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.; Chenet, M.; Agosta, Cécile; Fettweis, Xavier; Ho Tong Minh, D.; Ose, K.

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

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

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

Poster (2017, December 15)

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

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

Conference (2017, December 10)

Full Text
See detailAntarctica-Regional Climate and Surface Mass Budget
Favier, Vincent; Krinner, Gerhard; Amory, Charles; Gallée, Hubert; Beaumet, Hulien; Agosta, Cécile

in Current Climate Change Reports (2017)

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

in Journal of Climate (2017), in press

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

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

in Journal of Climate (2017)

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

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

in Cryosphere (2017), 11

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

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

Speech/Talk (2016)

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

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

Poster (2016, April 20)

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

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

in Nature Climate Change (2016)

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

Full Text
See detailCentury-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate
Cornford, S.L.; Martin, D.F.; Payne, A.J.; Ng, E.G.; Le Brocq, A.M.; Gladstone, R.M.; Edwards, T.L.; Shannon, S.R.; Agosta, Cécile; van den Broeke, M.R.; Hellmer, H.; Krinner, G.; Ligtenberg, S.R.M.; Timmermann, R.; Vaughan, D.G.

in Cryosphere (2015), 9

We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rate anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions and ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise.

Full Text
See detailComparison between observed and simulated aeolian snow mass fluxes in Adélie Land, East Antarctica
Amory, Charles; Trouvillez, A.; Gallée, H.; Favier, V.; Naaim-Bouvet, F.; Genthon, C.; Agosta, Cécile; Piard, L.; Bellot, H.

in Cryosphere (2015), 9

The regional climate model MAR including a coupled snow pack/aeolian snow transport parameterisation is compared with aeolian snow mass fluxes at a fine spatial resolution (5 km horizontally and 2 m vertically) and at a fine temporal resolution (30 min) over 1 month in Antarctica. Numerous feedbacks are taken into account in the MAR including the drag partitioning caused by the roughness elements. Wind speed is correctly simulated with a positive value of the Nash test (0.60 and 0.37) but the wind speeds above 10 m s−1 are underestimated. The aeolian snow transport events are correctly reproduced with a good temporal resolution except for the aeolian snow transport events with a particles' maximum height below 1 m. The simulated threshold friction velocity, calculated without snowfall, is overestimated. The simulated aeolian snow mass fluxes between 0 to 2 m have the same variations but are underestimated compared to the second-generation FlowCapt values and so is the simulated relative humidity at 2 m. This underestimation is not entirely due to the underestimation of the simulated wind speed. The MAR underestimates the aeolian snow quantity that pass through the first two meters by a factor ten compared to the second-generation FlowCapt value (13 990 kg m−1 and 151 509 kg m−1 respectively). It will conduct the MAR, with this parametrisation, to underestimate the effect of the aeolian snow transport on the Antarctic surface mass balance.

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

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

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

Conference (2015, June 25)

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

in Cryosphere (2015), 9

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

Full Text
See detailA novel experimental study of aeolian snow transport in Adelie Land (Antarctica)
Trouvilliez, Alexandre; Naaim-Bouvet, Florence; Genthon, Christophe; Piard, Luc; Favier, Vincent; Bellot, Hervé; Agosta, Cécile; Palerme, Cyril; Amory, Charles; Gallée, Hubert

in Cold Regions Science and Technology (2014), 108

None of the previous aeolian snow transport campaigns in Antarctica meet the requirements in terms of tempo- ral resolution, long-term series and qualified instruments for evaluations of meteorological and climate models including parameterization for aeolian snow transport. Consequently, determining the quantity of snow transported remains a challenge. A field campaign was therefore launched in January 2009, in Adélie Land, Antarctica, to acquire new model-evaluation-oriented observations within the European ICE2SEA project, with the logistical support of the French polar Institute (IPEV). The available aeolian snow transport sensors are reviewed and the sensor that best suited our specific needs was chosen: FlowCaptTM acoustic sensors. Three au- tomatic weather stations were deployed with FlowCaptsTM close to the coast. The stations' locations are distinct, ranging from 1 to 100 km inland, one of them with a 7-m mast with six levels of anemometers and thermohygrometers. The fluid and impact threshold friction velocities recorded were 0.48 ± 0.09 m s− 1 and 0.4 ± 0.09 m s−1, respectively, with a high standard deviation of 0.12 ± 0.03 m s−1 and 0.13 ± 0.03 m s−1, respectively. The aeolian snow transport frequency in Adélie Land was very high with seasonal variation of trans- port occurring with minima during the austral summer. Seven percent of the aeolian snow transport events were drifting snow (maximum particle's height, b1 m above the surface). The snow quantity transported was above 1 kiloton per year in the first meter above the surface.

