8 septembre 2012) (2012, September)

Chaque année, le nombre d'éoliennes dans le monde augmente de façon significative suite notamment aux politiques encourageant les productions d'énergie verte afin d’atténuer le réchauffement climatique. Toutefois, ce type d'énergie est tributaire de la météo. Cela implique que la production d'énergie éolienne est irrégulière à courte échelle de temps. Cependant, la disponibilité d’électricité de courtes périodes de temps est très importante à connaitre pour les producteurs d'énergie ainsi que pour les gestionnaires de réseaux. Pour ces raisons, il nous parait primordial d’analyser l’évolution de l’intermittence de la vitesse du vent sur les 30 dernières années (1979-2009). Pour ce faire nous utilisons le modèle WRF forcé par les réanalyses ERA-Interim, les réanalyses NCEP2 et certains modèles du GIEC (base de données CMIP5).

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See detailGreenland climate change: from the past to the future
Masson‐Delmotte, Valérie; Swingedouw, Didier; Landais, Amaëlle; Seidenkrantz, Marit‐Solveig; Gauthier, Emilie; Bichet, Vincent; Massa, Charly; Perren, Bianca; Jomelli, Vincent; Adalgeirsdottir, Gudfinna; Christensen, Jens Hesselbjerg; Arneborg, Jette; Bhatt, Uma; Walker, Donald A.; Elberling, Bo; Gillet‐Chaulet, Fabien; Ritz, Catherine; Gallée, Hubert; van den Broeke, Michiel; Fettweis, Xavier; de Vernal, Anne; Vinther, Bo

in Wiley Interdisciplinary Reviews. RNA (2012), 3(5), 427-449

Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics, and land surface changes in a broader perspective. We show that, during the last decade (2000s), atmospheric and sea surface temperatures are reaching levels last encountered millennia ago, when northern high latitude summer insolation was higher due to a different orbital configuration. Records from lake sediments in southern Greenland document major environmental and climatic conditions during the last 10,000 years, highlighting the role of soil dynamics in past vegetation changes, and stressing the growing anthropogenic impacts on soil erosion during the recent decades. Furthermore, past and present changes in atmospheric and oceanic heat advection appear to strongly influence both regional climate and ice sheet dynamics. Projections from climate models are investigated to quantify the magnitude and rates of future changes in Greenland temperature, which may be faster than past abrupt events occurring under interglacial conditions. Within one century, in response to increasing greenhouse gas emissions, Greenland may reach temperatures last time encountered during the last interglacial period, approximately 125,000 years ago. We review and discuss whether analogies between the last interglacial and future changes are reasonable, because of the different seasonal impacts of orbital and greenhouse gas forcings. Over several decades to centuries, future Greenland melt may act as a negative feedback, limiting regional warming albeit with global sea level and climatic impacts.

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See detailGreenland ice sheet albedo feedback: thermodynamics and atmospheric drivers
Box, J.; Fettweis, Xavier; Stroeve, J.; Tedesco, M.; Hal, D.; Steffen, K.

in Cryosphere (2012), 6

In this study, snowpack scenarios are modelled across the French Alps using dynamically downscaled variables from the ALADIN Regional Climate Model (RCM) for the control period (1961–1990) and three emission scenarios (SRES B1, A1B and A2) for the mid- and late 21st century (2021–2050 and 2071–2100). These variables are statistically adapted to the different elevations, aspects and slopes of the Alpine massifs. For this purpose, we use a simple analogue criterion with ERA40 series as well as an existing detailed climatology of the French Alps (Durand et al., 2009a) that provides complete meteorological fields from the SAFRAN analysis model. The resulting scenarios of precipitation, temperature, wind, cloudiness, longwave and shortwave radiation, and humidity are used to run the physical snow model CROCUS and simulate snowpack evolution over the massifs studied. The seasonal and regional characteristics of the simulated climate and snow cover changes are explored, as is the influence of the scenarios on these changes. Preliminary results suggest that the snow water equivalent (SWE) of the snowpack will decrease dramatically in the next century, especially in the Southern and Extreme Southern parts of the Alps. This decrease seems to result primarily from a general warming throughout the year, and possibly a deficit of precipitation in the autumn. The magnitude of the snow cover decline follows a marked altitudinal gradient, with the highest altitudes being less exposed to climate change. Scenario A2, with its high concentrations of greenhouse gases, results in a SWE reduction roughly twice as large as in the low-emission scenario B1 by the end of the century. This study needs to be completed using simulations from other RCMs, since a multi-model approach is essential for uncertainty analysis.

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See detailRefreezing on the Greenland ice sheet: a comparison of parameterizations
Reijmer, C.; van den Broeke, M.; Fettweis, Xavier; Ettema, J.; Stap, L.

in Cryosphere (2012), 6

Retention and refreezing of meltwater are acknowledged to be important processes for the mass budget of polar glaciers and ice sheets. Several parameterizations of these processes exist for use in energy and mass balance models. Due to a lack of direct observations, validation of these parameterizations is difficult. In this study we compare a set of 6 refreezing parameterizations against output of two Regional Climate Models (RCMs) coupled to an energy balance snow model, the Regional Atmospheric Climate Model (RACMO2) and the Modèle Atmosphérique Régional (MAR), applied to the Greenland ice sheet. In both RCMs, refreezing is explicitly calculated in a snow model that calculates vertical profiles of temperature, density and liquid water content. Between RACMO2 and MAR, the ice sheet-integrated amount of refreezing differs by only 4.9 mm w.e yr−1 (4.5 %), and the temporal and spatial variability are very similar. For consistency, the parameterizations are forced with output (surface temperature, precipitation and melt) of the RCMs. For the ice sheet-integrated amount of refreezing and its inter-annual variations, all parameterizations give similar results, especially after some tuning. However, the spatial distributions differ significantly and the spatial correspondence between the RCMs is better than with any of the parameterizations. Results are especially sensitive to the choice of the depth of the thermally active layer, which determines the cold content of the snow in most parameterizations. These results are independent of which RCM is used to force the parameterizations.

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See detailImpact of spatial resolution on the modelling of the Greenland ice sheet surface mass balance between 1990–2010, using the regional climate model MAR
Franco, Bruno; Fettweis, Xavier; Lang, Charlotte; Erpicum, Michel

in Cryosphere (2012), 6

With the aim to force an ice dynamical model, the Greenland ice sheet (GrIS) surface mass balance (SMB) was modelled at different spatial resolutions (15-50 km) for the period 1990-2010, using the regional climate model MAR (Modèle Atmosphérique Régional) forced by the ERA-INTERIM reanalysis. This comparison revealed that (i) the inter-annual variability of the SMB components is consistent within the different spatial resolutions investigated, (ii) the MAR model simulates heavier precipitation on average over the GrIS with diminishing spatial resolution, and (iii) the SMB components (except precipitation) can be derived from a simulation at lower resolution with an intelligent interpolation. This interpolation can also be used to approximate the SMB components over another topography/ice sheet mask of the GrIS. These results are important for the forcing of an ice dynamical model, needed to enable future projections of the GrIS contribution to sea level rise over the coming centuries.

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See detailEvaluation of the MAR and WRF regional climate models over Svalbard
Lang, Charlotte; Fettweis, Xavier; Doutreloup, Sébastien; Erpicum, Michel

Conference (2012, June 01)

It is well known that high latitude zones are very sensitive to climate change. As a result of global warming, ice sheet melting has increased which in turn has an influence on climate through modifications of the thermohaline circulation, feedback of ice albedo, sea level rise... Svalbard is an archipelago between 74 and 81°lat N and 60 percent of its area (62 248 km2) is covered with glaciers and ice sheets. The impact of global warming on the Svalbard cryosphere can be estimated with climate models. However, we need to use regional climate models as they offer the possibility of a higher resolution than general circulation models. We have ran two regional climate models (MAR and WRF) at a 10-kilometre resolution between 2006 and 2010 over Svalbard and compared their simulated climate to near surface measurements at several weather stations through the archipelago in order to determine which one of them could best represent the Svalbard climate.

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See detailAre the CMIP5 GCMs able to simulate atmospheric blocking situations over Europe ?
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

Poster (2012, April 26)

Some studies show that most General Circulation Models (GCMs) have difficulties to simulate the main observed circulation patterns and their frequencies. However, this does not only impact the GCM based projections for future climate, but also the results of downscaling methods using the circulation simulated by GCMs as forcing. Indeed, the downscaling methods are not able to correct the biases introduced by the GCM simulations in the free atmosphere. Here, we focus on the anticyclonic blocking situations over western Europe for summer (June, July and August). Indeed, these blocking situations, which are often related to droughts and heat waves, could become more frequent due to global warming. Moreover, their frequency and persistence depend on the variability of the circulation, which is known to be difficult to reproduce by the GCMs. In order to evaluate the ability of the GCMs to reproduce the observed frequency and persistence of blocking situations, we compare them with two reanalysis datasets (NCEP-NCAR 1 and ECMWF ERA-40) by using an automatic circulation type classification. The daily geopotential height at 500 hPa over the last 30 years of the current climate simulation (Historical experiment, 1976-2005) of all available CMIP5 GCMs prepared for the upcoming IPCC report AR5 is used here. The circulation type classification groups similar daily circulation situations together on basis of a leader-algorithm to obtain a few homogeneous circulation types describing the general circulation of the region. Thus, the frequency and the persistence of each circulation type can be analysed on a daily timescale. We show that the ability of the GCMs to reproduce the observed frequency and persistence of blocking situations is influenced by the anomalies in their circulation type frequency repartition. So, the GCMs which underestimate the frequency of the anticyclonic types tend to simulate less and shorter blocking situations. The contrary is observed for GCMs that overestimate the frequency of these circulation types. This rises questions about the reliability of the future projections for events related to blocking situations. Indeed, when applying the same approach as for the current climate to the future projections (experiments RCP4.5 and RCP8.5), it seems that the blocking situations become more frequent and persistent. However, when considering only the circulation patterns by removing the mean geopotential height increase due to global warming, there is no significant circulation change till 2100. This means that the GCMs conserve their circulation biases in spite of climate change and so, the frequency and the persistence of the blocking situations are projected to remain almost the same as those simulated for the current climate.

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See detailEvaluation of the regional climate model WRF over Svalbard
Lang, Charlotte; Fettweis, Xavier; Doutreloup, Sébastien; Erpicum, Michel

Poster (2012, April 24)

It is well known that high latitude zones are very sensitive to climate change. As a result of global warming, ice sheet melting has increased which in turn has an influence on climate through modifications of the thermohaline circulation, feedback of ice albedo, sea level rise, … Svalbard is an archipelago between 74 and 81°lat N and 60 percent of its area (62 248 km2) is covered with glaciers and ice sheets. The impact of global warming on the Svalbard cryosphere can be estimated with climate models. However, we need to use regional climate models as they offer the possibility of a higher resolution than general circulation models. We have ran two regional climate models (MAR and WRF) at a 10-kilometre resolution between 2006 and 2010 over Svalbard and compared their simulated climate to near surface measurements at several weather stations through the archipelago in order to determine which one of them could best represent the Svalbard climate.

