2024 |
Amory, C., Buizert, C., Buzzard, S., Case, E., Clerx, N., Culberg, R., et al. (2024). Firn On Ice Sheets. Nature Reviews Earth & Environment, .
Abstract: Most Of The Greenland And Antarctic Ice Sheets Are Covered With Firn – The Transitional Material Between Snow And Glacial Ice. Firn Is Vital For Understanding Ice-Sheet Mass Balance And Hydrology, And Palaeoclimate. In This Review, We Synthesize Knowledge Of Firn, Including Its Formation, Observation, Modelling And Relevance To Ice Sheets. The Refreezing Of Meltwater In The Pore Space Of Firn Currently Prevents 50% Of Meltwater In Greenland From Running Off Into The Ocean And Protects Antarctic Ice Shelves From Catastrophic Collapse. Continued Atmospheric Warming Could Inhibit Future Protection Against Mass Loss. For Example, Warming In Greenland Has Already Contributed To A 5% Reduction In Firn Pore Space Since 1980. All Projections Of Future Firn Change Suggest That Surface Meltwater Will Have An Increasing Impact On Firn, With Melt Occurring Tens To Hundreds Of Kilometres Further Inland In Greenland, And More Extensively On Antarctic Ice Shelves. Although Progress In Observation And Modelling Techniques Has Led To A Well-Established Understanding Of Firn, The Large Uncertainties Associated With Meltwater Percolation Processes (Refreezing, Ice-Layer Formation And Storage) Must Be Reduced Further. A Tighter Integration Of Modelling Components (Firn, Atmosphere And Ice-Sheet Models) Will Also Be Needed To Better Simulate Ice-Sheet Responses To Anthropogenic Warming And To Quantify Future Sea-Level Rise. A Firn Layer Covers The Earth'S Ice Sheets. This Review Outlines Techniques To Observe And Model Changes In Firn Properties And Meltwater Retention To Understand How This Firn Layer Will Respond To Climate Change.
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2023 |
Brun, F., King, O., Reveillet, M., Amory, C., Planchot, A., Berthier, E., et al. (2023). Everest South Col Glacier Did Not Thin During The Period 1984-2017. Cryosphere, 171(8), 3251–3268.
Abstract: The South Col Glacier Is A Small Body Of Ice And Snow (Approx. 0.2 Km(2)) Located At The Very High Elevation Of 8000Ma.S.L. (Above Sea Level) On The Southern Ridge Of Mt. Everest. A Recent Study By Potocki Et Al. (2022) Proposed That South Col Glacier Is Rapidly Losing Mass. This Is In Contradiction To Our Comparison Of Two Digital Elevation Models Derived From Aerial Photographs Taken In December 1984 And A Stereo Pleiades Satellite Acquisition From March 2017, From Which We Estimate A Mean Elevation Change Of 0.01 +/- 0.05M A(-1). To Reconcile These Results, We Investigate Some Aspects Of The Surface Energy And Mass Balance Of South Col Glacier. From Satellite Images And A Simple Model Of Snow Compaction And Erosion, We Show That Wind Erosion Has A Major Impact On The Surface Mass Balance Due To The Strong Seasonality In Precipitation And Wind And That It Cannot Be Neglected. Additionally, We Show That The Melt Amount Predicted By A Surface Energy And Mass Balance Model Is Very Sensitive To The Model Structure And Implementation. Contrary To Previous Findings, Melt Is Likely Not A Dominant Ablation Process On This Glacier, Which Remains Mostly Snow-Covered During The Monsoon.
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Otosaka, I., Shepherd, A., Ivins, E., Schlegel, N., Amory, C., Van Den Broeke, M., et al. (2023). Mass Balance Of The Greenland And Antarctic Ice Sheets From 1992 To 2020. Earth System Science Data, , 159711–161611.
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2022 |
Fang, G., Li, Z., Yang, J., Chen, Y., Duan, W., Amory, C., et al. (2022). Changes In Flooding In The Alpine Catchments Of The Tarim River Basin, Central Asia. Journal Of Flood Risk Management, .