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

Conference (2014, August 27)

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

Conference (2014, August 26)

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

Conference (2014, June)

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

Conference (2014, May 26)

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

Full Text
See detailOceanic forcing of Antarctic climate change: A study using a stretched-grid atmospheric general circulation model
Krinner, Gerhard; Largeron, Chloé; Ménégoz, Martin; Agosta, Cécile; Brutel-Vuilmet, Claire

in Journal of Climate (2014), 27

A variable-resolution atmospheric general circulation model (AGCM) is used for climate change projections over the Antarctic. The present-day simulation uses prescribed observed sea-surface conditions, while a set of five simulations for the end of the 21st century (2070-2099) under the SRES-A1B scenario uses sea- surface condition anomalies from selected CMIP3 coupled ocean-atmosphere climate models. Analysis of the results shows that the prescribed sea-surface condition anomalies have a very strong influence on the simulated climate change on the Antarctic continent, largely dominating the direct effect of the prescribed greenhouse gas concentration changes in the AGCM simulations. Complementary simulations with idealized forcings confirm these results. An analysis of circulation changes using self-organizing maps shows that the simulated climate change on regional scales is not principally caused by shifts of the frequencies of the dominant circulation patterns, except for precipitation changes in some coastal regions. The study illustrates that in some respects the use of bias-corrected sea- surface boundary conditions in climate projections with a variable-resolution atmospheric general circulation model has some distinct advantages over the use of limited-area atmospheric circulation models directly forced by generally biased coupled climate model output.

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

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

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

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

Scientific conference (2013, October 15)

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

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

Poster (2013, April)

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

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

Poster (2013, April)

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

Full Text
See detailImpact of uncertainty in climate forcing on projections of the West Antarctic ice sheet over the 21st and 22nd centuries
Payne, A.J.; Cornford, S.L.; Martin, D.F.; Agosta, Cécile; van den Broeke, M.R.; Edwards, T.L.; Gladstone, R.M.; Hellmer, H.H.; Le Brocq, A.M.; Ligtenberg, S.M.; Lipscomb, W.H.; Ng, E.G.; Shannon, S.R.; Timmerman, R.; Vaughan, D.G.

E-print/Working paper (2013)

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

Conference (2013, January)

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

Full Text
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; Genthon, Christophe; Favier, Vincent; Naaim-Bouvet, Florence

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 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.

Full Text
See detailAn updated and quality controlled surface mass balance dataset for Antarctica
Favier, Vincent; Agosta, Cécile; Parouty, Soazig; Durand, Gaël; Delaygue, Gilles; Gallée, Hubert; Drouet, Anne-Sophie; Trouvillez, Alexandre; Krinner, Gerhard

in Cryosphere (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 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.

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

Scientific conference (2012, December 04)

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

Full Text
See detailTransport de la neige par le vent en Terre Adélie (Antarctique) : observation et modélisation avec le Modèle Atmosphérique Régional (MAR).
Gallée, H.; Trouvilliez, A.; Agosta, Cécile; Genthon, C.; Favier; Naaim-Bouvet, F.

in Actes du 25e colloque de l’Association Internationale de Climatologie (2012, September 05)

Full Text
See detailEvolution of the Antarctic surface mass balance by high-resolution downscaling and impact on sea-level change for the next centuries
Agosta, Cécile; Favier, Vincent; Krinner, Gerhard; Gallée, Hubert; Genthon, Christophe

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 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.

Full Text
See detailA 40-year accumulation dataset for Adelie Land, Antarctica and its application for model validation
Agosta, Cécile; Favier, Vincent; Genthon, Christophe; Gallée, Hubert; Krinner, Gerhard

Poster (2012, July)

Full Text
See detailÉvolution du bilan de masse de surface Antarctique par régionalisation physique et conséquences sur les variations du niveau des mers
Agosta, Cécile

Doctoral thesis (2012)