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See detailSeveral years El Niño forecast using a wavelet-based mode decomposition
Nicolay, Samuel; Fettweis, Xavier

Conference (2012, April 23)

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See detailEstimation, prévision et contrôle du gisement solaire en région wallonne
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Poster (2012, April 23)

Présentation des activités et des recherches du Laboratoire de Climatologie et Topoclimatologie de l'Université de Liège dans le domaine du rayonnement solaire

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See detailFuture projections of the Greenland ice sheet climate simulated by the regional climate model MAR forced by 2 CMIP5 global models.
Fettweis, Xavier; Franco, Bruno

Conference (2012, February 14)

As part of the ICE2SEA project, the regional climate model MAR was forced by the global models HadCM3 and ECHAM5 for making future projections of the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) over 1980-2099 at a resolution of 25km. However, the comparison with MAR forced by the ERA-40 reanalysis over 1980-1999 shows that MAR forced by these GCMs is not able to represent reliably the current SMB due to biases in the general circulation and in the free atmosphere summer temperature modelled by these GCMs around the GrIS. <br /> <br /> That is why, we present here first results of MAR forced by the next generation of GCMs from the CMIP5 data base (CanESM2 and NorESM1 here). The comparison with the ERA-40 forced MAR simulations over current climate is a lot of better, which increases the reliability and the interest of these new MAR projections. In addition, the new scenarios (RCP 2.6, 4.5, 6.0 and 8.5) of the next IPCC Assessment Report (AR5) are used here. These new simulations show notably that the response of SMB to rising temperature is not a linear function of the temperature anomalies due to the positive albedo feedback which accelerates the surface melt. For 2100, in case of extreme rising temperature (RCP 8.5 scenario), MAR simulates a surface GrIS mass loss corresponding to a cumulated sea level rise of about 15 cm since 2000! Mainly the changes in SMB and in surface energy balance will be discussed here.

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See detailMeasurement and modeling of ablation of the bottom of supraglacial lakes in western Greenland
Tedesco, Marco; Luthje, M.; Steffen, K.; Steiner, N.; Fettweis, Xavier; Willis, I.; Bayou, N.; Banwell, A.

in Geophysical Research Letters (2012), 39(L02502), 5

We report measurements of ablation rates of the bottom of two supraglacial lakes and of temperatures at different depths collected during the summers of 2010 and 2011 in west Greenland. To our knowledge, this is the first time that such data sets are reported and discussed in the literature. The measured ablation rates at the bottom of the two lakes are of the order of ∼6 cm/day, versus a rate of ∼2.5–3 cm/day in the case of bare ice of surrounding areas. Though our measurements suggest the presence of a vertical temperature gradient, it is not possible to draw final conclusions as the measured gradient is smaller than the accuracy of our temperature sensors. In-situ measurements are compared with the results of a thermodynamic model forced with the outputs of a regional climate model. In general, the model is able to satisfactorily reproduce the measured quantities with RMSE of the order of 3–4 cm for the ablation and ∼1.5°C in the case of water temperature. Our results confirm that the ablation at the bottom of supraglacial lakes plays an important role on the overall lake volume with the ablation in the case of ice covered by a lake being 110–135% of that over bare ice at nearby locations. Beside ice sheet hydrological implications, melting at the bottom of a supraglacial lake might affect estimates of lake volume from spaceborne visible and near-infrared measurements.

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See detailÉtude fréquentielle de données via la transformée en ondelette : application aux cycles climatiques
Mabille, Georges; Fettweis, Xavier; Erpicum, Michel; Nicolay, Samuel

in Bulletin de la Société Géographique de Liège (2012), 58

Understanding the climate requires a complex study of time series connected to weather parameters. The climatologist frequently applies signal processing tools and often uses the harmonic analysis and the Fourier transform. This article is dedicated to the description of a new tool, elaborated by mathematicians, which completes the outfit of intruments intended for signal analysis. The scale spectrum, which synthetizes a part of the information supplied by the wavelet transform, possesses the property to reveal pseudo-cycles which evolves around an average period. When applied to air surface temperature time series obtained from more than one hundred weather stations, to reanalysis data and to climatic indices which characterize the tropospheric flows, the wavelet transforms and the scale spectra reveal cycles with periods close to 30 months and 42 months. The Solar parameters analysis also leads to the existence of pseudo-cycles with frequencies corresponding to those found in the temperature time series and climatic indices.

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See detailGreenland Ice Sheet - Arctic Report Card: Update for 2011
Box, J.; Cappelen, J.; Chen, C.; Decker, D.; Fettweis, Xavier; Hall, D.; Hanna, E.; Jorgensen, B.; Lipscomb, W.; Mernild, S.; Mote, T.; Steiner, N.; Tedesco, M.; van de Wal, R.; Wahr, J.

Report (2011)

A persistent and strong negative North Atlantic Oscillation (NAO) index was responsible for southerly air flow along the west of Greenland, which caused anomalously warm weather in winter 2010-11 and summer 2011. The area and duration of melting at the surface of the ice sheet in summer 2011 were the third highest since 1979. The lowest surface albedo observed in 12 years of satellite observations (2000-2011) was a consequence of enhanced surface melting and below normal summer snowfall. The area of marine-terminating glaciers continued to decrease, though at less than half the rate of the previous 10 years. In situ measurements revealed near record-setting mass losses concentrated at higher elevations on the western slope of the ice sheet, and at an isolated glacier in southeastern Greenland. Total ice sheet mass loss in 2011 was 70% larger than the 2003-09 average annual loss rate of -250 Gt y-1. According to satellite gravity data obtained since 2002, ice sheet mass loss is accelerating.

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See detailEvolution of wind behaviour and of its potential for wind power production in Belgium during the last 22 years : a comparison between WRF forced by NCEP2 reanalysis and WRF forced by ERA-INTERIM reanalysis
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Conference (2011, September 16)

The number of wind turbines in the world grows significantly every year due to politics proposing green energy productions as solutions to mitigate climate change effects. However, this kind of energy is dependent on the weather. This implies that the wind production is irregular at a very short time scale. But the short time scale availability of the wind-based energy is important to the producers of energy as well as to the electric grid managers because the wind energy production can rise or fall rapidly which creates some financial and voltage variations. For these reasons, we study the past evolution of the availability of the wind quantity by analysing the intermittence of the wind speed in Belgium during the last 22 years (1989-2010). To reach this goal, we use the regional model WRF (Weather and Research Forecast model) developed by the UCAR community users. In a first time, the WRF model is forced by the NCEP2 reanalysis outputs to obtain a regionalisation of the weather conditions over a domain centred on Belgium at a resolution of 10 km. This resolution allows to capture the minute-based time scale variability of wind speed and consequently the irregular behaviour of the wind power production. In a second time, the WRF model is forced by the ERA-Interim reanalysis outputs with the same configuration. To obtain a value of the wind intermittence, we calculate the persistence of a wind blowing continuously with a minimum speed of 1 ms-1, then the persistence of a wind blowing continuously with a minimum speed of 2 ms-1, etc. The persistence of the wind speed and its evolution over 22 years are characterised by : (a) the mean wind speed over a fixed period (monthly, seasonally, … ), (b) the mean duration of a wind speed above x ms-1 over the same fixed period and (c) the evolution of (a) and (b) during the studied period. This analysis is made with the outputs of WRF-NCEP2 and with the outputs of WRF-Interim allowing to evaluate the impact of forcing fields into WRF-based wind climatology.

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See detailInfluence de la résolution spatiale sur la modélisation du bilan de masse en surface de la calotte glaciaire du Groenland
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

in Fazzini, Massimiliano; Beltrando, Gérard (Eds.) Actes du XXIVe colloque annuel de l’Association Internationale de Climatologie : Climat Montagnard et Risques (2011, September 07)

By using the regional climate model MAR (Modèle Atmosphérique Régional), we have modelled the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) at 20, 25, 30, 40 and 50km resolution to assess the impact of the spatial resolution. As part of the ICE2SEA project, the 25km-resolution SMB outputs of the MAR model are used as forcing fields for ice sheet models, in order to produce projections of the GrIS contribution to sea-level rise over the next 200 years. However, the ice sheet models often run at a higher resolution (typically 5-10km) than the current MAR resolution (25km). Such higher-resolution runs of the MAR model on the same integration domain generate a significant additional computing time and are not doable until now. That is why several enhanced SMB interpolations are tested here in order to reduce biases when interpolating the MAR outputs onto higher resolution, in the framework of the ICE2SEA project.

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See detailInfluences de l'environnement d'un parc éolien sur la prévision de sa production électrique à l'aide des modèles GFS (50km/3h) et WRF (2km/15min) : Le cas du parc éolien d'Amel (Haute-Belgique)
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

in Fazzini, Massimiliano; Beltrando, Gérard (Eds.) Actes du XXIVème Colloque International AIC : Climat Montagnard et Risques (2011, September 06)

The economic and climate contexts require to use more electricity from wind farms. However this kind of production is intermittent, therefore it is necessary to forecast this resource at least 1 day ahead. Our laboratory has developed a forecasting model of wind-based electricity generation based on a global meteorological model (GFS) with a resolution of 50 km and 3 h. But this model has a resolution too coarse for a wind farm. So we have configured the regional model WRF with resolution of 2 km and 15 min to obtain better forecasts. Finally, the WRF model provides better forecasts, but both must be adjusted to take into account the direct environment of the wind farm.

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See detailCirculation atmosphérique simulée par les modèles de circulation générale en Europe de l'ouest : évaluation et projections futures
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

in Fazzini, Massimiliano; Beltrando, Gérard (Eds.) XXIVème colloque de l'Association Internationale de Climatologie - Climat montagnard et risques (2011, September)

Atmospheric circulation simulations from general circulation models are used as forcing for downscaling methods and for future projections. Thus, it is essential to evaluate them. An automatic circulation type classification is applied to daily 500 hPa geopotential height data. Firstly, the classification is done for the NCEP-NCAR 1 reanalysis, and then the main circulation types are imposed to the simulations of six general circulation models. For recent climate (20C3M scenario), it appears that most models are not able to simulate well the circulation over western Europe, due to biases in the mean geopotential height and an underestimation of the circulation variability. For future climate (A1B scenario), a general increase of the geopotential height is projected, leading to the emergence of new circulation types.

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See detailEstimation of the Sea Level Rise by 2100 Resulting from Changes in the Surface Mass Balance of the Greenland Ice Sheet
Fettweis, Xavier; Belleflamme, Alexandre; Erpicum, Michel; Franco, Bruno; Nicolay, Samuel

in Blanco, Juan; Kheradmand, Houshang (Eds.) Climate Change - Geophysical Foundations and Ecological Effects (2011)

We present here future projections of the Greenland climate performed by the regional climate model MAR coupled with a snow model and forced by two scenarios of greenhouse gas emissions from the global model CanESM2 of the next IPCC assessment report (AR5). Knowing that MAR forced by CanESM2 over the current climate (1970-1999) compares well with the reference MAR simulation performed by using the ERA-40 reanalysis as forcing, this gives us confidence in our future projections. For the RCP4.5 scenario (optimistic) and respectively RCP8.5 scenario (pessimistic), MAR projects a sea level rise in 2100 of 6.5 +/- 1.5 cm and respectively 14+/-2 cm as result of increasing surface melt of the Greenland ice sheet over 2000-2100. It is true that MAR projects a small increase of snowfall in the winter because the atmosphere will be warmer and therefore can contain more water vapor. But this is not sufficient to offset the acceleration of melt, notably for the scenario RCP8.5 which projects an increase of 10 °C in 2100 above the ice sheet. This work fits in the ICE2SEA project (http://www.ice2sea.eu) of the 7th Framework Program (FP7) which aims to improve the projections of the continental ice melting contribution to sea level rise.

