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

Conference (2014, May 26)

The Antarctic ice-sheet surface mass balance (SMB) is a significant contribution to sea level changes which may mitigate the rise in sea level in a warmer climate, but this term is still poorly known. The ... [more ▼]

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

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See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

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

About 75% of the Antarctic surface mass gain occurs over areas below 2000 m asl, which cover 40% of the grounded ice-sheet. As the topography is complex in many of these regions, SMB modelling is highly ... [more ▼]

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

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

Scientific conference (2013, October 15)

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu’on sache qu’il contribue de façon significative à l’évolution actuelle du niveau des mers et que sa contribution soit ... [more ▼]

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

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

Poster (2013, April)

We report future projections of Surface Mass Balance (SMB) over the Antarctic ice sheet obtained with the regional climate model MAR, for different warming scenarios. MAR forcing is carefully selected ... [more ▼]

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

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See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the 20th, 21st and 22nd centuries
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

Poster (2013, April)

Although areas below 2000 m above sea level (a.s.l.) cover 40% of the Antarctic grounded ice-sheet, they represent about 75% of the surface mass balance (SMB) of the continent. Because the topography is ... [more ▼]

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

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See detailImportant role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet
Fettweis, Xavier ULg; Hanna, Edward; Lang, Charlotte ULg et al

in Cryosphere (The) (2013), 7

Since 2007, there has been a series of surface melt records over the Greenland ice sheet (GrIS), continuing the trend towards increased melt observed since the end of the 1990's. The last two decades are ... [more ▼]

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

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See detailModélisation du bilan de masse de surface Antarctique : quelle stratégie et quelle validation ?
Agosta, Cécile ULg; Favier, Vincent; Fettweis, Xavier ULg et al

Conference (2013, January)

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu'on sache qu'il contribue de façon significative à l'évolution actuelle du niveau des mers et que sa contribution soit supposée ... [more ▼]

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

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See detailAn updated and quality controlled surface mass balance dataset for Antarctica
Favier, Vincent; Agosta, Cécile ULg; Parouty, Soazig et al

in Cryosphere (The) (2013), 7

We present an updated and quality controlled surface mass balance (SMB) database for the Antarctic ice sheet. We retrieved a total of 5284 SMB data documented with important meta-data, to which a filter ... [more ▼]

We present an updated and quality controlled surface mass balance (SMB) database for the Antarctic ice sheet. We retrieved a total of 5284 SMB data documented with important meta-data, to which a filter was applied to discard data with limited spatial and temporal representativeness, too small measurement accuracy, or lack of quality control. A total of 3438 reliable data was obtained, which is about four times more than by applying the same data filtering process to previously available databases. New important data with high spatial resolution are now available over long traverses, and at low elevation in some areas. However, the quality control led to a considerable reduction in the spatial density of data in several regions, particularly over West Antarctica. Over interior plateaus, where the SMB is low, the spatial density of mea- surements remained high. This quality controlled dataset was compared to results from ERA-Interim reanalysis to assess model representativeness over Antarctica, and also to identify large areas where data gaps impede model validation. Except for very few areas (e.g. Adelie Land), the elevation range between 200 m and 1000 m a.s.l. is not correctly sampled in the field, and measurements do not allow a thorough validation of models in regions with complex topography, where the highest scattering of SMB values is reported. Clearly, increasing the spatial density of field measurements at low elevations, in the Antarctic Peninsula and in West Antarctica remains a scientific priority. [less ▲]

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See detailTransport of Snow by the Wind: A Comparison Between Observations in Adélie Land, Antarctica, and Simulations Made with the Regional Climate Model MAR
Gallée, Hubert; Trouvillez, Alexandre; Agosta, Cécile ULg et al

in Boundary-Layer Meteorology (2013), 146(1), 133--147

For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Adélie Land, Antarctica. A regional climate model featuring a ... [more ▼]