Abstract: Floods Are One Of The Most Affective Climate-Related Disasters, And Climate Change Has Altered Their Intensity And Frequency Worldwide. This Study Examined Long-Term Changes In Flood Characteristics (Including Magnitude, Frequency, And Timing) In 30 Alpine Headwaters Of The Large Endorheic Tarim River Basin, Central Asia. The Contributions Of Climatic Factors To Flood (Magnitude And Timing) Changes Were Investigated Using Numerical Experiments In Combination With The Random Forest Approach. The Following Results Were Obtained: (1) Annual Maximum Flood Peaks Increased At Most Stations (89% Stations) During 1961-2015 With Increased Flood Frequency. Earlier Flood Peaks Were Observed In Spring With A Rate Of 1.38 Day Per Decade; For Other Seasons, Changes In The Occurrence Time Of Flood Peaks Showed Strong Spatial Variability. (2) Precipitation Was The Dominant Factor For The Increased Flood Magnitude In Most Catchments Of The Southern Slope Of The Tianshan Mountains, And Temperature Played A Greater Role In The Northern Kunlun Mountains. (3) For Flood Timing Changes, Melt Level Height And Precipitation Were The Most Influential Factors In The Alpine Catchments In The Tarim River Basin. The Results Provide Information On The Spatiotemporal Variations Of Floods And Their Driving Factors In This Alpine Basin Under Climate Change.
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Fang, G., Yang, J., Li, Z., Chen, Y., Duan, W., Amory, C., et al. (2022). Shifting In The Global Flood Timing. Scientific Reports, 121(1).
Abstract: Climate Change Will Have An Impact On Not Only Flood Magnitude But Also On Flood Timing. This Paper Studies The Shifting In Flood Timing At 6167 Gauging Stations From 1970 To 2010, Globally. The Shift In Flood Timing And Its Relationship With Three Influential Factors (Maximum 7-Day Precipitation, Soil Moisture Excess, And Snowmelt) Are Investigated. There Is A Clear Global Pattern In The Mean Flooding Date: Winter (Dec-Feb) Across The Western Coastal America, Western Europe And The Mediterranean Region, Summer (Jun-Aug) In The North America, The Alps, Indian Peninsula, Central Asia, Japan, And Austral Summer (Dec-Feb) In South Africa And North Australia Area. The Shift In Flood Timing Has A Trend From – 22 Days Per Decade (Earlier) To 28 Days Per Decade (Delayed). Earlier Floods Were Found Extensively In The North America, Europe And Northeast Australia While Delayed Floods Were Prevailing In The Amazon, Cerrado, South Africa, India And Japan. Earlier Flood Timing In The North America And Europe Was Caused By Earlier Snowmelt While Delayed Extreme Soil Moisture Excess And Precipitation Have Jointly Led To Delayed Floods Around The Monsoon Zone, Including South Africa, India And Japan. This Study Provides An Insight On The Shifting Mechanism Of Flood Timing, And Supports Decisions On The Global Flood Mitigation And The Impact From Future Climate Change.
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Kittel, C., Amory, C., Hofer, S., Agosta, C., Jourdain, N. C., Gilbert, E., et al. (2022). Clouds Drive Differences In Future Surface Melt Over The Antarctic Ice shelves. Cryosphere, 161(7), 2655–2669.