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See detailAnalysis of the past (1970-1999) and future (2046-2065 and 2081-2100) evolutions of precipitation and temperature, in the Province of Binh Thuan, South East Vietnam, based on IPCC models
Doutreloup, Sébastien; Erpicum, Michel; Fettweis, Xavier; Ozer, Pierre

in Proceedings of the 1st International Conference on Energy, Environment and Climate Change (2011, August)

Natural resource-dependent societies in developing countries are facing increased pressures linked to global climate change. The Province of Binh Thuan, in South East Vietnam, where rainfall is on average 500 to 700 mm but can drop as low as 350 mm in some years, knows a recent increase of agricultural activities in order to contribute to reduce poverty although the technical efficiency of Binh Thuan is still very low. Within this framework of higher dependency of the local economy on the agricultural sector, there is growing evidence that changes in climate extremes are increasing exposure of currently vulnerable rural populations. In order to assess the future climate of the province of Binh Thuan, only three models able to simulate the current climate in the study area were used out of the 24 selected by the IPCC: CCCMA-T47, INGV and IPSL. The future climate projections (that is 2046-2065 and 2081-2100 compared to historical data 1970-1999) were focused on two targets: [i] assessing changes in climate statistics, and [ii] analysing the beginning and the end of the rainy season. [i] The first analysis indicates an increase of mean temperature of about 1.6°C (over 2046-2065) and 2.5°C (over 2081-2100) and an increase of extreme temperatures and extreme rainfall events. However, no significant changes about the evolution of the annual amount of precipitation were found. [ii] The second analysis indicates that the dry season is likely to be longer in 2046-2065 owing to a delay in the onset of the rainy season (up to 15 days) accompanied by an earlier end of the rainy season (up to 30 days). Although it must be kept in mind that precipitations are the most difficult climate variable to predict, it is likely that increasing water needs to support expending agriculture within the context of climate change in the Province of Binh Thuan will be a challenge. Indeed, extreme rainfall events are likely to increase and unchanged yearly amounts of precipitation should be concentrated in time.

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See detailGreenland [in "State of the Climate in 2010"]
Box, J. E.; Ahlstrøm, A.; Cappelen, J.; Fettweis, Xavier; Decker, D.; Mote, T.; van As, D.; van de Wal, R. S. W.; Vinther, B.; Wahr, J.

in Bulletin of the American Meteorological Society (2011), 92(6), 161-171

Record warm air temperatures were observed over Greenland in 2010. This included the warmest year on record for Greenland's capital, Nuuk, in at least 138 years. The duration of the melt period on Greenland’s inland ice sheet was exceptional, being 1 month longer than the average over the past 30 years, and led to an extended period of amplified summer melt. All of the additional melt water very likely contributing to a faster rate of crevasse widening. Glacier loss along the Greenland margins was also exceptional in 2010, with the largest single glacier area loss (110 square miles, at Petermann glacier) equivalent to an area four times that of Manhattan Island. There is now no doubt that Greenland ice losses have not just increased above past decades, but have accelerated. The implication is that sea level rise projections will again need to be revised upward.

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See detailMelting trends over the Greenland ice sheet (1958–2009) from spaceborne microwave data and regional climate models
Fettweis, Xavier; Tedesco, Marco; van den Broeke, Michiel; Ettema, Janneke

in Cryosphere (2011), 5

To study near-surface melt changes over the Greenland ice sheet (GrIS) since 1979, melt extent estimates from two regional climate models were compared with those obtained from spaceborne microwave brightness temperatures using two different remote sensing algorithms. The results from the two models were consistent with those obtained with the remote sensing algorithms at both daily and yearly time scales, encouraging the use of the models for analyzing melting trends before the satellite era (1958–1979), when forcing data is available. Differences between satellite-derived and model-simulated results still occur and are used here to identify (i) biases in the snow models (notably in the albedo parametrization, in the thickness of a snow layer, in the maximum liquid water content within the snowpack and in the snowfall impacting the bare ice appearance in summer) and (ii) limitations in the use of passive microwave data for snowmelt detection at the edge of the ice sheet due to mixed pixel effect (e.g., tundra or rock nearby the ice sheet). The results from models and spaceborne microwave sensors confirm a significant (p-value = 0.01) increase in GrIS surface melting since 1979. The melt extent recorded over the last years (1998, 2003, 2005 and 2007) is unprecedented in the last 50 yr with the cumulated melt area in the 2000's being, on the average, twice that of the 1980's.

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See detailEvaluation of the present and future general circulation over western Europe simulated by the IPCC AR5/CMIP5 GCMs with the help of a circulation type classification
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

Conference (2011, April 08)

Downscaling methods forced by General Circulation Model (GCM) simulations are not able to correct the biases in the general circulation simulated by the GCMs. Moreover, since the GCMs have a coarse spatial resolution, they have difficulties to simulate reliably ground variables like temperature and precipitation which are affected by topography, land use and local features. So, we can attempt that they simulate better the large-scale atmospheric circulation. That is why it is of special interest to evaluate the GCM simulations of atmospheric circulation for current climate by comparing them with the NCEP-NCAR 1 and the ECMWF reanalysis data over 1961-1990. This analysis is done over western Europe for summer (JJA) and winter (DJF) for the GCMs (available on http://cmippcmdi.llnl.gov/cmip5/) proposed by the IPCC for its upcoming report (AR5). The method used is an automated circulation type classification based on the daily geopotential height at 500 hPa. It is a leader-algorithm correlation based method taking part of the COST733CAT classification catalogue. Unlike the usually used methods based on the monthly mean circulation, this approach allows a precise analysis of each circulation type. So, it gives much more information on the ability of the GCMs to simulate the different circulation types and consequently the climatic variability of a region. In order to allow a direct comparison between the GCM simulations and the reanalysis data, the classification is done first only for the reanalysis dataset over 1961-1990. Then, the main types individualised here are imposed for the classification of the GCM outputs. Since the circulation types are the same, the comparison between the datasets can be made on the basis of the differences of the frequency distribution throughout the classes. Moreover, the mean intraclass repartition of the circulation situations may differ from one dataset to another. So, the study of this mean and its standard deviation gives an idea of the differences between the reanalysis and the GCMs within each class. Firstly, this approach is applied to current climate (1961-1990) for evaluating the ability of the GCMs driven by the historical experiment to simulate the climate of the last decades over western Europe. In fact, if one GCM is not able to reproduce reliably the main characteristics of the current climate, its future projections may be questionable. Then, the best matching GCMs are retained and the same approach is applied to the future simulations driven by RCP concentrations or emissions (2011-2040, 2041-2070 and 2071-2100). So, the evolution of the frequency of the circulation types and maybe the appearance of new types can be analysed under climate change conditions. Moreover, it is interesting to compare the uncertainty of the current climate simulations to the projected changes for future climate. If the uncertainty is of the same order or higher than the projected changes, the reliability of the simulations for future climate may be very questionable.

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See detailExplanation of the extreme low surface mass balance over the Greenland ice sheet in 2010 with the help of a regional climate model and a circulation type classification.
Fettweis, Xavier; van den Broeke, Michiel; van de Berg, Willem Jan; Tedesco, Marco; Belleflamme, Alexandre; Franco, Bruno; Erpicum, Michel

Conference (2011, April 06)

The regional climate models MAR and RCMO show that the surface mass balance (SMB) rate of the whole Greenland ice sheet (GrIS) is the lowest in 2010 since 50 years. This record is a combination of an abnormal dry year and an exceptional melt in summer confirmed by ground measurements and satellite-derived observations. An automated circulation type classification (CTC) is used for detecting anomalies in the daily atmospheric circulation at 500hPa over the Greenland ice sheet during 2010. The CTC reveals that the low snow accumulation is due to the general circulation (negative NAO index) while the record melt in summer is rather a consequence of the well known surface albedo-temperature feedback induced by - a warmer and thinner than normal snowpack above the bare ice at the end of the spring. - an earlier beginning of the melt season. - a drier summer. - an exceptional persistence of atmospheric circulations inducing warm and dry conditions over the GrIS. All these anomalies induced in summer 2010 an exceptional time exposure of bare ice areas (with a lower albedo than snow) over the GrIS which impacts the surface melt. Sensitivity experiments carried out by the MAR model allow to estimate the importance of each anomaly in the record simulated melt of summer 2010.

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See detailEvaluation of the present and future general circulation over Greenland simulated by the IPCC AR5/CMIP5 GCMs with the help of a circulation type classification
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

Poster (2011, April 05)

Future projections of the Greenland ice sheet melt are based on General Circulation Model (GCM) simulations. In particular, the reliability of downscaling methods forced by these simulations depends on the quality of the atmospheric circulation simulated by GCMs. Therefore, it is essential to analyse and evaluate the GCMs modelled general circulation for current climate (1961-1990). Atmospheric circulation type classifications offer a very interesting approach for evaluating the GCM-based circulation at a daily time scale compared to the most used methods based only on monthly means. Indeed, the circulation type classification allows a precise and detailed analysis of each circulation type and so, it gives much more information on the ability of GCMs to simulate the different circulation types and consequently the climatic variability of a region. In fact, exceptional circulation events over Greenland, which cannot be taken into account by the monthly mean approach, have much more impact on the melt than the mean atmospheric state. Thus, an automated correlation-based atmospheric circulation type classification (CTC) is used for evaluating the new GCM outputs (available on http://cmip-pcmdi.llnl.gov/cmip5/) computed for the upcoming IPCC report (AR5). The daily geopotential height at 500 hPa simulations of the GCMs for current climate are compared to the NCEP-NCAR 1 and the ECMWF reanalysis data for the summer months (JJA), when melt is the most important. To achieve this, the classification is first done for the reanalysis data over 1961-1990 and afterwards, the types of the reanalysis based CTC are imposed for classifying the GCM datasets over 1961-1990 (from the historical experiment) to allow a direct type per type comparison based on the frequency distribution of each dataset. This approach also gives the opportunity to study the intraclass repartition differences between the reanalysis and the GCMs. After the evaluation of the GCM simulations for current climate, the future projections driven by RCP concentrations or emissions (2011-2040, 2041-2070 and 2071-2100) from the best matching GCMs are analysed in the same way. For current climate, it clearly appears that only a few GCMs are able to reproduce reliably the variability of the atmospheric circulation over Greenland during summer. The differences of frequency between the GCMs and the reanalysis are mainly due to biases of the geopotential height which is systematically over or underestimated by most GCMs and to the underestimation of the variability of the circulation by most GCMs. For future projections, no new circulation types are detected, but rather a general increase of the mean geopotential height regardless of the circulation type. It is also important to note that for many GCMs, the uncertainty of the current climate simulations (given by the differences of the classification results between the GCM simulations for current climate and the reanalysis data for the same time) are of the same order than the projected changes for future climate. Therefore, these projections may be questionable.

