Publications of Xavier Fettweis
Bookmark and Share    
Full Text
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, Achim; Eisen, O.; MacFerrin, Michael et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 122 (8 ULiège)
Full Text
See detailAssessing the future evolution of climate extremes favouring floods using the regional climate model MAR over the CORDEX.be domain
Wyard, Coraline ULiege; Doutreloup, Sébastien ULiege; Fettweis, Xavier ULiege

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 62 (7 ULiège)
Full Text
See detailInfluence of the recent circulation change in summer on future surface mass balance of Greenland ice sheet
Delhasse, Alison ULiege; Fettweis, Xavier ULiege; Kittel, Christoph ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 46 (10 ULiège)
Full Text
See detailEtude des séries temporelles : exemple de la température de l’eau
Richir, Jonathan ULiege; Borges, Alberto ULiege; Champenois, Willy ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 185 (6 ULiège)
Full Text
See detailGlobal warming and natural variability
Reynaerts, Laura ULiege; Fettweis, Xavier ULiege; Nicolay, Samuel ULiege

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 94 (13 ULiège)
Full Text
See detailExtreme temperature events on Greenland in observations and the MAR regional climate model
Leeson, A.; Eastoe, E.; Fettweis, Xavier ULiege

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 150 (5 ULiège)
Full Text
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. et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 32 (6 ULiège)
Full Text
See detailCyclone Activity in the Arctic From an Ensemble of Regional Climate Models (Arctic CORDEX)
Akperov, M.; Rinke, A.; Mokhov, I. et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 26 (2 ULiège)
Full Text
See detailCombining regional downscaling expertise in Belgium: CORDEX and beyond
Termonia, Piet; Van Schaeybroeck, Bert; De Cruz, Lesley et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 113 (2 ULiège)
Full Text
See detailSensitivity of the Antarctic surface mass balance to oceanic perturbations
Kittel, Christoph ULiege; Amory, Charles ULiege; Agosta, Cécile ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 71 (17 ULiège)
Full Text
See detailInterests of using the RCM MAR to downscale CMIP6 outputs
Amory, Charles ULiege; Kittel, Christoph ULiege; Delhasse, Alison ULiege et al

Conference (2017, December 10)

Detailed reference viewed: 39 (6 ULiège)
Full Text
See detailWinter season changes in Belgium: the MAR model contribution to the CORDEX.be project
Wyard, Coraline ULiege; Doutreloup, Sébastien ULiege; Scholzen, Chloé ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 57 (11 ULiège)
Full Text
See detailGreenland Ice Sheet Surface Mass Loss: Recent Developments in Observation and Modeling
van den Broeke, M.; Box, J.; Fettweis, Xavier ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 71 (7 ULiège)
Full Text
See detailDark ice dynamics of the south-west Greenland Ice Sheet
Tedstone, A.; Bamber, J.; Cook, J. et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 34 (1 ULiège)
Full Text
See detailThe MAR model : CORDEX.be and EURO-CORDEX results
Wyard, Coraline ULiege; Doutreloup, Sébastien ULiege; Fettweis, Xavier ULiege

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 42 (8 ULiège)
Full Text
See detailSurface solar radiation modelling over 1900-2014: comparison between the regional climate model MAR and reanalyses
Wyard, Coraline ULiege; Doutreloup, Sébastien ULiege; Belleflamme, Alexandre ULiege et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 101 (6 ULiège)
Full Text
See detailApplication of a two-step approach for mapping ice thickness to various glacier types on Svalbard
Fürst, Johannes Jakob; Gillet-Chaulet, Fabien; Benham, Toby J. et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 67 (8 ULiège)
Full Text
See detailNoël 2010 en Belgique : neige en Flandre et pluie en Haute-Ardenne
Fettweis, Xavier ULiege; Wyard, Coraline ULiege; Doutreloup, Sébastien ULiege et al

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 ... [more ▼]

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 [less ▲]

Detailed reference viewed: 110 (30 ULiège)
Full Text
See detailEvaluating Model Simulations of Twentieth-Century Sea Level Rise. Part 2: Regional Sea-Level Changes
Meyssignac, B.; Slangen, A.; Melet, A. et al

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 40 (4 ULiège)