The Antarctic surface mass balance (SMB, i.e. the snow accumulation from which we sub- tract ablation by sublimation, run-off or erosion) is a major yet poorly known contribution to changes in the present-day sea level. Water storage by snow accumulation at the top of the ice- sheet is expected to increase during the 21st century, which would moderate the rise in sea level. Three-quarters of the Antarctic SMB are concentrated below 2000 m above sea level whereas this area represents only 40% of the grounded ice sheet area. Orographic precipitation is a major contributor to snow accumulation in this region, which is why a better estimation of this term is important. The representation of this process by models depends to a great extent on the resolu- tion of the model, since precipitation amounts depend on the ice sheet slopes. Sublimation and snowmelt also depend on elevation. Global and regional atmospheric climate models are unable to achieve a 40-km resolution over Antarctica at a century time scale, due to their computing cost. At this resolution, ice-sheet margins are still badly resolved. <br />That is why we developed the downscaling model SMHiL (surface mass balance high-resolution downscaling), which estimates the Antarctic SMB components at a high resolution (∼15 km) from large-scale atmospheric forcings. We compute the impact of the high-resolution topography on orographic precipitation amounts and on the boundary-layer processes that lead to sublima- tion, melting and refreezing. To validate SMHiL, we compare our results with more than 2700 field data recently updated and quality-controlled. However, we exhibit that field data below 2000 m above sea level are too scarce to settle SMHiL efficiency. In light of this, we show that the GLACIOCLIM-SAMBA stake lines located on the ice sheet coast-to-plateau area is an ap- propriate reference to evaluate model performance. Finally, we downscale the atmospheric global climate model LMDZ4 to estimate the SMB changes during the 21st and 22nd centuries. The high-resolution SMB is significantly different from the SMB given by LMDZ4. Our results sug- gest that running LMDZ4 at a finer resolution may give a future increase in SMB in Antarctica between 15% to 30% higher than at its standard resolution. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and ru- noff. The downscaling model is a powerful tool that can be applied to climate models for a better assessment of a future rise in sea level.

Full Text
See detailImpact of model resolution on simulated wind, drifting snow and surface mass balance in Terre Adelie, East Antarctica
Lenaerts, J. T. M.; van den Broeke, Michiel R.; Scarchilli, Claudio; Agosta, Cécile

in Journal of Glaciology (2012), 58(211), 821

This paper presents the impact of model resolution on the simulated wind speed, drifting snow climate and surface mass balance (SMB) of Terre Ade ́lie and its surroundings, East Antarctica. We compare regional climate model simulations at 27 and 5.5 km resolution for the year 2009. The wind speed maxima in Terre Ade ́lie and the narrow glacial valleys of Victoria Land are better represented at 5.5 km resolution, because the topography is better resolved. Drifting snow sublimation is >100 mm a−1 in regions with high wind speeds. Our results indicate a strong feedback between topography, wind gradients and drifting snow erosion. As a result, SMB shows much more local spatial variability at 5.5 km resolution that is controlled by drifting snow erosion, whereas the large-scale SMB gradient is largely determined by precipitation. Drifting snow processes lead to ablation in the narrow glacial valleys of Victoria Land. The integrated SMB equals 86 Gt. Although wind climate, drifting snow processes and SMB variability are better represented at 5.5 km, the area-integrated SMB is not significantly different between the simulations at 27 and 5.5 km. A horizontal resolution of 27 km is sufficient to realistically simulate ice-sheet wide SMB.

Full Text
See detailA 40-year accumulation dataset for Adelie Land, Antarctica and its application for model validation
Agosta, Cécile; Favier, Vincent; Genthon, Christophe; Gallée, Hubert; Krinner, Gerhard; Lenaerts, Jan T. M.; van den Broeke, Michiel R.

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-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.

Full Text
See detail21st century Antarctic surface mass balance downscaling from global circulation models
Agosta, Cécile; Favier, Vincent; Genthon, Christophe; Gallée, Hubert; Krinner, Gerhard; Lenaerts, Jan T. M.; van den Broeke, Michiel R.

Poster (2011, October)

Full Text
See detail21st century high-resolution downscaling of the Antarctic surface mass balance from global circulation models
Agosta, Cécile; Favier, Vincent; Genthon, Christophe; Gallée, Hubert; Krinner, Gerhard

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 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.

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

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 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.

Full Text
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; Favier, Vincent; Krinner, Gerhard; Gallée, Hubert; Genthon, Christophe

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 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.

Full Text
See detailA Downscaling Approach Toward High-resolution Surface Mass Balance Over Antarctica
Gallée, Hubert; Agosta, Cécile; Gential, Luc; Favier, Vincent; Krinner, Gerhard

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 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.

Full Text
See detailModeling the mass and surface heat budgets in a coastal blue ice area of Adelie Land, Antarctica
Favier, Vincent; Agosta, Cécile; Genthon, Christophe; Arnaud, Laurent; Trouvillez, Alexandre; Gallée, Hubert

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 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.