Abstract: Recent warm atmospheric conditions have damaged the ice shelves of the Antarctic Peninsula through surface melt and hydrofracturing and could potentially initiate future collapse of other Antarctic ice shelves. However, model projections with similar greenhouse gas scenarios suggest large differences in cumulative 21st-century surface melting. So far it remains unclear whether these differences are due to variations in warming rates in individual models or whether local feedback mechanisms of the surface energy budget could also play a notable role. Here we use the polar-oriented regional climate model MAR (Modele Atmospherique Regional) to study the physical mechanisms that would control future surface melt over the Antarctic ice shelves in high-emission scenarios RCP8.5 and SSP5-8.5. We show that clouds enhance future surface melt by increasing the atmospheric emissivity and longwave radiation towards the surface. Furthermore, we highlight that differences in meltwater production for the same climate warming rate depend on cloud properties and particularly cloud phase. Clouds containing a larger amount of supercooled liquid water lead to stronger melt, subsequently favouring the absorption of solar radiation due to the snowmelt-albedo feedback. As liquid-containing clouds are projected to increase the melt spread associated with a given warming rate, they could bea major source of uncertainties in projections of the future Antarctic contribution to sea level rise.
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Wever, N., Keenan, E., Amory, C., Lehning, M., Sigmund, A., Huwald, H., et al. (2022). Observations And Simulations Of New Snow Density In The Drifting Snow-Dominated Environment Of Antarctica. Journal Of Glaciology, .
Abstract: Owing To Drifting Snow Processes, Snow Accumulation And Surface Density In Polar Environments Are Variable In Space And Time. We Present New Field Data Of Manual Measurements, Repeat Terrestrial Laser Scanning And Snow Micro-Penetrometry From Dronning Maud Land, Antarctica, Showing The Density Of New Snow Accumulations. We Combine These Data With Published Drifting Snow Mass Flux Observations, To Evaluate The Performance Of The 1-D, Detailed, Physics-Based Snow Cover Model Snowpack In Representing Drifting Snow And Surface Density. For Two Sites In East Antarctica With Multiple Years Of Data, We Found A Coefficient Of Determination For The Simulated Drifting Snow Of R(2) = 0.42 And R(2) = 0.50, Respectively. The Field Observations Show The Existence Of Low-Density Snow Accumulations During Low Wind Conditions. Successive High Wind Speed Events Generally Erode These Low-Density Layers While Producing Spatially Variable Erosion/Deposition Patterns With Typical Length Scales Of A Few Metres. We Found That A Model Setup That Is Able To Represent Low-Density Snow Accumulating During Low Wind Speed Conditions, As Well As Subsequent Snow Erosion And Redeposition At Higher Densities During Drifting Snow Events Was Mostly Able To Describe The Observed Temporal Variability Of Surface Density In The Field.
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2021 |
Amory, C., Kittel, C., Le Toumelin, L., Agosta, C., Delhasse, A., Favier, V., et al. (2021). Performance of MAR (v3.11) in simulating the drifting-snow climate and surface mass balance of Adelie Land, East Antarctica. Geoscientific Model Development, 14(6), 3487–3510.
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Donat-Magnin, M., Jourdain, N., Kittel, C., Agosta, C., Amory, C., Gallee, H., et al. (2021). Future surface mass balance and surface melt in the Amundsen sector of the West Antarctic Ice Sheet. Cryosphere, 15(2), 571–593.
Abstract: We present projections of West Antarctic surface mass balance (SMB) and surface melt to 2080-2100 under the RCP8.5 scenario and based on a regional model at 10 km resolution. Our projections are built by adding a CMIP5 (Coupled Model Intercomparison Project Phase 5) multi-model-mean seasonal climate-change anomaly to the present-day model boundary conditions. Using an anomaly has the advantage to reduce CMIP5 model biases, and a perfect-model test reveals that our approach captures most characteristics of future changes despite a 16 %-17 % underestimation of projected SMB and melt rates. SMB over the grounded ice sheet in the sector between Getz and Abbot increases from 336 Gt yr(-1) in 1989-2009 to 455 Gt yr(-1) in 2080-2100, which would reduce the global sea level changing rate by 0.33 mm yr(-1). Snowfall indeed increases by 7.4 % degrees C-1 to 8.9 % degrees C-1 of near-surface warming due to increasing saturation water vapour pressure in warmer conditions, reduced sea-ice concentrations, and more marine air intrusion. Ice-shelf surface melt rates increase by an order of magnitude in the 21st century mostly due to higher downward radiation from increased humidity and to reduced albedo in the presence of melting. There is a net production of surface liquid water over eastern ice shelves (Abbot, Cosgrove, and Pine Island) but not over western ice shelves (Thwaites, Crosson, Dotson, and Getz). This is explained by the evolution of the melt-to-snowfall ratio: below a threshold of 0.60 to 0.85 in our simulations, firn air is not entirely depleted by melt water, while entire depletion and net production of surface liquid water occur for higher ratios. This suggests that western ice shelves might remain unaffected by hydrofracturing for more than a century under RCP8.5, while eastern ice shelves have a high potential for hydrofracturing before the end of this century.
