Publications of Xavier Fettweis
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See detailSurface meltwater runoff on the Greenland ice sheet estimated from remotely sensed supraglacial lake infilling rate
Yang, K.; Smith, L.; Fettweis, Xavier ULiege et al

in Remote Sensing of Environment (2019)

Surface runoff constitutes a large percentage of Greenland Ice Sheet (GrIS) mass loss at present but is difficult to measure directly. This study provides a novel method to estimate surface runoff through ... [more ▼]

Surface runoff constitutes a large percentage of Greenland Ice Sheet (GrIS) mass loss at present but is difficult to measure directly. This study provides a novel method to estimate surface runoff through remote sensing of supraglacial lake volumes. Because terminal, non-draining (consistently expanding during the melt season) lakes impound runoff from their surrounding contributing catchments, such changes reflect runoff produced within the catchment. To estimate supraglacial lake volumes, multi-temporal lake maps derived from Landsat-8 images are intersected with dry lake-bed topographic depressions (showing lake bathymetry) identified for two supraglacial catchments (~10 km2) in southwestern GrIS, using high-resolution (2 m) ArcticDEMs. Intersecting remotely sensed lake shorelines with their underlying ice surface topography yields multi-temporal lake volume changes, which are then compared with cumulative runoff as simulated by four Surface Mass Balance (SMB) models (HIRHAM5, MAR3.6, RACMO2.3, and MERRA-2). Comparison of cumulative lake infilling with SMB simulations for these two lakes over the period 8–31 July 2015 indicates that SMB models overestimated surface runoff by 106 – 123%. These large offsets improved after early July, overestimating runoff by 40 – 55%. The runoff delay function incorporated into the MAR3.6 model improves simulation of early melt season runoff, signifying the importance of integrating meltwater routing schemes into SMB models for improved understanding of Greenland supraglacial hydrology and surface mass balance. [less ▲]

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See detailEvaluating a regional climate model simulation of Greenland ice sheet snow and firn density for improved surface mass balance estimates
Alexander, P.; Tedesco, M.; Koening, L. et al

in Geophysical Research Letters (2019)

Modeling vertical profiles of snow and firn density near the surface of the Greenland ice sheet (GrIS) is key to estimating GrIS mass balance, and by extension, global sea level change. To understand ... [more ▼]

Modeling vertical profiles of snow and firn density near the surface of the Greenland ice sheet (GrIS) is key to estimating GrIS mass balance, and by extension, global sea level change. To understand sources of error in simulated GrIS density, we compare GrIS density profiles from a leading regional climate model with coincident in situ measurements. We identify key contributors to model density and mass balance biases, including underestimated simulated fresh snow density (which leads to underestimation of density in the top 1 m of snow by ~10%). In areas undergoing frequent melting, positive density biases (of 7% in the top 1 m, and 10% between 1 and 10 m) are likely associated with errors in representing meltwater production, retention and refreezing. The results highlight the importance of accurately capturing fresh snow density and meltwater processes in models used to estimate GrIS mass balance change. [less ▲]

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See detailEstimating Greenland tidewater glacier retreat driven by submarine melting
Slater, D.; Straneo, F.; Felikson, D. et al

in Cryosphere (2019), (13), 24892509

The effect of the North Atlantic Ocean on the Greenland Ice Sheet through submarine melting of Greenland's tidewater glacier calving fronts is thought to be a key driver of widespread glacier retreat ... [more ▼]

The effect of the North Atlantic Ocean on the Greenland Ice Sheet through submarine melting of Greenland's tidewater glacier calving fronts is thought to be a key driver of widespread glacier retreat, dynamic mass loss and sea level contribution from the ice sheet. Despite its critical importance, problems of process complexity and scale hinder efforts to represent the influence of submarine melting in ice-sheet-scale models. Here we propose parameterizing tidewater glacier terminus position as a simple linear function of submarine melting, with submarine melting in turn estimated as a function of subglacial discharge and ocean temperature. The relationship is tested, calibrated and validated using datasets of terminus position, subglacial discharge and ocean temperature covering the full ice sheet and surrounding ocean from the period 1960–2018. We demonstrate a statistically significant link between multi-decadal tidewater glacier terminus position change and submarine melting and show that the proposed parameterization has predictive power when considering a population of glaciers. An illustrative 21st century projection is considered, suggesting that tidewater glaciers in Greenland will undergo little further retreat in a low-emission RCP2.6 scenario. In contrast, a high-emission RCP8.5 scenario results in a median retreat of 4.2 km, with a quarter of tidewater glaciers experiencing retreat exceeding 10 km. Our study provides a long-term and ice-sheet-wide assessment of the sensitivity of tidewater glaciers to submarine melting and proposes a practical and empirically validated means of incorporating ocean forcing into models of the Greenland ice sheet. [less ▲]