For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Adélie Land, Antarctica. A regional climate model featuring a coupled atmosphere/blowing snow/snowpack model is forced laterally by meteorological re-analyses. The vertical grid spacing was 2 m from 2 to 20 m above the surface and the horizontal grid spacing was 5 km. The simulation was validated by comparing the occurrence of blowing snow events and other meteorological parameters at two automatic weather stations. The Nash test allowed us to compute effi- ciencies of the simulation. The regional climate model simulated the observed wind speed with a positive efficiency (0.69). Wind speeds higher than 12 m s−1 were underestimated. Positive efficiency of the simulated wind speed was a prerequisite for validating the blowing snow model. Temperatures were simulated with a slightly negative efficiency (−0.16) due to overestimation of the amplitude of the diurnal cycle during one week, probably because the cloud cover was underestimated at that location during the period concerned. Snowfall events were correctly simulated by our model, as confirmed by field reports. Because observations suggested that our instrument (an acoustic sounder) tends to overestimate the blowing snow flux, data were not sufficiently accurate to allow the complete validation of snow drift val- ues. However, the simulation of blowing snow occurrence was in good agreement with the observations made during the first 20 days of January 2010, despite the fact that the blowing snow flux may be underestimated by the regional climate model during pure blowing snow events. We found that blowing snow occurs in Adélie Land only when the 30-min wind speed value at 2 m a.g.l. is >10 m s−1. The validation for the last 10 days of January 2010 was less satisfactory because of complications introduced by surface melting and refreezing. [less ▲]

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See detailFuture surface mass balance contribution of the Antarctic ice-sheet to sea level rise
Agosta, Cécile ULg; Fettweis, Xavier ULg; Krinner, Gerhard et al

Scientific conference (2012, December 04)

Most of the IPCC-AR4 global circulation models predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. High-resolution modeling ... [more ▼]

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

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See detailGreenland climate change: from the past to the future
Masson‐Delmotte, Valérie; Swingedouw, Didier; Landais, Amaëlle et al

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

Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics ... [more ▼]

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

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See detailEvolution of the Antarctic surface mass balance by high-resolution downscaling and impact on sea-level change for the next centuries
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

Conference (2012, July)

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present ... [more ▼]

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present accumulation and predicted change are largest at the ice sheet margins because they are driven by snowfall, which mostly comes from warm, moist air arising over the land slopes. The coastal belt is also where complex processes of sublimation, melt and refreezing occur. Thus, high-resolution modelling is necessary to adequately capture the effects of small-scale variations in topography on the atmospheric variables in this area, but limitations in computing resources prevent such resolution at the scale of Antarctica in full climate models. We present here a downscaling method leading to 15-km SMB resolution for century time-scales over Antarctica. We compute the effect of the fine topography on orographic precipitation and on boundary layer processes that lead to sublimation, melt and refreezing. We first display the SMB downscaled from ERA-Interim and show that the downscaling improves the agreement between modelled and observed SMB for the end of the 20th century. We then present hi-resolution features of the Antarctic SMB evolution during the 21st century downscaled from LMDZ4 for different scenarios. We show that a higher resolution induce at the same time more run-off but a significantly higher mitigation of sea level rise. [less ▲]

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See detailA 40-year accumulation dataset for Adelie Land, Antarctica and its application for model validation
Agosta, Cécile ULg; Favier, Vincent; Genthon, Christophe et al

Poster (2012, July)

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See detailA 40-year accumulation dataset for Adelie Land, Antarctica and its application for model validation
Agosta, Cécile ULg; Favier, Vincent; Genthon, Christophe et al

in Climate Dynamics (2012), 38(1-2), 75-8686

The GLACIOCLIM-SAMBA (GS) Antarctic accumulation monitoring network, which extends from the coast of Adelie Land to the Antarctic plateau, has been surveyed annually since 2004. The network includes a 156 ... [more ▼]