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See detailImpact of the spatial resolution on the Greenland Ice Sheet Surface Mass Balance modelling using the regional climate model MAR with the aim to force an ice sheet model.
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Poster (2011, April 05)

By using the regional climate model MAR (Modèle Atmosphérique Régional), validated for Greenland at 25km resolution and forced every 6 hours with the ERA-INTERIM reanalysis (Fettweis 2007, Fettweis et al. 2010), we have modelled the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) at 20, 25, 30, 40 and 50km resolution to assess the impact of the spatial resolution. As part of the ICE2SEA project, the 25km-resolution SMB outputs of the MAR model are used as forcing fields for ice sheet models, in order to produce future projections of the GrIS contribution to sea-level rise over the next 200 years. Although the current spatial resolution of the MAR model (25km) is much higher than the general circulation models (GCM) resolution (150-300km), the ice sheet models often run at a higher resolution (typically 5-10km). Nevertheless, such higher-resolution runs of the MAR model on the same integration domain generate a significant additional computing time and are not doable until now. Moreover, conventional linear interpolations of the SMB outputs onto a higher-resolution grid, generally induce biases because ice sheet masks at different spatial resolutions do not match and the SMB is a very complex function of the spatial resolution and the topography . That is why several enhanced SMB interpolations are tested here in order to reduce biases when interpolating the MAR outputs onto higher resolution, in the framework of the ICE2SEA project.

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See detailAssimilation of the 1979-2009 microwave satellite data into the regional climate MAR model for studying the Greenland ice sheet melt extent
Fettweis, Xavier; Tedesco, Marco; van den Broeke, Michiel

Conference (2011, April 05)

The 1979-2009 melt extent derived from the amount of produced meltwater by day simulated by the regional climate model MAR and derived from the spaceborne microwave 19GHz horizontal polarizated (T19H) brightness temperature compares well over the Greenland ice sheet (GrIS). However, some disagreements still occur in some pixels for any days. Therefore, we run the MAR model in an assimilation mode, constrained by the daily SMMR-SSM/I derived melt extent over 1979-2009. As assimilation, we change the MAR near-surface snowpack temperature for the pixels where MAR and satellite disagree. This correction allows to conserve the water equivalent of the snowpack mass in MAR while having a full agreement between model and satellite derived melt extent. The assimilation helps to improve the meltwater production simulation as well as the matching with other satellite data sets (MODIS, GRACE, ...), with the objective to reduce the uncertainties of the current SMB model-based estimates over the GrIS.

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See detailEvaluation of the Greenland ice sheet surface mass balance simulated by a regional climate model forced by some selected IPCC AR5/CMIP5 AOGCMs over the current climate.
Fettweis, Xavier; van den Broeke, Michiel; van de Berg, Willem Jan; Belleflamme, Alexandre; Franco, Bruno; Erpicum, Michel

Conference (2011, April 05)

Abstract. As part of the ICE2SEA project, the regional climate model MAR was forced by the general circulation model ECHAM5 for making future projections of the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) over 1980-2099 at a resolution of 25km. For the A1B scenario, MAR projects a highly negative (-500 GT/yr) SMB rate at the end of this century and a induced mass loss corresponding to a sea level rise of ~7 cm over 2000-2100. However, the comparison with MAR forced by the ERA-40 reanalysis over 1980-1999 shows that MAR forced by the 20C3M scenario is not able to represent reliably the current SMB due to biases in the general circulation and in the free atmosphere summer temperature modeled by ECHAM5 around the GrIS. These biases induce in MAR an underestimation of the snow accumulation and an overestimation of the surface melt. Therefore, this questions the reliability of these ECHAM5-forced future projections, knowing that i) these biases could be amplified in future and that ii) the MAR outputs are used to force ice sheets models for the ICE2SEA project. That is why, by waiting the outputs from the next generation of GCMs (CMIP5), we investigate the impacts of current climate biases over the future projections and we suggest corrections of ECHAM5 forcing files for having a better agreement with the ERA-40 forced simulation. This is useful for the ice sheet model wanting to use the absolute values of MAR future projections instead of anomalies.

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See detailEvolution of wind behaviour and of its potential for wind power production in Belgium during the last 30 years.
Doutreloup, Sébastien; Fettweis, Xavier; Sacré, Bernard; Erpicum, Michel

Poster (2011, April 04)

The number of wind turbines in the world grows significantly every year due to politics proposing green energy productions as solutions to mitigate climate change effects. However, this kind of energy is dependent on the weather. This implies that the wind production is irregular at a very short time scale. But the short time scale availability of the wind-based energy is important to the producers of energy as well as to the electric grid managers because the wind energy production can rise or fall rapidly which creates some financial and voltage variations. For these reasons, we study the past evolution of the availability of the wind quantity by analysing the intermittence of the wind speed in Belgium during the last 30 years. To reach this goal, we use the regional model WRF (Weather and Research Forecast model) developed by the UCAR community users. The WRF model is forced by the NCEP2 Reanalysis model to obtain a regionalisation of the weather conditions over a domain centred on Belgium at a spatiotemporal resolution of 10 km and 1 min. This resolution allows to capture the minute-based time scale variability of wind speed and consequently the irregular behaviour of the wind power production. To obtain a value of the wind intermittence, we calculate the persistence of a wind blowing continuously with a minimum speed of 1 ms-1, then the persistence of a wind blowing continuously with a minimum speed of 2 ms-1, etc. The persistence of the wind speed and its evolution over 30 years are characterised by : (a) the mean wind speed over a fixed period (monthly, seasonally, … ), (b) the mean duration of a wind speed above x ms-1 over the same fixed period and (c) the evolution of (a) and (b) during the studied period. This study will show the evolution during the last decades of the wind behaviour in Belgium and its potential for electricity production.

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See detailSurface Energy Balance
van den Broeke, Michiel; Fettweis, Xavier; Molg, Thomas

in Encyclopedia of Snow, Ice and Glaciers (2011)

Definition of surface energy balance over a snowpack

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See detailRecord Summer Melt in Greenland in 2010
Tedesco, Marco; Fettweis, Xavier; van den Broeke, Michiel; van de Wal, Roderik; Smeets, Paul; van de Berg, Willem Jan; Serreze, Mark; Box, Jason

in EOS (2011), 92(15), 126

As Arctic temperatures increase, there is growing concern about the melting of the Greenland ice sheet, which reached a new record during the summer of 2010. Understanding the changing surface mass balance of the Greenland ice sheet requires appreciation of the close links among changes in surface air temperature, surface melting, albedo, and snow accumulation. Increased melting accelerates surface snow grain growth, leading to a decrease in surface albedo, which then fosters further melt. In turn, winter accumulation contributes to determining how much snow is required before a dark (e.g., lower albedo), bare ice surface is exposed in spring (Figure 1).

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See detailUnderstanding the climatic signal in the water stable isotope records from the NEEM shallow firn/ice cores in northwest Greenland
Steen-Larsen; Masson-Delmotte; Sjolte; Johnsen; Vinther; Bréon; Clausen; Dahl-Jensen; Falourd; Fettweis, Xavier; Gallée; Jouzel; Kageyama; Lerche; Minster; Picard; Punge; Risi; Salas; Schwander; Steffen; Sveinbjörnsdóttir; Svensson; White

in Journal of Geophysical Research. Atmospheres (2011), 116

Samples of precipitation and atmospheric water vapor were collected together with shallow firn/ice cores as part of the new deep drilling project in northwest Greenland: the NEEM project. These samples were analyzed for their isotope composition to understand the processes affecting the climatic signal archived in the water stable isotope records from the NEEM deep ice core. The dominant moisture source for the snow deposited at the NEEM-site may be originating as far south as 35°N from the western part of the Atlantic Ocean. The surface atmospheric water vapor appears in isotopic equilibrium with the snow surface indicating a large water exchange between the atmosphere and snowpack. The interannual variability of NEEM shallow firn/ice cores stable isotope data covering the last ∼40 years shows an unexpectedly weak NAO signal. Regional to global atmospheric models simulate a dominant summer precipitation in the NEEM area, suggesting that the intermittency of modern winter precipitation is responsible for the lack of a strong NAO imprint. The interannual variability of NEEM isotope data however shows a strong correlation with interannual variations of Baffin Bay sea ice cover, a relationship consistent with air mass trajectories. NEEM deep ice core isotopic records may therefore provide detailed information on past Baffin Bay sea ice extent. NEEM stable water isotope content increasing trend points to a local warming trend of ∼3.0°C over the last 40 years.

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See detailThe role of albedo and accumulation in the 2010 melting record in Greenland
Tedesco, Marco; Fettweis, Xavier; van den Broeke, Michiel; van de Wal, Roderik; Smeets, Paul; van de Berg, Willem Jan; Serreze, Mark; Box, Jason

in Environmental Research Letters (2011), 6(1),

Analyses of remote sensing data, surface observations and output from a regional atmosphere model point to new records in 2010 for surface melt and albedo, runoff, the number of days when bare ice is exposed and surface mass balance of the Greenland ice sheet, especially over its west and southwest regions. Early melt onset in spring, triggered by above-normal near-surface air temperatures, contributed to accelerated snowpack metamorphism and premature bare ice exposure, rapidly reducing the surface albedo. Warm conditions persisted through summer, with the positive albedo feedback mechanism being a major contributor to large negative surface mass balance anomalies. Summer snowfall was below average. This helped to maintain low albedo through the 2010 melting season, which also lasted longer than usual.

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See detailImpact of the spatial resolution on the Greenland ice sheet surface mass balance modelling using the regional climate model MAR.
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Conference (2011, January 14)

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See detailPresent and future climates of the Greenland ice sheet according to the IPCC AR4 models
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel; Nicolay, Samuel

in Climate Dynamics (2011), 36

The atmosphere–ocean general circulation models (AOGCMs) used for the IPCC 4th Assessment Report (IPCC AR4) are evaluated for the Greenland ice sheet (GrIS) current climate modelling. The most suited AOGCMs for Greenland climate simulation are then selected on the basis of comparison between the 1970–1999 outputs of the Climate of the twentieth Century experiment (20C3M) and reanalyses (ECMWF, NCEP/NCAR). This comparison indicates that the representation quality of surface parameters such as temperature and precipitation are highly correlated to the atmospheric circulation (500 hPa geopotential height) and its interannual variability (North Atlantic oscillation). The outputs of the three most suitable AOGCMs for present-day climate simulation are then used to assess the changes estimated by three IPCC greenhouse gas emissions scenarios (SRES) over the GrIS for the 2070–2099 period. Future atmospheric circulation changes are projected to dampen the zonal flow, enhance the meridional fluxes and therefore provide additional heat and moisture to the GrIS, increasing temperature over the whole ice sheet and precipitation over its northeastern area. We also show that the GrIS surface mass balance anomalies from the SRES A1B scenario amount to −300 km3/year with respect to the 1970–1999 period, leading to a global sea-level rise of 5 cm by the end of the 21st century. This work can help to select the boundaries conditions for AOGCMs-based downscaled future projections.