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See detailRapid expansion of Greenland’s low-permeability ice slabs
MacFerrin, Michael; Machguth, Horst; van As, D. et al

in Nature (2019), 573

In recent decades, meltwater runoff has accelerated to become the dominant mechanism for mass loss in the Greenland ice sheet1,2,3. In Greenland’s high-elevation interior, porous snow and firn accumulate ... [more ▼]

In recent decades, meltwater runoff has accelerated to become the dominant mechanism for mass loss in the Greenland ice sheet1,2,3. In Greenland’s high-elevation interior, porous snow and firn accumulate; these can absorb surface meltwater and inhibit runoff4, but this buffering effect is limited if enough water refreezes near the surface to restrict percolation5,6. However, the influence of refreezing on runoff from Greenland remains largely unquantified. Here we use firn cores, radar observations and regional climate models to show that recent increases in meltwater have resulted in the formation of metres-thick, low-permeability ‘ice slabs’ that have expanded the Greenland ice sheet’s total runoff area by 26 ± 3 per cent since 2001. Although runoff from the top of ice slabs has added less than one millimetre to global sea-level rise so far, this contribution will grow substantially as ice slabs expand inland in a warming climate. Runoff over ice slabs is set to contribute 7 to 33 millimetres and 17 to 74 millimetres to global sea-level rise by 2100 under moderate- and high-emissions scenarios, respectively—approximately double the estimated runoff from Greenland’s high-elevation interior, as predicted by surface mass balance models without ice slabs. Ice slabs will have an important role in enhancing surface meltwater feedback processes, fundamentally altering the ice sheet’s present and future hydrology. [less ▲]

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See detailRepresentation of the rain shadow effect in Patagonia using an orographic‐derived regional climate model
Damseaux, Adrien ULiege; Fettweis, Xavier ULiege; Lambert, Marius ULiege et al

in International Journal of Climatology (2019)

This study focuses on Patagonia, where Foehn events observed in the lee of the Andes mountains are not yet well simulated by state‐of‐the‐art climate models. It has been agreed that one source of this ... [more ▼]

This study focuses on Patagonia, where Foehn events observed in the lee of the Andes mountains are not yet well simulated by state‐of‐the‐art climate models. It has been agreed that one source of this shortcoming is related to the poor relief representation in models. To resolve this need, a common method used is to enhance the spatial resolution of the model to retrieve a more complex surface elevation, at the expense of calculation time or surface area covered. This paper tackles the problem from a different angle by addressing the Digital Elevation Model (DEM) generalization, .i.e. the altitudes generalization from a high‐resolution DEM to a coarser resolution grid model. Most current climate models use DEM generalization methods that smooth the relief, a key controlling factor in Foehn events modeling. The aim of this study is to compare three original methods of DEM generalization (percentile 90 (P90), envelope maximum (EM), and thalweg and crests (TC)) and to evaluate their impact on simulated precipitation and temperature fields on the eastern part of Patagonia, where warm and dry air masses are expected. Thanks to MAR, a Regional Climate Model, we validate the models at 10 and 5 km resolutions against the Climate Research Unit and perform three sensitivity experiments involving a change in the DEM generalization. Our results show that (i) a finer spatial resolution can slightly improve the temperature biases, however, it cannot resolve the precipitation biases and (ii) a more appropriate use of DEM generalization induces a significant decrease in precipitation for the P90 and EM methods and an increase in mean temperature for all three methods in the study area. This study serves as a recommendation for a better use of DEM generalization in climate models performing in Patagonia, but also regions sharing the same orographic features as the Patagonian relief. [less ▲]

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See detailGreenland Ice Sheet late-season melt: investigating multiscale drivers of K-transect events
Ballinger, T.; Mote, T.; Mattingly, K. et al

in Cryosphere (2019), 13

One consequence of recent Arctic warming is an increased occurrence and longer seasonality of above-freezing air temperature episodes. There is significant disagreement in the literature concerning ... [more ▼]