The GLACIOCLIM-SAMBA (GS) Antarctic accumulation monitoring network, which extends from the coast of Adelie Land to the Antarctic plateau, has been surveyed annually since 2004. The network includes a 156-km stake-line from the coast inland, along which accumulation shows high spatial and interannual variability with a mean value of 362 mm water equivalent a -1. In this paper, this accumulation is compared with older accumulation reports from between 1971 and 1991. The mean and annual standard deviation and the km-scale spatial pattern of accumulation were seen to be very similar in the older and more recent data. The data did not reveal any significant accumulation trend over the last 40 years. The ECMWF analysis-based forecasts (ERA-40 and ERA-Interim), a stretched-grid global general circulation model (LMDZ4) and three regional circulation models (PMM5, MAR and RACMO2), all with high resolution over Antarctica (27-125 km), were tested against the GS reports. They qualitatively reproduced the meso-scale spatial pattern of the annual-mean accumulation except MAR. MAR significantly underestimated mean accumulation, while LMDZ4 and RACMO2 overestimated it. ERA-40 and the regional models that use ERA-40 as lateral boundary condition qualitatively reproduced the chronology of interannual variability but underestimated the magnitude of interannual variations. Two widely used climatologies for Antarctic accumulation agreed well with the mean GS data. The model-based climatology was also able to reproduce the observed spatial pattern. These data thus provide new stringent constraints on models and other large-scale evaluations of the Antarctic accumulation. [less ▲]

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See detail21st century Antarctic surface mass balance downscaling from global circulation models
Agosta, Cécile ULg; Favier, Vincent; Genthon, Christophe et al

Poster (2011, October)

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See detail21st century high-resolution downscaling of the Antarctic surface mass balance from global circulation models
Agosta, Cécile ULg; Favier, Vincent; Genthon, Christophe et al

Conference (2011, April)

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present ... [more ▼]

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present accumulation and predicted change are largest at the ice sheet margins because they are driven by snowfall, which mostly comes from warm, moist air arising over the land slopes. The coastal belt is also where complex processes of sublimation, melt and redistribution by the wind occur. Thus, high-resolution modelling (5 to 10 km) is necessary to adequately capture the effects of small-scale variations in topography on the atmospheric variables in this area, but limitations in computing resources prevent such resolution at the scale of Antarctica in full climate models. We present here a downscaling method leading to 10-km SMB resolution for century time-scales over Antarctica. We compute the effect of the fine topography on orographic precipitation and on boundary layer processes that lead to melt and sublimation. We show that the accumulation downscaled from ERA-Interim is in good agreement with field measurements for the last 40 years. We then display the SMB downscaled from LMDZ4 AGCM outputs (~60-km resolution), and show that the downscaling improves the agreement between present modelled and observed SMB. Finally, we present hi-resolution features of the Antarctic SMB evolution during the 21st century downscaled from LMDZ4 and discuss the effect of the resolution on the Antarctic SMB contribution to sea level change. The downscaling model is a powerful tool that will be applied to others climate models for a better assessment of future sea level rise. [less ▲]

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See detailEvolution of Antarctic Surface Mass Balance by high-resolution downscaling of LMDZ4 AGCM and contribution to sea-level change
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

Conference (2011, March)

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present ... [more ▼]

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century that would mitigate global sea level rise. Present accumulation and predicted change are largest at the ice sheet margins because they are driven by snowfall, which mostly comes from warm, moist air arising over the land slopes. The coastal belt is also where complex processes of sublimation, melt and redistribution by the wind occur. Thus, high-resolution modelling (5 to 15 km) is necessary to adequately capture the effects of small-scale variations in topography on the atmospheric variables in this area, but limitations in computing resources prevent such resolution at the scale of Antarctica in full climate models. We present here a downscaling method leading to 15-km SMB resolution for century time-scales over Antarctica. We compute precipitation fields by considering orographic processes induced by the broad-scale and the fine-scale topography, and we estimate sublimation, melting and refreezing with a surface scheme validated for snow and ice-covered land surface. We display the SMB downscaled from LMDZ4 AGCM outputs (~60-km resolution), and compare the agreement of the broad-scale SMB and the downscaled SMB with 20th century observations. Then, we present hi-resolution features of the Antarctic SMB evolution during the 21st century downscaled from LMDZ4 and discuss the effect of the resolution on the Antarctic SMB contribution to sea level change. The downscaling model is a powerful tool that will be applied to others climate models for a better assessment of future sea level rise. [less ▲]

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See detailDésagrégation du Bilan de Masse et du Bilan d’Energie en Surface de la calotte polaire Antarctique, application pour le 21ème siècle
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

Conference (2011, February)

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century, driven by an increase of snow falls, which would ... [more ▼]