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Fettweis, X., Hofer, S., Seferian, R., Amory, C., Delhasse, A., Doutreloup, S., et al. (2021). Brief communication: Reduction in the future Greenland ice sheet surface melt with the help of solar geoengineering. Cryosphere, 15(6), 3013–3019.
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Hofer, S., Amory, C., Kittel, C., Carlsen, T., Le Toumelin, L., & Storelvmo, T. (2021). The Contribution of Drifting Snow to Cloud Properties and the Atmospheric Radiative Budget Over Antarctica. Geophysical Research Letters, 48(22).
Abstract: The Antarctic Ice Sheet experiences perpetual katabatic winds, transporting snow, and moisture from the interior towards the periphery. However, the impacts of Antarctic moisture and drifting snow on cloud structure and surface energy fluxes have not been widely investigated. Here, we use a regional climate model with a newly developed drifting snow scheme to show that accounting for drifting snow notably alters the spatial distribution, vertical structure and radiative effect of clouds over Antarctica. Overall, we find that accounting for drifting snow leads to a greater cloud cover providing an increase of +2.74 Wm(-2) in the surface radiative energy budget. Additionally, a comparison with 20 weather stations reveals a 2.17 Wm(-2) improvement in representing the radiative energy fluxes. Our results highlight the need to study the impact of drifting snow processes on the future evolution of clouds, the surface energy budget and the vertical atmospheric structure over Antarctica.
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Kittel, C., Amory, C., Agosta, C., Jourdain, N., Hofer, S., Delhasse, A., et al. (2021). Diverging future surface mass balance between the Antarctic ice shelves and grounded ice sheet. Cryosphere, 15(3), 1215–1236.
Abstract: The future surface mass balance (SMB) will influence the ice dynamics and the contribution of the Antarctic ice sheet (AIS) to the sea level rise. Most of recent Antarctic SMB projections were based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5). However, new CMIP6 results have revealed a C1:3 degrees C higher mean Antarctic near-surface temperature than in CMIP5 at the end of the 21st century, enabling estimations of future SMB in warmer climates. Here, we investigate the AIS sensitivity to different warmings with an ensemble of four simulations performed with the polar regional climate model Modele Atmospherique Regional (MAR) forced by two CMIP5 and two CMIP6 models over 1981-2100. Statistical extrapolation enables us to expand our results to the whole CMIP5 and CMIP6 ensembles. Our results highlight a contrasting effect on the future grounded ice sheet and the ice shelves. The SMB over grounded ice is projected to increase as a response to stronger snowfall, only partly offset by enhanced meltwater run-off. This leads to a cumulated sealevel-rise mitigation (i.e. an increase in surface mass) of the grounded Antarctic surface by 5.1 +/- 1.9 cm sea level equivalent (SLE) in CMIP5-RCP8.5 (Relative Concentration Pathway 8.5) and 6.3 +/- 2.0 cm SLE in CMIP6-ssp585 (Shared Socioeconomic Pathways 585). Additionally, the CMIP6 low-emission ssp126 and intermediate-emission ssp245 scenarios project a stabilized surface mass gain, resulting in a lower mitigation to sea level rise than in ssp585. Over the ice shelves, the strong run-off increase associated with higher temperature is projected to decrease the SMB (more strongly in CMIP6-ssp585 compared to CMIP5-RCP8.5). Ice shelves are however predicted to have a close-to-present-equilibrium stable SMB under CMIP6 ssp126 and ssp245 scenarios. Future uncertainties are mainly due to the sensitivity to anthropogenic forcing and the timing of the projected warming. While ice shelves should remain at a close-to-equilibrium stable SMB under the Paris Agreement, MAR projects strong SMB decrease for an Antarctic near-surface warming above C2:5 degrees C compared to 1981-2010 mean temperature, limiting the warming range before potential irreversible damages on the ice shelves. Finally, our results reveal the existence of a potential threshold (C7:5 degrees C) that leads to a lower groundedSMB increase. This however has to be confirmed in following studies using more extreme or longer future scenarios.