One consequence of recent Arctic warming is an increased occurrence and longer seasonality of above-freezing air temperature episodes. There is significant disagreement in the literature concerning potential physical connectivity between high-latitude open water duration proximate to the Greenland Ice Sheet (GrIS) and late-season (i.e., end-of-summer and autumn) GrIS melt events. Here, a new date of sea ice advance (DOA) product is used to determine the occurrence of Baffin Bay sea ice growth along Greenland's west coast for the 2011–2015 period. Over the 2-month period preceding the DOA, northwest Atlantic Ocean and atmospheric conditions are analyzed and linked to late-season melt events observed at a series of on-ice automatic weather stations (AWSs) along the K-transect in southwestern Greenland. Surrounding ice sheet, tundra, and coastal winds from the Modèle Atmosphérique Régional (MAR) and Regional Atmospheric Climate Model (RACMO) provide high-resolution spatial context to AWS observations and are analyzed along with ERA-Interim reanalysis fields to understand the meso-to-synoptic-scale (thermo)dynamic drivers of the melt events. Results suggest that late-season melt events, which primarily occur in the ablation area, are strongly affected by ridging atmospheric circulation patterns that transport warm, moist air from the subpolar North Atlantic toward west Greenland. Increasing concentrations of North Atlantic water vapor are shown to be necessary to produce melt conditions as autumn progresses. While thermal conduction and advection off south Baffin Bay open waters impact coastal air temperatures, local marine air incursions are obstructed by barrier flows and persistent katabatic winds along the western GrIS margin. [less ▲]

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See detailFuture projections of cyclone activity in the Arctic for the 21st century from regional climate models (Arctic-CORDEX)
Akperov, M.; Rinke, A.; Mokhov, I. et al

in Global and Planetary Change (2019)

Changes in the characteristics of cyclone activity (frequency, depth and size) in the Arctic are analyzed based on simulations with state-of-the-art regional climate models (RCMs) from the Arctic-CORDEX ... [more ▼]

Changes in the characteristics of cyclone activity (frequency, depth and size) in the Arctic are analyzed based on simulations with state-of-the-art regional climate models (RCMs) from the Arctic-CORDEX initiative and global climate models (GCMs) from CMIP5 under the Representative Concentration Pathway (RCP) 8.5 scenario. Most of RCMs show an increase of cyclone frequency in winter (DJF) and a decrease in summer (JJA) to the end of the 21st century. However, in one half of the RCMs, cyclones become weaker and substantially smaller in winter and deeper and larger in summer. RCMs as well as GCMs show an increase of cyclone frequency over the Baffin Bay, Barents Sea, north of Greenland, Canadian Archipelago, and a decrease over the Nordic Seas, Kara and Beaufort Seas and over the sub-arctic continental regions in winter. In summer, the models simulate an increase of cyclone frequency over the Central Arctic and Greenland Sea and a decrease over the Norwegian and Kara Seas by the end of the 21st century. The decrease is also found over the high-latitude continental areas, in particular, over east Siberia and Alaska. The sensitivity of the RCMs' projections to the boundary conditions and model physics is estimated. In general, different lateral boundary conditions from the GCMs have larger effects on the simulated RCM projections than the differences in RCMs' setup and/or physics. [less ▲]

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See detailRegional modeling of surface mass balance on the Cook Ice Cap, Kerguelen Islands
Verfaillie, D.; Favier, V.; Gallée, H. et al

in Climate Dynamics (2019)

We assess the ability of the regional circulation model MAR to represent the recent negative surface mass balance (SMB) observed over the Kerguelen Islands ( 49∘S , 69∘E ) and evaluate the uncertainties ... [more ▼]

We assess the ability of the regional circulation model MAR to represent the recent negative surface mass balance (SMB) observed over the Kerguelen Islands ( 49∘S , 69∘E ) and evaluate the uncertainties in SMB projections until the end of the century. The MAR model forced by ERA-Interim reanalysis shows a good agreement with meteorological observations at Kerguelen, particularly after slight adjustment of the forcing fields (+ 10% humidity, +0.8∘C , all year round) to improve precipitation occurrence and intensity. The modeled SMB and surface energy balance (SEB) are also successfully evaluated with observations, and spatial distributions are explained as being largely driven by the elevation gradient and by the strong west to east foehn effect occurring on the ice cap. We select five general circulation models (GCMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5) by evaluating their ability to represent temperature and humidity in the southern mid-latitudes over 1980–1999 with respect to ERA-Interim and use them to force the MAR model. These simulations fail to replicate SMB observations even when outputs from the best CMIP5 model (ACCESS1-3) are used as forcing because all GCMs fail in accurately reproducing the circulation changes observed at Kerguelen since the mid-1970s. Global models chosen to represent extreme values of SMB drivers also fail in producing extreme values of SMB, suggesting that more rigorous modeling of present and future circulation changes with GCMs is still needed to accurately assess future changes of the cryosphere in this area. [less ▲]