Most of the IPCC-AR4 Atmospheric Global Circulation Models (AGCM) predict an increase of the Antarctic Surface Mass Balance (SMB) during the 21st century, driven by an increase of snow falls, which would mitigate the sea level rise. Much of the SMB change is expected to happen in the Antarctic coastal area, which concentrates the major part of the snow falls. This area is also were we find complex processes of precipitation, sublimation, melt and redistribution by the wind. High-resolution modeling (5 to 10 km) is necessary to accurately capture the effects of the fine topography on the atmospheric variables but limitations in computing resources prevent such resolution at the scale of Antarctica in full climate models. We present here a downscaling method yielding to a 10-km resolution of the SMB for the 21st century, from ~60-km resolution LMDZ4 AGCM outputs. We compute orographic precipitation induced by the finer topography, as well as the boundary layer processes leading to melt and sublimation. It shows a clear improvement of the SMB distribution in coastal regions with consequences on the grounded ice sheet SMB estimation. [less ▲]

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See detailModeling the mass and surface heat budgets in a coastal blue ice area of Adelie Land, Antarctica
Favier, Vincent; Agosta, Cécile ULg; Genthon, Christophe et al

in Journal of Geophysical Research. Earth Surface (2011), 116

Meteorological data recorded from 12 December 2008 to 30 June 2010 were analyzed to assess the surface energy balance (SEB) in a blue ice area of Cap Prudhomme, Adelie Land (66 degrees 41'S, 139 degrees ... [more ▼]

Meteorological data recorded from 12 December 2008 to 30 June 2010 were analyzed to assess the surface energy balance (SEB) in a blue ice area of Cap Prudhomme, Adelie Land (66 degrees 41'S, 139 degrees 55'E). The SEB was computed with a newly developed model forced by direct measurements and with a voluntarily limited number of parameters to better assess model sensitivity. Incoming short-wave radiation was corrected for the slope and orientation of the local terrain assuming direct and diffuse radiation components. Turbulent heat fluxes were assessed using the bulk aerodynamic approach. Heat conduction in the ice was computed by solving the thermal diffusion equation. Snow accumulation was modeled using ERA interim total precipitation and a one-dimensional erosion model. The surface heat budget and accumulation/erosion model accurately reproduced field observations. The occurrence of blue ice is linked with higher rates of erosion than in the surrounding snow covered areas, which may be caused by local flow divergence or snow not being redistributed from higher elevations. Melting occurs between December and February when incoming short-wave radiation is high. However, the SEB was closely linked to air temperature through the incoming long-wave radiation and the turbulent sensible heat flux. Several warm events caused by cyclones intruding into the continent led to significant warming of the ice and high melting rates. Intruding cyclones were also associated with high precipitation that led to significant accumulation. Except in blue ice areas, modeling suggests that expected higher precipitation in a warmer climate will result in more accumulation. [less ▲]

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See detailA Downscaling Approach Toward High-resolution Surface Mass Balance Over Antarctica
Gallée, Hubert; Agosta, Cécile ULg; Gential, Luc et al

in Surveys in Geophysics (2011), 32(4-5), 507-518518

The Antarctic ice sheet surface mass balance shows high spatial variability over the coastal area. As state-of-the-art climate models usually require coarse resolutions to keep computational costs to a ... [more ▼]

The Antarctic ice sheet surface mass balance shows high spatial variability over the coastal area. As state-of-the-art climate models usually require coarse resolutions to keep computational costs to a moderate level, they miss some local features that can be captured by field measurements. The downscaling approach adopted here consists of using a cascade of atmospheric models from large scale to meso-gamma scale. A regional climate model (Modegravele Atmospheacuterique Reacutegional) forced by meteorological reanalyses provides a diagnostic physically-based rain- and snowfall downscaling model with meteorological fields at the regional scale. Although the parameterizations invoked by the downscaling model are fairly simple, the knowledge of small-scale topography significantly improves the representation of spatial variability of precipitation and therefore that of the surface mass balance. Model evaluation is carried out with the help of shallow firn cores and snow height measurements provided by automatic weather stations. Although downscaling of blowing snow still needs to be implemented in the model, the net accumulation gradient across Law Dome summit is shown to be induced mostly by orographic effects on precipitation. [less ▲]

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