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See detailThe 1958–2009 Greenland ice sheet surface melt and the mid-tropospheric atmospheric circulation
Fettweis, Xavier; Mabille, Georges; Erpicum, Michel; Nicolay, Samuel; Van den Broeke, Michiel

in Climate Dynamics (2011), 36(1 (2011)), 139-159

In order to assess the impact of the mid-tropospheric circulation over the Greenland ice sheet (GrIS) on surface melt, as simulated by the regional climate model MAR, an automatic Circulation type classification (CTC) based on 500 hPa geopotential height from reanalyses is developed. General circulation correlates significantly with the surface melt anomalies for the summers in the period 1958–2009. The record surface melt events observed during the summers of 2007–2009 are linked to the exceptional persistence of atmospheric circulations favouring warm air advection. The CTC emphasizes that summer 500 hPa circulation patterns have changed since the beginning of the 2000s; this process is partly responsible for the recent warming observed over the GrIS.

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See detailEvaluation over Greenland of WRF with GC-NET observations (1995-2005) by comparison with 2 other RCMs
Sacré, Bernard; Fettweis, Xavier; Doutreloup, Sébastien; Franco, Bruno; van den Broeke, Michiel; Erpicum, Michel

Conference (2011)

In the context of climate change, the Greenland Ice Sheet (GrIS) plays an important role in sea level variation and oceanic thermohaline circulation changes. Unfortunately, Global Climate Models do not illustrate enough the characteristics of Greenland. To solve that, specific RCMs have been developed to take into account the features of polar regions. In this project, we compare three RCMs : the MAR model, the RACMO model and the Weather Research and Forecasting (WRF) model. WRF is an open source model developed by the Mesoscale and Microscale Meteorology Division of NCAR. We use here the standard WRF (version 3.2.1) and its polar optimization (called polar WRF). The MAR version tuned for the GrIS and coupled with a 1D surface scheme called SISVAT (for Soil Ice Snow Vegetation Atmosphere Transfer) is compared here. The version of RACMO is a specific version for the Greenland climate, RACMO2/GR. This model contains a special surface module for snow-ice treatment and other modifications concerning, for example, the surface turbulence heat flux or the surface roughness. The comparison is made on a domain centered on Greenland at a 25-km horizontal resolution over the 1995-2005 period when Automatic Weather Station (AWS) measurements are available from the Greenland Climate NETwork (GC-NET). Statistics (mean, bias, RMSE, correlation coefficient) are calculated for the near-surface temperature, surface pressure, 10m-wind speed and specific humidity for winter (October to April) and summer (May to September). In addition, the modeled snowfall are evaluated with ice core-based snow accumulation climatologies. Comparison shows a significant improvement from RCMs compared to the reanalyses (NCEP2 and ERA-INTERIM) in respect to the AWS measurements. RACMO and MAR seem to compare better with observations than WRF.

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See detailUsing a circulation type classification to analyse the general circulation over Greenland simulated by GCMs
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

Conference (2010, November 23)

Future projections of the Greenland ice sheet melt are mainly based on General Circulation Model (GCM) outputs. The atmospheric circulation type classification offers a unique opportunity for validating the GCM-based circulations. Six GCMs used in the last IPCC report are analysed here. A correlation-based classification is constructed for each model using daily geopotential height at 500 hPa over Greenland. It is applied to a dataset combining the GCM-based outputs (20C3M scenario) for the current climate and the NCEP-NCAR 1 reanalysis data over the period 1961-1990 allowing a direct comparison for each circulation type. Most of the analysed models are able to reproduce the main circulation types, but they fail to reproduce their frequencies because they underestimate the climate variability. In addition, some biases in the mean geopotential height remain. However, we use our atmospheric circulation type classification for analysing future projections made by GCMs. As for the 20th century climate, a combined classification is made integrating reanalysis data over 1971-1990, GCM-based outputs over 1971-1990 (using 20C3M scenario) and GCM-based outputs over 2046-2065 and 2081-2100 (using A1B scenario). No new circulation types are individualised knowing that the main changes are just a general increase of the geopotential height. Furthermore, the changes in frequency observed between the 20th century climate and the first future period (2046-2065) are of the same order than the uncertainties of the models for simulating the current climate by comparison with the reanalysis data. Therefore, the circulation type classification is a useful tool to give a precise analysis of the atmospheric circulation simulated by GCMs knowing that most of downscaling techniques are dependent on the general circulation simulated by the GCMs.

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See detailAnalysis of the atmospheric circulation simulated by GCMs over Greenland
Belleflamme, Alexandre; Fettweis, Xavier; Erpicum, Michel

Conference (2010, November 05)

The variability of the geopotential height at 500 hPa simulated by General Circulation Models (GCMs) over Greenland is evaluated using an atmospheric circulation type classification. The GCM outputs for the current climate (20C3M) are first compared to reanalysis data over 1961-1990. The comparison shows that most of them simulate well the main circulation types but fail to reproduce their frequencies because of underestimations of circulation variability and biases in the mean geopotential height. GCM-based future projections do not individualise new circulation types but show a general increase of the geopotential height. Based on this approach, the correlation between surface temperature and atmospheric circulation offers a new way for estimating the Greenland ice sheet melt.

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See detailHigh resolution modelling of the Greenland ice sheet surface mass balance using the regional climate model MAR coupled with a downscaling interface
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Poster (2010, November 05)

We are developing a coupling interface downscaling the 25km-atmosphere fields simulated by the regional climate MAR (Modèle Atmosphérique Régional) model onto a 5km-grid in order to resolve the surface processes at high resolution with the SISVAT (Sea Ice Soil Vegetation Atmosphere Transfer) snow-ice module. This coupling interface improves the representation of the topography and ablation zone of the Greenland ice sheet (GrIS) in the MAR model, and therefore will provide higher resolution estimations of the GrIS surface mass balance (SMB) without additional computing time. By using outputs from previously-gauged global circulation models (GCM) as forcing fields, the MAR model coupled with the downscaling interface will then perform 5km future simulations of the GrIS SMB for different IPCC greenhouse gas emissions scenarios for the 21st century.

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See detailInterests of regional modelisation for wind power forecasting
Doutreloup, Sébastien; Fettweis, Xavier; Erpicum, Michel

Conference (2010, October 22)

European policies have decided to reduce the greenhouse gas emissions of 20% and to reach 20% of renewable power production by 2020. Increasing wind power is one of the numerous solutions to reach these goals. However, this kind of energy production depends on the meteorological conditions and gives it an intermittent behaviour. The wind speed variations cause voltage and frequency fluctuations that are unacceptable for the power grid. Therefore, forecasting production will become essential with the aim of integrating this kind of energy production into the power grid. We have developed and compared two forecasting models which give as outputs the wind power production every 15 minutes over the Belgian territory: the first one uses the outputs from the global model GFS (available at a horizontal resolution of 0.5° every 3h) and the second one uses the regional climate model WRF-NMM (using a horizontal resolution of 4km). Both of these models predict the wind speed and transform wind speed into wind power production, using a power curve which depends on the wind turbines and their characteristics. The first model using the GFS outputs is not precise enough in space and time to correctly forecast the wind speed in punctual wind farms. That is why we apply some specific tunings on these forecasts. These tunings depend on the air density, the wind direction and the stability of the air mass. The second model using the WRF-NMM outputs runs over the Belgian territory. Initial conditions are forced by the GFS outputs at 0.5° and WRF computes a physical based spatio-temporal downscaling of the meteorological variables. The outputs have a spatial resolution of 4 km and a time resolution of 15 minutes. Some tunings are also needed to adjust the wind power forecasts by comparison to the wind power observations. We present here some results of both models and the interest of using a regional model for more precise wind power forecasting.

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See detailGreenland [in Arctic Report Card 2010]
Box, J.; Cappelen, J.; Decker, D.; Fettweis, Xavier; Mote, T.; Tedesco, M.; van de Wal, R.

Report (2010)

Record warm air temperatures were observed over Greenland in 2010. This included the warmest year on record for Greenland's capital, Nuuk, in at least 138 years. The duration of the melt period on Greenland’s inland ice sheet was exceptional, being 1 month longer than the average over the past 30 years, and led to an extended period of amplified summer melt. All of the additional melt water very likely contributing to a faster rate of crevasse widening. Glacier loss along the Greenland margins was also exceptional in 2010, with the largest single glacier area loss (110 square miles, at Petermann glacier) equivalent to an area four times that of Manhattan Island. There is now no doubt that Greenland ice losses have not just increased above past decades, but have accelerated. The implication is that sea level rise projections will again need to be revised upward.

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See detailAnalysis of the evolution of the climate parameters, especially precipitations and temperatures, in the province of Binh Thuan in Southern Vietnam based on IPCC models
Doutreloup, Sébastien; Fettweis, Xavier; Ozer, Pierre

Report (2010)

This research is implied into the BELSPO / Vietnamese desertification project and the aim of this work is to analyse the future evolution of the temperatures and precipitations in the region of Binh Thuan thanks to the IPCC models (CMIP3).

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See detailGreenland [in "State of the Climate in 2009"]
Box, J.; Bhattacharya, I.; Cappelen, J.; Decker, D.; Fettweis, Xavier; Kezek, K.; Mote, T.; Tedesco, M.

in Bulletin of the American Meteorological Society (2010), 91(6), 121-124

The summer minimum ice extent in the Arctic was the third-lowest recorded since 1979. The 2008/09 boreal snow cover season marked a continuation of relatively shorter snow seasons, due primarily to an early disappearance of snow cover in spring. Preliminary data indicate a high probability that 2009 will be the 19th consecutive year that glaciers have lost mass. Below normal precipitation led the 34 widest marine terminating glaciers in Greenland to lose 101 km2 ice area in 2009, within an annual loss rate of 106 km2 over the past decade. Observations show a general increase in permafrost temperatures during the last several decades in Alaska, northwest Canada, Siberia, and Northern Europe. Changes in the timing of tundra green-up and senescence are also occurring, with earlier green-up in the High Arctic and a shift to a longer green season in fall in the Low Arctic.

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See detailMicrowave data and a regional climate model for studying the Greenland ice sheet surface mass balance over 1979-2009
Fettweis, Xavier; Marco, Tedesco

Poster (2010, May 03)

Results (melt extent and winter accumulation) from an atmosphere-snow coupled regional climate model are compared with microwave brightness temperatures-derived estimates to study the surface mass balance (SMB) changes over the Greenland ice sheet (GrIS) since 1979. Two simple algorithms are selected to retrieve the melt extent from the brightness temperatures. The first one is sensible to the production of surface meltwater as suggests the regional model and the second one is rather sensible to the presence of liquid water content into the snowpack. Both algorithms compare very well with model outputs and they are unanimous to show a significant increase of the surface melt over 1979-2009.We found also a good correlation between the March-April mean brightness temperatures and the simulated winter snow accumulation although no significant changes are found in both simulated and microwave-derived snow accumulation. The interannual variability of the brightness temperature-derived SMB components compare very well with the model results. This suggests that the variability of the model is reliable and that the model can be used to detect SMB changes over longer periods where no satellite data is available. Finally, both model and satellite agree to confirm the acceleration of the GrIS surface melting since 30 years.