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See detailGreenland ice sheet mass balance assessed by PROMICE (1995–2015)
Colgan, W.; Mankoff; Kjeldsen, K. et al

in GEUS Bulletin (2019), 43

The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) has measured ice-sheet elevation and thickness via repeat airborne surveys circumscribing the ice sheet at an average elevation of 1708 ± ... [more ▼]

The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) has measured ice-sheet elevation and thickness via repeat airborne surveys circumscribing the ice sheet at an average elevation of 1708 ± 5 m (Sørensen et al. 2018). We refer to this 5415 km survey as the ‘PROMICE perimeter’. Here, we assess ice-sheet mass balance following the input-output approach of Andersen et al. (2015). We estimate ice-sheet output, or the ice discharge across the ice-sheet grounding line, by applying downstream corrections to the ice flux across the PROMICE perimeter. We subtract this ice discharge from ice-sheet input, or the area-integrated, ice sheet surface mass balance, estimated by a regional climate model. While Andersen et al. (2015) assessed ice-sheet mass balance in 2007 and 2011, this updated input-output assessment now estimates the annual sea-level rise contribution from eighteen sub-sectors of the Greenland ice sheet over the 1995–2015 period. [less ▲]

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See detailCloud microphysics and circulation anomalies control differences in future Greenland melt
Hofer, Stefan ULiege; Tedstone, A.; Fettweis, Xavier ULiege et al

in Nature Climate Change (2019), 9

Recently, the Greenland Ice Sheet (GrIS) has become the main source of barystatic sea-level rise. The increase in the GrIS melt is linked to anticyclonic circulation anomalies, a reduction in cloud cover ... [more ▼]

Recently, the Greenland Ice Sheet (GrIS) has become the main source of barystatic sea-level rise. The increase in the GrIS melt is linked to anticyclonic circulation anomalies, a reduction in cloud cover and enhanced warm-air advection. The Climate Model Intercomparison Project fifth phase (CMIP5) General Circulation Models (GCMs) do not capture recent circulation dynamics; therefore, regional climate models (RCMs) driven by GCMs still show significant uncertainties in future GrIS sea-level contribution, even within one emission scenario. Here, we use the RCM Modèle Atmosphèrique Règional to show that the modelled cloud water phase is the main source of disagreement among future GrIS melt projections. We show that, in the current climate, anticyclonic circulation results in more melting than under a neutral-circulation regime. However, we find that the GrIS longwave cloud radiative effect is extremely sensitive to the modelled cloud liquid-water path, which explains melt anomalies of +378 Gt yr–1 (+1.04 mm yr–1 global sea level equivalent) in a +2 °C-warmer climate with a neutral-circulation regime (equivalent to 21% more melt than under anticyclonic circulation). The discrepancies between modelled cloud properties within a high-emission scenario introduce larger uncertainties in projected melt volumes than the difference in melt between low- and high-emission scenarios. [less ▲]

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See detailPrecipitation Evolution over Belgium by 2100 and Sensitivity to Convective Schemes Using the Regional Climate Model MAR
Doutreloup, Sébastien ULiege; Kittel, Christoph ULiege; Wyard, Coraline et al

in Atmosphere (2019), 10(321),

The first aim of this study is to determine if changes in precipitation and more specifically in convective precipitation are projected in a warmer climate over Belgium. The second aim is to evaluate if ... [more ▼]

The first aim of this study is to determine if changes in precipitation and more specifically in convective precipitation are projected in a warmer climate over Belgium. The second aim is to evaluate if these changes are dependent on the convective scheme used. For this purpose, the regional climate model Modèle Atmosphérique Régional (MAR) was forced by two general circulation models (NorESM1-M and MIROC5) with five convective schemes (namely: two versions of the Bechtold schemes, the Betts–Miller–Janjić scheme, the Kain–Fritsch scheme, and the modified Tiedtke scheme) in order to assess changes in future precipitation quantities/distributions and associated uncertainties. In a warmer climate (using RCP8.5), our model simulates a small increase of convective precipitation, but lower than the anomalies and the interannual variability over the current climate, since all MAR experiments simulate a stronger warming in the upper troposphere than in the lower atmospheric layers, favoring more stable conditions. No change is also projected in extreme precipitation nor in the ratio of convective precipitation. While MAR is more sensitive to the convective scheme when forced by GCMs than when forced by ERA-Interim over the current climate, projected changes from all MAR experiments compare well. [less ▲]