Full Text
See detailDésagrégation du bilan de masse en surface de la calotte polaire Antarctique
Agosta, Cécile

Scientific conference (2009, November)

Le bilan de masse de surface (BMS) de la calotte polaire Antarctique est une composante majeure de l’évolution du niveau des mers. Le modèle de circulation général LMDZ4 et la grande majorité des modèles atmosphériques utilisés dans l’IPCC-AR4 prévoient une augmentation du BMS en Antarctique au cours du siècle prochain, menée par une augmentation des précipitations neigeuses, qui contribuerait à une modération de l’élévation du niveau des mers. Les changements prévus sont particulièrement importants sur la zone côtière Antarctique, qui concentre la majorité des précipitations du continent. Il est donc crucial d’avoir une connaissance plus fine de l’évolution du BMS Antarctique, particulièrement en zone côtière. Les modèles actuels atteignent une résolution de 60 km à l’échelle de l’Antarctique pour des simulations longues (> 20 ans). L’objectif de ma thèse est d’estimer le BMS à une résolution inférieure à 15 km sur l’ensemble du continent, en désagrégeant les sorties des modèles de climat avec un modèle à temps de calcul réduit. La validation des modèles désagrégés est un point clé pour estimer la qualité des simulations futures. Or cette validation n’est pas aisée, du fait de l’impossibilité de mesurer directement les précipitations neigeuses et de la faible quantité de données d’accumulation à l’échelle du continent. Nous commencerons donc par une évaluation de la base de données Glacioclim-SAMBA et son utilisation pour valider différents modèles de climat en région côtière. Nous nous concentrerons ensuite sur la validation des modèles à l’échelle de l’Antarctique, en proposant une modification de la méthode utilisée par Arthern et al. (2006) pour fournir une climatologie du BMS à partir de données de terrain et de données satellite. Enfin, nous verrons le fonctionnement des modèles de désagrégation des précipitations et du bilan d’énergie en surface de la calotte polaire Antarctique. Je préciserai ma contribution au développement des modèles et présenterai des résultats préliminaires de désagrégation du BMS ainsi que les modifications que je prévoie d’apporter aux modèles.

Full Text
See detailSpatial distribution of accumulation in the Adélie Land - Comparison of the Antarctic GLACIOCLIM-SAMBA observation data with remote sensing techniques and high-resolution climate models
Agosta, Cécile; Genthon, Christophe; Favier, Vincent; Gallée, Hubert; Krinner, Gerhard; Picard, Ghislain; Six, Delphine

Conference (2009, July)

The Surface Mass Balance (SMB) of Antarctica is typically much larger within the first 200 km from the coast than further inland. This is also likely where much of the SMB change is expected due to climate warming. That is why since 2004 the GLACIOCLIM- SAMBA French observatory monitors the SMB in this region with spatial scales resolving those of climate models. A stakes line made of 91 stakes was deployed along a 150 km transect from the coast of Adélie Land towards Dome C on the plateau. This transect is surveyed each year and 3 complete records are now available. We first compare recent SMB measurements with older reports from the 1972-1992 period. We show that a significant fraction of the observed kilometer-scale spatial variability is stationary with time. Moreover the large spatial and inter-annual variabilities of recent measurements are consistent with the historical ones. The lateral spatial significance of the transect is then evaluated using the background remote-sensing- based model of Arthern et al. (2006). Finally, results from 3 high-resolution climate models are presented and compared with the field reports: the global atmosphere-surface climate model LMDZ4 zoomed to reach a 35 km resolution over Antarctica, the regional atmospheric climate model MAR developed for polar regions (20 km resolution) and the ECMWF analysis (50 km resolution). We show large differences between the 3 models, which don’t have the same ability to reproduce the meso-scale characteristics of the distribution of the observed SMB in the critical coast-to-plateau area.

Full Text
See detailTransect GLACIOCLIM-SAMBA : observation et modélisation du Bilan de Masse de Surface sur la zone de transition côte-plateau Antarctique
Agosta, Cécile; Genthon, Christophe; Favier, Vincent; Six, Delphine

Conference (2008, October)

Full Text
See detailSensibilité aux variations climatiques d'un bassin à forts enjeux - Le cas de la Garonne
Dupeyrat, Anne; Agosta, Cécile; Sauquet, Eric; Hendrickx, Frederic

in IWRA 13th World Water Congress (2008, September)