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See detailModélisation à haute résolution de l'impact actuel et futur du réchauffement climatique sur le bilan de masse en surface de la calotte glaciaire du Groenland
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Conference (2010, January 21)

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See detailMulti-Months Cycles Observed in Climatic Data
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

in Simard, Suzanne (Ed.) Climate Change and Variability (2010)

Climatic variations happen at all time scales and since the origins of these variations are usually of very complex nature, climatic signals are indeed chaotic data. The identification of the cycles induced by the natural climatic variability is therefore a knotty problem, yet the knowing of these cycles is crucial to better understand and explain the climate (with interests for weather forecasting and climate change projections). Due to the non-stationary nature of the climatic time series, the simplest Fourier-based methods are inefficient for such applications (see e.g. Titchmarsh (1948)). This maybe explains why so few systematic spectral studies have been performed on the numerous datasets allowing to describe some aspects of the climate variability (e.g. climatic indices, temperature data). However, some recent studies (e.g. Matyasovszky (2009); Paluš & Novotná (2006)) show the existence of multi-year cycles in some specific climatic data. This shows that the emergence of new tools issued from signal analysis allows to extract sharper information from time series. Here, we use a wavelet-based methodology to detect cycles in air-surface temperatures obtained from worldwide weather stations, NCEP/NCAR reanalysis data, climatic indices and some paleoclimatic data. This technique reveals the existence of universal rhythms associated with the periods of 30 and 43 months. However, these cycles do not affect the temperature of the globe uniformly. The regions under the influence of the AO/NAO indices are influenced by a 30 months period cycle, while the areas related to the ENSO index are affected by a 43 months period cycle; as expected, the corresponding indices display the same cycle. We next show that the observed periods are statistically relevant. Finally, we consider some mechanisms that could induce such cycles. This chapter is based on the results obtained in Mabille & Nicolay (2009); Nicolay et al. (2009; 2010).

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See detailA statistical validation for the cycles found in air temperature data using a Morlet wavelet-based method
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

in Nonlinear Processes in Geophysics (2010), 17

Recently, new cycles, associated with periods of 30 and 43 months, respectively, have been observed by the authors in surface air temperature time series, using a wavelet-based methodology. Although many evidences attest the validity of this method applied to climatic data, no systematic study of its efficiency has been carried out. Here, we estimate confidence levels for this approach and show that the observed cycles are significant. Taking these cycles into consideration should prove helpful in increasing the accuracy of the climate model projections of climate change and weather forecast.

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See detailCOST733CAT - a database of weather and circulation
Philipp, Andreas; Bartholy, Judit; Beck, Christoph; Erpicum, Michel; Esteban, Pere; Fettweis, Xavier; Huth, Radan; James, Paul; Jourdain, Sylvie; Kreienkamp, Frank; Krennert, Thomas; Lykoudis, Spyros; Michalides, Silas C.; Pianko-Kluczynska, Krystyna; Post, Piia; Álvarez, Domingo Rasilla; Schiemann, Reinhard; Spekat, Arne; Tymvios, Filippos S.

in Physics and Chemistry of the Earth (2010), 35(9-12), 360-373

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See detailThe 1958-2008 Greenland ice sheet surface mass balance variability simulated by the regional climate model MAR
Fettweis, Xavier

Poster (2009, April 24)

Results made with the regional climate model MAR over 1958-2008 show a very high interannual variability of the Greenland ice sheet (GrIS) surface mass balance (SMB) modelled in average to be 330 +/- 130 km^3/yr. To a first approximation, the SMB variability is driven by the annual precipitation anomaly minus the meltwater run-off rate variability. Sensitivity experiments carried out by the MAR model evaluate the impacts on the surface melt of (i) the summer SST around the Greenland, (ii) the snow pack temperature at the beginning of the spring, (iii) the winter snow accumulation, (iv) the solid and liquid summer precipitations and (v) the summer atmospheric circulation. This last one, by forcing the summer air temperature above the ice sheet, explains mainly the surface melt anomalies.

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See detailThe 1958-2008 Greenland ice sheet surface melt and the mid-tropospheric atmospheric circulation
Fettweis, Xavier

Conference (2009, April 23)

With the aim to study the impact of the mid-tropospheric circulation on the Greenland ice sheet (GrIS) surface melt simulated by the regional climate model MAR, we developed an automatic Circulation Type Classification (CTC) based on the 500hPa geopotential height from reanalyses over the period 1958-2008. This CTC shows that the dominant mode of the regional atmospheric summer variability around the GrIS is linked to the North Atlantic Oscillation (NAO) and that the surface melt anomalies are highly correlated to the general circulation. It explains notably why a record surface melt was observed during the summers 2007 and 2008. In addition, the climate conditions occurring the 27th August of 2003, where the GrIS temperature was 10°C higher than the normal, were the consequence of an almost unique 500 hPa circulation in the 50 last years.

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See detailGreenland ice sheet projections from IPCC AR4 global models
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel; Nicolay, Samuel

Poster (2009, April)

The atmosphere-ocean general circulation models (AOGCMs) used for the IPCC 4th Assessment Report (IPCC AR4) are evaluated for the Greenland ice sheet (GrIS) current climate modelling. The most efficient AOGCMs are chosen by comparison between the 1970-1999 outputs of the Climate of the twentieth Century experiment (20C3M) and reanalyses (ECMWF, NCEP/NCAR). This comparison reveals that surface parameters such as temperature and precipitation are highly correlated to the atmospheric circulation (500 hPa geopotential height) and its interannual variability (North Atlantic oscillation). The outputs of the three most efficient AOGCMs are then used to assess the changes planned by three IPCC greenhouse gas emissions scenarios (SRES) for the 2070-2099 period. Future atmospheric circulation changes should dampen the west-to-east circulation (zonal flow) and should enhance the Meridional Overturning Circulation (MOC). As a consequence, this provides more heat and moisture to the GrIS, increasing temperature on the whole ice sheet and precipitation on the north-eastern region. It is also shown that the GrIS surface mass balance (SMB) anomalies from the SRES A1B scenario are about -300 km³/yr with respect to the 1970-1999 period, leading to 5 cm of global sea-level rise (SLR) for the end of the 21st century. This work helps to choose the boundaries conditions for AOGCMs downscaled future projections.

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See detailCycles in the Temperature Data Are Induced by the Sun
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

Poster (2009, April)

The presence of two cycles of period of 30 and 42 months approximatively has been observed in temperature records and climatic indices. Moreover, it has been shown that these cycles are statistically significant. Here we outline the role played by the Sun in the presence of these cycles, observed in time series. To do so, we use IPCC AR4 climatic models, sunspot number data and the Morlet wavelet method.

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See detailConfidence Levels for the Cycles Found in Air Temperature Data
Mabille, Georges; Fettweis, Xavier; Erpicum, Michel; Nicolay, Samuel

Poster (2009, April)

Recently, new cycles have been observed in air temperature data and proxy series using a wavelet-based methodology. Although many evidences attest the validity of this method applied to climatic data, no systematic study of its efficiency has been carried out. Here, we estimate the confidence levels for this approach and show that the observed cycles are significant.

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See detailCycles found in air temperature data and proxy series using the Morlet wavelet method
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

Scientific conference (2009, February 25)

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See detailGreenland ice sheet surface mass balance projections from IPCC AR4 global models
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Poster (2009, January 28)

Results from atmosphere-ocean general circulation models (AOGCM's) for the IPCC 4th Assessment Report are used to investigate surface mass balance (SMB) future projections of the Greenland ice sheet (GrIS). The most efficient models for the GrIS climate modeling are chosen by comparison between the 1970-1999 outputs (averages and trends) from the Climate of the twentieth Century Experiment (20C3M) and reanalyses (ECMWF, NCEP) as well as observations (ice core measurements). The outputs from these most efficient models are after used to assess changes planned by the IPCC greenhouse gas emissions scenarios (SRES) for the 2070-2099 period. The GrIS SMB projections are estimated from changes in precipitation and temperatures from these AOGCM's outputs. However, large uncertainties remain in these SMB projections based on simplified physics and huge model outputs. High resolution simulations made with regional models (which simulate explicitly the SMB by taking into account the surface feedbacks) forced at their boundaries by a GrIS well-adapted AOGCM could bring more precise brief replies.

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See detailNew cycles found in air temperature data and proxy series using the Morlet wavelet method
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

Conference (2009, January 13)

We use the wavelet transform to detect cycles in air temperature data and proxy series

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See detail30 and 43 months period cycles found in air temperature time series using the Morlet wavelet
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

in Climate Dynamics (2009), 33

A wavelet-based methodology is applied to relevant climatic indices and air temperature records and allow to detect the existence of unexpected cycles. The scale spectrum shows the presence of two cycles of about 30 and 43 months, respectively, in the air–temperature time series, in addition to the well-known cycles of 1 day and 1 year. The two cycles do not affect the globe uniformly: some regions seem to be more influenced by the period of 30 months (e.g. Europe), while other areas are affected by the period of 43 months (e.g. North-West of the USA). Similar cycles are found in the indices and the regions influenced by these indices: the NAO index and the Western Europe display a cycle of 30 months, while the cycle of 43 months can be found in the ENSO index and in regions where it is known to have an impact.

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See detailHydrologic response of the Greenland ice sheet: the role of oceanographic warming
Hanna, E.; Cappelen, J.; Fettweis, Xavier; Huybrechts, P.; Luckman, A.; Ribergaard, M. H.

in Hydrological Processes (2009), 23(1), 7-30

The response of the Greenland ice sheet to ongoing climate change remains an area of great uncertainty, with most previous studies having concentrated on the contribution of the atmosphere to the ice mass-balance signature. Here we systematically assess for the first time the influence of oceanographic changes on the ice sheet. The first part of this assessment involves a statistical analysis and interpretation of the relative changes and variations in sea-surface temperatures (SSTs) and air temperatures around Greenland for the period 1870-2007. This analysis is based on HadISST1 and Reynolds OI.v2 SST analyses, in situ SST and deeper ocean temperature series, surface-air-temperature records for key points located around the Greenland coast, and examination of atmospheric pressure and geopotential height from NCEP/NCAR reanalysis. Second, we carried out a novel sensitivity experiment in which SSTs were perturbed as input to a regional climate model, and document the resulting effects on simulated Greenland climate and surface mass balance. We conclude that sea-surface/ocean temperature forcing is not sufficient to strongly influence precipitation/snow accumulation and melt/runoff of the ice sheet. Additional evidence from meteorological reanalysis suggests that high Greenland melt anomalies of summer 2007 are likely to have been primarily forced by anomalous advection of warm air masses over the ice sheet and to have therefore had a more remote atmospheric origin. However, there is a striking correspondence between ocean warming and dramatic accelerations and retreats of key Greenland outlet glaciers in both southeast and southwest Greenland during the late 1990s and early 2000s.

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See detailGreenland [in "State of the Climate in 2008"]
Box, J.; Bai, L.; Benson, R.; Bhattacharya, I.; Bromwich, D.; Cappelen, J.; Decker, D.; DiGirolamo, N.; Fettweis, Xavier; Hall, D.; Hanna, E.; Mote, T.; Tedesco, M.; van de Wal, R.; van den Broeke, M.

in Bulletin of the American Meteorological Society (2009), 90

An abnormally cold winter across the southern half of Greenland led to substantially higher west coast sea ice thickness and concentration. Even so, record-setting summer temperatures around Greenland, combined with an intense melt season (particularly across the northern ice sheet), led the 2008 Greenland climate to be marked by continued ice sheet mass deficit and floating ice disintegration.