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See detailUtilité du modèle atmosphérique régional MAR pour le suivi des précipitations en Afrique : cas de la région du Lac Victoria et du Burkina Faso
Doutreloup, Sébastien ULiege; Wyard, Coraline ULiege; Fettweis, Xavier ULiege

Poster (2019, June)

Les lacs et retenues d’eau d’Afrique sont primordiaux à la vie dans les régions qu’ils bordent. Le niveau de ces lacs est fortement dépendant des précipitations qui tombent directement à leur surface et ... [more ▼]

Les lacs et retenues d’eau d’Afrique sont primordiaux à la vie dans les régions qu’ils bordent. Le niveau de ces lacs est fortement dépendant des précipitations qui tombent directement à leur surface et dans leur bassin versant. Dans un but de gestion de la ressource en eau, il est important d’analyser et de prévoir l’évolution passée, en temps réel (mode prévision) et future des quantités de précipitations qui l’alimentent. Ce poster montre que l’utilisation du modèle climatique régional MAR et de schémas convectifs appropriés permet de modéliser des quantités de précipitation journalières et saisonnières ? proches de celles obtenues par des observations dans la région du Lac Victoria et au Burkina Faso. De plus, la variabilité diurne est bien représentée dans les simulations du MAR suggérant que le modèle pourra être utilisé en mode prévision météo sur d’autres régions d’Afrique. [less ▲]

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See detailComplementarity Assessment of South Greenland Katabatic Flows and West Europe Wind Regimes
Radu, David-Constantin ULiege; Berger, Mathias ULiege; Fonteneau, Raphaël ULiege et al

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 ... [more ▼]

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. [less ▲]

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See detailEvaluation of the near-surface climate of the Greenland ice sheet as modelled by the climate model MAR and the ERA-Interim, ERA5 and Arctic System reanalyses
Delhasse, Alison ULiege; Kittel, Christoph ULiege; Amory, Charles ULiege et al

Conference (2019, April 12)

The new reanalysis produced by the ECMWF, ERA5, is currently available over the period 2000-2017. Ultimately, it will cover the period 1950 to the present-time and will replace the ERA-Interim, which is ... [more ▼]

The new reanalysis produced by the ECMWF, ERA5, is currently available over the period 2000-2017. Ultimately, it will cover the period 1950 to the present-time and will replace the ERA-Interim, which is by many considered as one of the best reanalyses over the Greenland ice sheet (GrIS). We first compare the ERA5 reanalysis to ERA-Interim and ASR (Arctic System Reanalysis), which is a regional reanalysis specifically developed for the Arctic area at a finer resolution. We evaluate them against a set of near-surface climate observations from the AWS of the PROMICE network covering the GrIS. This observation data set is not assimilated in these reanalyses. We furthermore assess the ability of the state-of-the-art regional climate model (RCM) MAR, forced by the ECMWF reanalyses, ERA-Interim and ERA5, to represent the AWS observations. Finally, we demonstrate the advantages of using MAR compared to the forcing reanalyses alone. ERA5 improves ERA-Interim almost for radiative fluxes, but not significantly. ASR, which is more specific for Arctic region and has a finer spatial resolution, outclasses other reanalyses for wind speed and near-surface temperature. The comparison of results from MAR simulations forced by ERA-Interim and ERA5 reanalysis shows that the near-surface climate variables are closed to each other and then not significantly different according to the forcing used. ERA5 which should replace ERA-Interim after 2018, can be used to force a RCM such as MAR in the same way than ERA-Interim now. Although the reanalyses seem to be sufficient to study the surface climate of Greenland, the RCM MAR has the best representation of the near-surface temperature even without any data assimilation. This is mainly due to a better representation of the snowpack and interactions between surface and atmosphere by MAR, resulting in a better representation of the surface melt and the GrIS surface mass balance (SMB). Near-surface temperature and SMB are both very useful for constraining glacial dynamics models in order to represent the current and future evolution of the ice dynamics of the GrIS. [less ▲]

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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 ULiege et al

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 ... [more ▼]

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. [less ▲]

