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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 ULg; Chevrier, R. et al

in Journal of Climate (2016), 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 ... [more ▼]

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. [less ▲]

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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. et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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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. et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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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 ULg; Fettweis, Xavier ULg; Scholzen, Chloé ULg

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 ... [more ▼]

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. [less ▲]

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See detailAnnual Greenland accumulation rates (2009–2012) from airborne snow radar
Koenig, L.; Ivanoff, A.; Alexander, P. et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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See detailGreenland Ice sheet [in "State of the Climate in 2015"]
Tedesco, M.; Box, J.; Cappelen, J. et al

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 et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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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 ULg et al

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 ... [more ▼]

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. [less ▲]

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

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 ... [more ▼]

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). [less ▲]

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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 ULg; Agosta, Cécile ULg; 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 ... [more ▼]

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. [less ▲]

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

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 ... [more ▼]

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. [less ▲]

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See detailAnthropogenic forcing dominates global mean sea-level rise since 1970
Slagen, A.; Church, J.; Agosta, Cécile ULg et al

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 ... [more ▼]

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). [less ▲]

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

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” ... [more ▼]

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. [less ▲]

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See detailThe darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)
Tedesco, M.; Doherty, S.; Fettweis, Xavier ULg et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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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. et al

in Cryosphere (The) (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 ... [more ▼]

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. [less ▲]

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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 ULg; Scholzen, Chloé ULg; Fettweis, Xavier ULg et al

in International Journal of Climatology (2016)

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 ... [more ▼]

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. [less ▲]

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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 ULg; Knudsen, E. et al

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 ... [more ▼]

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. [less ▲]

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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 ULg; Fettweis, Xavier ULg; Erpicum, Michel ULg

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 ... [more ▼]

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. [less ▲]

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See detailWhat Darkens the Greenland Ice Sheet?
Tedesco, M; Doherty, S.; Warren, S. et al

in EOS : Transactions, American Geophysical Union (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 ... [more ▼]

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). [less ▲]

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See detailGreenland ice sheet [in "State of the Climate in 2014"]
Tedesco, M.; Box, J.; Cappelen, J. et al

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

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