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See detailExtreme snowmelt in Northern Greenland during summer 2008
Tedesco, Marco; Fettweis, Xavier; van den Broeke, Michiel; van de Wal, Roderik; Smeets, Paul

in EOS (2008), 89(41), 391

Extreme snowmelt occurred during summer 2008 over the northern part of the Greenland ice sheet, according to the analysis of microwave data recorded by the Special Sensor Microwave Imager (SSM/I) on board the F13 satellite of the U.S. Defense Meteorological Satellite Program (DMSP). New records of the number of melting days were also observed over large portions of the same areas (letters A and B in Figure 1).

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See detailAnalysis of climatic conditions of wildfires in Belgium with an automatic daily atmospheric circulation patterns classification
Erpicum, Michel; Fettweis, Xavier; Mabille, Georges

Conference (2008, October 02)

An automatic daily atmospheric circulation patterns classification was built using the geopotential heights of 850 hPa level from the ECMWF (re)analysis over the period 1958-2007. The classification is based on a similarity index between two 850hPa geopotential maps centred on Belgium, taking into account the slope difference between both daily geopotential surfaces as well as the absolute geopotential difference between both surfaces. Wildfire occurrences are analysed in April and September together with monthly frequencies and persistences of daily atmospheric circulation patterns types as well as with monthly variability of weather climate conditions.

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See detailNew cycles found in air temperature data and proxy series
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

Conference (2008, October 01)

We apply the wavelet transform tool to detect unsuspected cycles in several climatic time series

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See detailSolar activity in connection with a 2.5 years periode cycle in air temperature time series using the Morlet wavelet method
Nicolay, Samuel; Mabille, Georges; Fettweis, Xavier; Erpicum, Michel

Conference (2008, April 17)

The Morlet wavelet is applied to air temperature time series obtained from several weather stations and reveals the existence of a period cycle of 20-30 months since 1950, with an estimated amplitude of 0.5 C. The origin of this period is investigated by computing the scale spectra associated to the principal indices that characterize air mass flows in the troposphere and the stratosphere, as well as the signals related to the sunspot number and the solar flux. Each analysed signal shows this period of approximatively 2.5 years. This suggests that the 2.5 years-cycle could be resulted from the solar activity.

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See detailA record negative Greenland ice sheet surface mass balance rate in 2007
Fettweis, Xavier

Poster (2008, April 15)

Results made with the regional climate model MAR show a record surface melt (592 km³/yr = a global sea level rise of 1.6 mm/yr) of the Greenland ice sheet (GrIS) during summer 2007 compared with 1970-2006. This record melt, detected also in the microwave satellite data, is associated with very low snowfall (508 km³/yr) inducing a negative Surface Mass Balance (SMB) rate of -65 km³/yr. Such a negative simulated SMB rate is unprecedented in the recent Greenland history. The summer 2007 is associated with a positive SST anomaly, a negative 2006-2007 GrIS winter accumulation and anomalous advection of warm air masses over the GrIS. Sensitivity experiments carried out by the MAR model evaluate the impacts of these anomalies on the Greenland climate and SMB. The main impacts of a warmer SST anomaly in the MAR model are more precipitation over Greenland due to an enhanced evaporation above the ocean and, an increase of surface melt induced by the advection of warmer oceanic air (>0°C) into the continent by the atmospheric part of MAR. A negative winter accumulation anomaly exposes ice and old snow (with a lower albedo) earlier than previous years in the ablation zone which significantly increases the melting given the albedo feedback. Finally, changes in the boundaries forcing of the MAR model test the consequence of the anomalous persistent southerly airflow during June and July.

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See detailEstimation of the 1900-2100 Greenland ice sheet surface mass balance
Fettweis, Xavier

Conference (2008, April 15)

Results from a 37-year simulation (1970-2006) over the Greenland ice sheet (GrIS) with the regional climate model MAR reveals that more than 97% of the interannual variability of the modelled Surface Mass Balance (SMB) is explained by the GrIS summer temperature anomaly and the GrIS annual precipitation anomaly. This dependence is also fully confirmed by another model using the ECMWF (re)analysis. This multiple regression is then used to empirically estimate the GrIS SMB since 1900 from climatological time series and reanalysises. The projected SMB changes in the 21st century are investigated with the set of simulations performed with AOGCM's for the IPCC Fourth Assessment Report. These estimations show that the high surface mass loss rates of these last years (1998, 2003, 2006) are not unprecedented in the GrIS history of the last hundred years. The minimum SMB rate seems to be occurred in the 1930's due to a combination of dryer and warmer years than now although the effect of the man-induced global warming was not perceptible at that time. The AOGCM's project that the SMB rate of the 1930s would be common at the end of this century. The temperature would be higher than in the 1930s but the increase of accumulation would partly offset the acceleration of surface melt due to the temperature increase. If no change will occur in the iceberg discharge rate, such negative SMB rates would be not large enough to significantly increase in the future the fresh meltwater flux from the GrIS into the ocean. However, these assumptions are based on an empirical multiple regression only currently validated and the accuracy and time homogeneity of the data sets and AOGCM results used in these estimations constitute a large uncertainty.

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See detailImpacts of ice sheet mask and resolution on estimating the surface mass balance of the Greenland ice sheet
Fettweis, Xavier

Poster (2008, April 15)

The impacts of the spatial resolution and a Greenland ice sheet (GrIS) mask on modelling the Surface Mass Balance (SMB) are studied with the regional climate model MAR coupled with a complex energy balance/snowpack model. On the one hand, too coarse resolution prevents the model from resolving adequately the steep ice sheet margin and the ablation zone, not wider than 100 km in Greenland, where substantial seasonal melting occurs. The resolution affects also the precipitation modelling. On the other hand, a too large ice sheet mask (i.e. with low-altitude ice pixels in the model, where there is no ice in reality) leads to an overestimation of the run-off. In addition, due to the albedo feedback, biases in the ice sheet mask have also consequences on the surface energy balance.

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See detailIs the Greenland ice sheet beginning to melt?
Gallée, Hubert; Fettweis, Xavier

in La Lettre « Changement Global » PIGB - PMRC France (2008), 21

Using a new evaluation of satellite data, and simula- tions carried out with a regional climate model, it has been shown that the acceleration of surface melting of the Greenland ice sheet during the 1979-2005 period was twice as rapid as earlier studies had estimated. Between 1979 and 2005, the area of Greenland affected by melt at least one day per year in fact increased by 42%, and the average summer tempera- ture rose by 2.4°C.

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See detailAutomatic 1958-2007 daily weather pattern classification applied to an analysis of climatic conditions of wildfires in eastern Belgium
Erpicum, Michel; Fettweis, Xavier; Mabille, Georges; François, Louis

Poster (2008, April)

The daily atmospheric circulation patterns classification is founded on a 100 km regular grid centred on Belgium. The geopotential heights of 500, 850 and 1000 hPa levels were extracted <br />from the ERA-40 database on the period 1958-2002 and from ECMWF operational analysis until the end of year 2007. The classification was based on a similarity index calculated on the orientation of exaggerated slopes of different daily geopotential fields. Wildfire occurrences were analyzed in April and September (which are the two months with the most frequent wildfire-days in Belgium) together with monthly frequencies and persistences of daily atmospheric circulation pattern classes as well as with yearly variability of weather climate conditions.

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See detailLast century Greenland ice sheet surface mass balance projections from IPCC AR4 global models
Franco, Bruno; Fettweis, Xavier; Erpicum, Michel

Poster (2008, April)

Results from atmosphere-ocean general circulation models (AOGCM's) for the IPCC 4th Assessment Report are used to investigate surface mass balance (SMB) future projections of the Greenland ice sheet (GrIS). The most efficient models for the GrIS climate modeling are chosen by comparison between the 1970-1999 outputs (averages and trends) from the 20C3M Experiment outputs, and the reanalyses (ECMWF, NCEP) as well as climatologies. The SMB is estimated from the summer temperature (from which is deduced the run-off) and annual snowfall from the well-adapted AOGCM's. It is validated with 1970-1999 results from the regional climate model MAR by interpolating the AOGCM's outputs on the MAR grid. However, large uncertainties remain in these SMB projections due to the simplified physic and coarse AOGCM's resolution. High resolution simulations made with the MAR model (which simulates explicitly the SMB by taking into account the surface feedbacks) forced at its boundaries by a GrIS well-adapted AOGCM could bring more precise brief replies.

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See detailEstimation of the 1900-2100 Greenland ice sheet surface mass balance
Fettweis, Xavier; Hanna, E.; Gallée, H.; Erpicum, Michel

Conference (2008, April)

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See detailCirculations atmosphériques et anomalies de fonte à la surface de la calotte glaciaire du Groenland
Fettweis, Xavier; Mabille, Georges; Erpicum, Michel

in Bulletin de la Société Géographique de Liège (2008), 51

With the aim to study the impact of the 500hPa general circulation on the Greenland ice sheet surface melt simulated by the regional climate model MAR, we developed a new Circulation Type Classification (CTC) based on the 500hPa geopotential height from the ECMWF (re)analysis over the period 1958-2007. This CTC shows that the dominant mode of the regional atmospheric variability around the Greenland is linked to the North Atlantic Oscillation (NAO) and that the surface anomalies are highly correlated to the general circulation. It explains also why a record surface melt was observed during the summer 2007. The 27th August of 2003, where the temperature was 10°C higher than the normal, is the consequence of an almost unique 500 hPa circulation in the 50 last years.

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See detailDiagnosing the extreme surface melt event over southwestern Greenland in 2007
Tedesco, Marco; Serreze, Marc; Fettweis, Xavier

in Cryosphere (2008), 2

Analysis of passive microwave brightness temperatures from the space-borne Special Sensor Microwave Imager (SSM/I) documents a record surface snowmelt over high elevations (above 2000 m) of the Greenland ice sheet during summer of 2007. To interpret this record, results from the SSM/I are examined in conjunction with fields from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and output from a regional climate model. The record surface melt reflects unusually warm conditions, seen in positive summertime anomalies of surface air temperatures, downwelling longwave radiation, 1000–500 hPa atmospheric thickness, and the net surface energy flux, linked in turn to southerly airflow over the ice sheet. Low snow accumulation may have contributed to the record through promoting anomalously low surface albedo.

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See detailEstimation of the Greenland ice sheet surface mass balance for the 20th and 21st centuries
Fettweis, Xavier; Hanna, Edward; Gallée, Hubert; Huybrechts, Philippe; Erpicum, Michel

in Cryosphere (2008), 2

Results from a regional climate simulation (1970–2006) over the Greenland ice sheet (GrIS) reveals that more than 97% of the interannual variability of the modelled Surface Mass Balance (SMB) can be explained by the GrIS summer temperature anomaly and the GrIS annual precipitation anomaly. This multiple regression is then used to empirically estimate the GrIS SMB since 1900 from climatological time series. The projected SMB changes in the 21st century are investigated with the set of simulations performed with atmosphere-ocean general circulation models (AOGCMs) of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). These estimates show that the high surface mass loss rates of recent years are not unprecedented in the GrIS history of the last hundred years. The minimum SMB rate seems to have occurred earlier in the 1930s and corresponds to a zero SMB rate. The AOGCMs project that the SMB rate of the 1930s would be common at the end of 2100. The temperature would be higher than in the 1930s but the increase of accumulation in the 21st century would partly offset the acceleration of surface melt due to the temperature increase. However, these assumptions are based on an empirical multiple regression only validated for recent/current climatic conditions, and the accuracy and time homogeneity of the data sets and AOGCM results used in these estimations constitute a large uncertainty.