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Le Toumelin, L., Amory, C., Favier, V., Kittel, C., Hofer, S., Fettweis, X., et al. (2021). Sensitivity of the surface energy budget to drifting snow as simulated by MAR in coastal Adelie Land, Antarctica. Cryosphere, 15(8), 3595–3614.
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Pohl, B., Favier, V., Wille, J., Udy, D., Vance, T., Pergaud, J., et al. (2021). Relationship Between Weather Regimes and Atmospheric Rivers in East Antarctica. Journal Of Geophysical Research-Atmospheres, 126(24).
Abstract: Here, we define weather regimes in the East Antarctica-Southern Ocean sector based on daily anomalies of 700 hPa geopotential height derived from ERA5 reanalysis during 1979-2018. Most regimes and their preferred transitions depict synoptic-scale disturbances propagating eastwards off the Antarctic coastline. While regime sequences are generally short, their interannual variability is strongly driven by the polarity of the Southern Annular Mode (SAM). Regime occurrences are then intersected with atmospheric rivers (ARs) detected over the same region and period. ARs are equiprobable throughout the year, but clearly concentrate during regimes associated with a strong atmospheric ridges/blockings on the eastern part of the domain, which act to channel meridional advection of heat and moisture from the lower latitudes towards Antarctica. Both regimes and ARs significantly shape climate variability in Antarctica. Regimes favorable to AR occurrences are associated with anomalously warm and humid conditions in coastal Antarctica and, to a lesser extent, the hinterland parts of the Antarctic plateau. These anomalies are strongly enhanced during AR events, with warmer anomalies and dramatically amplified snowfall amounts. Large-scale conditions favoring AR development are finally explored. They show weak dependency to the SAM, but particularly strong atmospheric ridges/blockings over the Southern Ocean appear as the most favorable pattern, in which ARs can be embedded, and to which they contribute.
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Veillon, F., Dumont, M., Amory, C., & Fructus, M. (2021). A versatile method for computing optimized snow albedo from spectrally fixed radiative variables: VALHALLA v1.0. Geoscientific Model Development, 14(12), 7329–7343.
Abstract: In climate models, the snow albedo scheme generally calculates only a narrowband or broad-band albedo, which leads to significant uncertainties. Here, we present the Versatile ALbedo calculation metHod based on spectrALLy fixed radiative vAriables (VALHALLA version 1.0) to optimize spectral snow albedo calculation. For this optimization, the energy absorbed by the snowpack is calculated by the spectral albedo model Two-streAm Radiative TransfEr in Snow (TARIES) and the spectral irradiance model Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART). This calculation takes into account the spectral characteristics of the incident radiation and the optical properties of the snow based on an analytical approximation of the radiative transfer of snow. For this method, 30 wavelengths, called tie points (TPs), and 16 reference irradiance profiles are calculated to incorporate the absorbed energy and the reference irradiance. The absorbed energy is then interpolated for each wavelength between two TPs with adequate kernel functions derived from radiative transfer theory for snow and the atmosphere. We show that the accuracy of the absorbed energy calculation primarily depends on the adaptation of the irradiance of the reference profile to that of the simulation (absolute difference < 1 W m(-2) for broadband absorbed energy and absolute difference < 0.005 for broadband albedo). In addition to the performance in terms of accuracy and calculation time, the method is adaptable to any atmospheric input (broadband, narrowband) and is easily adaptable for integration into a radiative scheme of a global or regional climate model.