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See detailRefining the outputs of a dynamic vegetation model (CARAIB):Research at ULiège, Belgium
Hambuckers, Alain ULiege; Paillet, Marc ULiege; Henrot, Alexandra-Jane ULiege et al

Scientific conference (2019, March 19)

Dynamic vegetation models (DVMs) are process-based models combining the inputs and the outputs of sub-models, possibly in feedback loops, to simulate the plant functions. The sub-models compute conditions ... [more ▼]

Dynamic vegetation models (DVMs) are process-based models combining the inputs and the outputs of sub-models, possibly in feedback loops, to simulate the plant functions. The sub-models compute conditions outside and inside the plant and physiological reactions from the environmental data (climate, light intensity, air CO2 concentration, soil properties). DVMs are tools of choice to predict the future and the past of the vegetation taking into account climatic variations. The emergence of new questions in the context of climate change, particularly on threatened species or on commercial species, compels to apply DVMs to species while the information to parameterize and validate them is largely lacking. Of particular importance are the morpho-physiological traits. These were intensively studied within the hypothesis that they could be used to predict plant performances. This hypothesis finally revealed not very suitable, but it brought to light that important traits controlling photosynthesis and water relationships could strongly vary within each species in response to environmental conditions. We studied the Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière), in Morocco (northern Africa). It is a threatened tree species of important economic value. We also studied the English oak (Quercus robur L.) and the sessile oak (Quercus petraea (Matt.) Liebl.) in eastern Belgium. In a series of localities, we determined several traits (specific leaf area, leaf C/N, sapwood C/N, as well as for the cedar, leaf longevity) and we assessed biomass and net primary productivity as validation data, thanks to forest inventories, dendrochronology analyses and allometric equations combined with leaf area index estimations. We compared the model simulations of the CARAIB DVM when varying the set of traits (direct site estimates or default values) to the field estimates of biomass and net primary productivity. We found that trait default values provide sufficient information for the DVM to compute mean output values but low ability to reproduce between site variations. On the contrary, the in situ traits improve drastically this ability, which indicates that the plant performances are the results of acclimation to the evolving local environmental conditions. [less ▲]

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See detailRefining the outputs of a dynamic vegetation model (CARAIB): the importance of plant traits to improve prediction accuracy at tree species level
Hambuckers, Alain ULiege; Paillet, Marc ULiege; Henrot, Alexandra-Jane ULiege et al

Conference (2019, March 11)

Dynamic vegetation models (DVMs) are process-based models combining the inputs and the outputs of sub-models, possibly in feedback loops, to simulate the plant functions. The sub-models compute conditions ... [more ▼]

Dynamic vegetation models (DVMs) are process-based models combining the inputs and the outputs of sub-models, possibly in feedback loops, to simulate the plant functions. The sub-models compute conditions outside and inside the plant and physiological reactions from the environmental data (climate, light intensity, air CO2 concentration, soil properties). DVMs are tools of choice to predict the future and the past of the vegetation taking into account climatic variations. The emergence of new questions in the context of climate change, particularly on threatened species or on commercial species, compels to apply DVMs to species while the information to parameterize and validate them is largely lacking. Of particular importance are the morpho-physiological traits. These were intensively studied within the hypothesis that they could be used to predict plant performances. This hypothesis finally revealed not very suitable, but it brought to light that important traits controlling photosynthesis and water relationships could strongly vary within each species in response to environmental conditions. We studied the Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière), in Morocco (northern Africa). It is a threatened tree species of important economic value. We also studied the English oak (Quercus robur L.) and the sessile oak (Quercus petraea (Matt.) Liebl.) in eastern Belgium. In a series of localities, we determined several traits (specific leaf area, leaf C/N, sapwood C/N, as well as for the cedar, leaf longevity) and we assessed biomass and net primary productivity as validation data, thanks to forest inventories, dendrochronology analyses and allometric equations combined with leaf area index estimations. We compared the model simulations of the CARAIB DVM when varying the set of traits (direct site estimates or default values) to the field estimates of biomass and net primary productivity. We found that trait default values provide sufficient information for the DVM to compute mean output values but low ability to reproduce between site variations. On the contrary, the in situ traits improve drastically this ability, which indicates that the plant performances are the results of acclimation to the evolving local environmental conditions. [less ▲]

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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. et al

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 ... [more ▼]

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. [less ▲]

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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 ULiege; Amory, Charles ULiege; Kittel, Christoph ULiege et al

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 ... [more ▼]

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. [less ▲]

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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. et al

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 ... [more ▼]

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. [less ▲]

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