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See detailReconstruction of the 1979-2005 Greenland ice sheet surface mass balance using satellite data and the regional climate model MAR
Fettweis, Xavier

Conference (2007, April 17)

In order to improve our knowledge on the current state and variability of the Greenland ice sheet surface mass balance (SMB), a 27-year simulation (1979-2005) has been performed with the coupled atmosphere-snow regional model MAR. This simulation reveals an increase in the main factors of the SMB which are, on the one hand, the snowfall (+ 1.0 ± 1.5 km^3 yr^-2, not significant) in winter and on the other hand, the run-off of the melt water (+ 5.3 ± 3.0 km^3 yr^-2, significant) in summer. The net effect of these two competing factors leads to a SMB loss rate of – 4.1 ± 4.1 km^3 yr^-2, which has a significance of 95%. The melt extent derived from the passive microwave satellite data since 1979 also shows the acceleration of the surface melt. The contribution of changes in the net water vapour fluxes to the SMB variability is negligible. The melt water supply has increased because the Greenland ice sheet has been warming up by + 0.08 ± 0.04 °C yr^-1 since 1979. Latent heat flux, sensible heat flux and net solar radiation have not varied significantly over the last three decades. However, the simulated summer downward infra-red flux has increased by 7.1 W m^-2 since 1979. The natural climate variability (e.g. the North Atlantic Oscillation) does not fully explain these changes on the Greenland ice sheet. These changes result very likely from the global warming induced by human activities. The increase of +137 km^3 in the melt water run-off in the period 1979-2005 suggests that the overall ice sheet mass balance has been increasingly negative, given the observed melt-induced outlet glacier acceleration.

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See detailThe 1979-2005 Greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval XPGR algorithm
Fettweis, Xavier

Poster (2007, April 16)

Analysis of passive microwave satellite observations over the Greenland ice sheet reveals a significant increase in surface melt over the period 1979-2005. Since 1979, the total melt area was found to have increased +1.22 x 10ˆ7 kmˆ2. An improved version of the cross-polarized gradient ratio (XPGR) technique is used to identify the melt from the brightness temperatures. The improvements in the melt retrieval XPGR algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere-snow regional climate model. From 1979 to 2005, the ablation period increases everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall. Indeed, more snowfall in summer decreases the liquid water content of the snowpack, raises the albedo and therefore reduces the melt. Finally, this melt acceleration over the Greenland ice sheet is highly correlated with both Greenland and global warming suggesting a continuing surface melt increase in the future.

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See detailThe 1979–2005 Greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval XPGR algorithm
Fettweis, Xavier; van Ypersele, Jean-Pascal; Gallée, Hubert; Lefebre, Filip; Lefebvre, Wouter

in Geophysical Research Letters (2007)

Analysis of passive microwave satellite observations over the Greenland ice sheet reveals a significant increase in surface melt over the period 1979–2005. Since 1979, the total melt area was found to have increased by +1.22 × 107 km2. An improved version of the cross-polarized gradient ratio (XPGR) technique is used to identify the melt from the brightness temperatures. The improvements in the melt retrieval XPGR algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere-snow regional climate model. From 1979 to 2005, the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall. Indeed, more snowfall in summer decreases the liquid water content of the snowpack, raises the albedo and therefore reduces the melt. Finally, the observed melt acceleration over the Greenland ice sheet is highly correlated with both Greenland and global warming suggesting a continuing surface melt increase in the future.

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See detailReconstruction of the 1979–2006 Greenland ice sheet surface mass balance using the regional climate model MAR
Fettweis, Xavier

in Cryosphere (2007), 1

Results from a 28-year simulation (1979–2006) over the Greenland ice sheet (GrIS) reveal an increase of solid precipitation (+0.4±2.5 km3 yr−2) and run-off (+7.9±3.3 km3 yr−2) of surface meltwater. The net effect of these competing factors is a significant Surface Mass Balance (SMB) loss of −7.2±5.1 km3 yr−2. The contribution of changes in the net water vapour flux (+0.02±0.09 km3 yr−2) and rainfall (+0.2±0.2 km3 yr−2) to the SMB variability is negligible. The meltwater supply has increased because the GrIS surface has been warming up +2.4°C since 1979. Sensible heat flux, latent heat flux and net solar radiation have not varied significantly over the last three decades. However, the simulated downward infrared flux has increased by 9.3 W m−2 since 1979. The natural climate variability (e.g. the North Atlantic Oscillation) does not explain these changes. The recent global warming, due to the greenhouse gas concentration increase induced by human activities, could be a cause of these changes. The doubling of surface meltwater flux into the ocean over the period 1979–2006 suggests that the overall ice sheet mass balance has been increasingly negative, given the likely meltwater-induced acceleration of outlet glaciers. This study suggests that increased melting overshadows over an increased accumulation in a warming scenario and that the GrIS is likely to keep losing mass in the future. An enduring GrIS melting will probably affect in the future an certain effect on the stability of the thermohaline circulation and the global sea level rise.

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See detailReconstruction of the 1979-2005 Greenland ice sheet surface mass balance using satellite data and the regional climate model MAR
Fettweis, Xavier

Doctoral thesis (2006)

In order to improve our knowledge on the current state and variability of the Greenland ice sheet surface mass balance (SMB), a 27-year simulation (1979-2005) has been performed with the coupled atmosphere-snow regional model MAR. This simulation reveals an increase in the main factors of the SMB which are, on the one hand, the snowfall (+ 1.6 ± 1.8 km3 yr-1) in winter and on the other hand, the run-off (+ 4.2 ± 1.9 km3 yr-1) in summer. The net effect of these two competing factors leads to a SMB loss rate of – 2.7 ± 3.0 km3 yr-1, which has a significance of 87%. The melt extent derived from the passive microwave satellite data since 1979 also shows this trend. The melt water supply has increased because the Greenland ice sheet has been warming up by + 0.09 ± 0.04 °C yr-1 since 1979. This warming comes from a uniform increase of downward infra-red radiation which can not be explained by the natural variability. These changes result very likely from the global warming induced by human activities. As a result, it seems that: i) increased melting dominates over increased accumulation in a warming scenario, ii) the Greenland ice sheet has been significantly losing mass since the beginning of the 1980's by an increasing melt water run-off as well as by a probable increase of iceberg discharge into the ocean due to the "Zwally effect" (the melt water-induced ice sheet flow acceleration) and iii) the Greenland ice sheet is projected to continue to lose mass in the future. The Greenland ice sheet melting could have an effect on the stability of the thermohaline circulation (THC) and the global sea level rise. On the one hand, increases in the freshwater flux from the Greenland ice sheet (glacier discharge and run-off) could perturb the THC by reducing the density contrast driving it. On the other hand, the melting of the whole Greenland ice sheet would account for a global mean sea level rise of 7.4 m.

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See detailThe 1988-2003 Greenland ice sheet melt extent by passive microwave satellite data and a regional climate model
Fettweis, Xavier; Gallée, Hubert; Lefebre, Filip; van Ypersele, Jean-Pascal

in Climate Dynamics (2006), 27(5), 531-541

Measurements from ETH-Camp and JAR1 AWS (West Greenland) as well as coupled atmosphere-snow regional climate simulations have highlighted flaws in the cross-polarized gradient ratio (XPGR) technique used to identify melt from passive microwave satellite data. It was found that dense clouds (causing notably rainfall) on the ice sheet severely perturb the XPGR melt signal. Therefore, the original XPGR melt detection algorithm has been adapted to better incorporate atmospheric variability over the ice sheet and an updated melt trend for the 1988–2003 period has been calculated. Compared to the original algorithm, the melt zone area increase is eight times higher (from 0.2 to 1.7% year−1). The increase is higher with the improved XPGR technique because rainfall also increased during this period. It is correlated to higher atmospheric temperatures. Finally, the model shows that the total ice sheet runoff is directly proportional to the melt extent surface detected by satellites. These results are important for the understanding of the effect of Greenland melting on the stability of the thermohaline circulation.

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See detailEvaluation of a high-resolution regional climate simulation over Greenland
Lefebre, Filip; Fettweis, Xavier; Gallée, Hubert; van Ypersele, Jean-Pascal; Marbaix, Philippe; Greuell, Wouter; Calanca, Pierluigi

in Climate Dynamics (2005), 25(1), 99-116

A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model (RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than 60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations.

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See detailGreenland surface mass balance simulated by a regional climate model and comparison with satellite derived data in 1990-1991
Fettweis, Xavier; Gallée, Hubert; Lefebre, Filip; van Ypersele, Jean-Pascal

in Climate Dynamics (2005), 24

The 1990 and 1991 ablation seasons over Greenland are simulated with a coupled atmosphere-snow regional climate model with a 25-km horizontal resolution. The simulated snow water content allows a direct comparison with the satellite-derived melt signal. The model is forced with 6-hourly ERA-40 reanalysis at its boundaries. An evaluation of the simulated precipitation and a comparison of the modelled melt zone and the surface albedo with remote sensing observations are presented. Both the distribution and quantity of the simulated precipitation agree with observations from coastal weather stations, estimates from other models and the ERA-40 reanalysis. There are overestimations along the steep eastern coast, which are most likely due to the “topographic barrier effect”. The simulated extent and time evolution of the wet snow zone compare generally well with satellite-derived data, except during rainfall events on the ice sheet and because of a bias in the passive microwave retrieved melt signal. Although satellite-based surface albedo retrieval is only valid in the case of clear sky, the interpolation and the correction of these data enable us to validate the simulated albedo on the scale of the whole Greenland. These two comparisons highlight a large sensitivity of the remote sensing observations to weather conditions. Our high-resolution climate model was used to improve the retrieval algorithms by taking more fully into account the atmosphere variability. Finally, the good agreement of the simulated melting surface with the improved satellite signal allows a detailed estimation of the melting volume from the simulation.

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See detailGreenland ice sheet melt: intercomparison between SSM/I and a regional climate model
Fettweis, Xavier

Conference (2004, April 27)

The daily melt extent on the Greenland ice sheet can easily be retrieved from satellite observations and therefore is a very useful index to study the surface mass balance (SMB) evolution of the last years. It is also particularly helpful for the validation of a model because there is little in-situ observations on the Greenland ice sheet. The remote sensing melt-detection algorithms use the changes in microwave brightness temperatures during snowmelt. The most used one on Greenland is the cross-polarized gradient ratio (XPGR) method from Abdalati and Steffen (1997)*. It was found from a comparison with simulations made by the regional climate model MAR (Modèle Atmosphérique Régional) that the rainfall on the ice sheet in summer perturbs the melt signal detected by XPGR via the 37-Ghz vertical channel. An improved XPGR algorithm was developed. We present here our motivation to modify the XPGR. An intercomparaison between the SSM/I derived observations and the MAR is performed. The aim is to validate our model, in order to study the SMB for future climate. The simulated extent and time evolution of the wet snow zone compares better with satellite derived data when the modified XPGR method is used.

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See detailModelling of the 1991 Greenland Summer with the coupled atmosphere-snow regional climate model MAR
Fettweis, Xavier

Poster (2003, May 12)

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See detailModelling of Greenland summer 1991 with a coupled atmosphere-snow regional climate model in view of surface melt calculations
Fettweis, Xavier

Poster (2002, September 30)