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Verjans, V., Leeson, A., Mcmillan, M., Stevens, C., Van Wessem, J., Van De Berg, W., et al. (2021). Uncertainty in East Antarctic Firn Thickness Constrained Using a Model Ensemble Approach. Geophysical Research Letters, 48(7).
Abstract: Mass balance assessments of the East Antarctic ice sheet (EAIS) are highly sensitive to changes in firn thickness, causing substantial disagreement in estimates of its contribution to sea-level. To better constrain the uncertainty in recent firn thickness changes, we develop an ensemble of 54 model scenarios of firn evolution between 1992 and 2017. Using statistical emulation of firn-densification models, we quantify the impact of firn compaction formulation, differing climatic forcing, and surface snow density on firn thickness evolution. At basin scales, the ensemble uncertainty in firn thickness change ranges between 0.2 and 1.0 cm yr(-1) (15%-300% relative uncertainty), with the choice of climate forcing having the largest influence on the spread. Our results show the regions of the ice sheet where unexplained discrepancies exist between observed elevation changes and an extensive set of modeled firn thickness changes estimates, marking an important step toward more accurately constraining ice sheet mass balance.
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2020 |
Donat-Magnin, M., Jourdain, N., Gallee, H., Amory, C., Kittel, C., Fettweis, X., et al. (2020). Interannual variability of summer surface mass balance and surface melting in the Amundsen sector, West Antarctica. Cryosphere, 14(1), 229–249.
Abstract: Understanding the interannual variability of surface mass balance (SMB) and surface melting in Antarctica is key to quantify the signal-to-noise ratio in climate trends, identify opportunities for multi-year climate predictions and assess the ability of climate models to respond to climate variability. Here we simulate summer SMB and surface melting from 1979 to 2017 using the Regional Atmosphere Model (MAR) at 10 km resolution over the drainage basins of the Amundsen Sea glaciers in West Antarctica. Our simulations reproduce the mean present-day climate in terms of near-surface temperature (mean overestimation of 0.10 degrees C), near-surface wind speed (mean underestimation of 0.42 m s(-1)), and SMB (relative bias < 20 % over Thwaites glacier). The simulated interannual variability of SMB and melting is also close to observation-based estimates. For all the Amundsen glacial drainage basins, the interannual variability of summer SMB and surface melting is driven by two distinct mechanisms: high summer SMB tends to occur when the Amundsen Sea Low (ASL) is shifted southward and westward, while high summer melt rates tend to occur when ASL is shallower (i.e. anticyclonic anomaly). Both mechanisms create a northerly flow anomaly that increases moisture convergence and cloud cover over the Amundsen Sea and therefore favors snowfall and downward longwave radiation over the ice sheet. The part of interannual summer SMB variance explained by the ASL longitudinal migrations increases westward and reaches 40 % for Getz. Interannual variation in the ASL relative central pressure is the largest driver of melt rate variability, with 11 % to 21 % of explained variance (increasing westward). While high summer SMB and melt rates are both favored by positive phases of El Nino-Southern Oscillation (ENSO), the Southern Oscillation Index (SOI) only explains 5 % to 16 % of SMB or melt rate interannual variance in our simulations, with moderate statistical significance. However, the part explained by SOI in the previous austral winter is greater, suggesting that at least a part of the ENSO-SMB and ENSO-melt relationships in summer is inherited from the previous austral winter. Possible mechanisms involve sea ice advection from the Ross Sea and intrusions of circumpolar deep water combined with melt-induced ocean overturning circulation in ice shelf cavities. Finally, we do not find any correlation with the Southern Annular Mode (SAM) in summer.
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2019 |
Agosta, C., Amory, C., Kittel, C., Orsi, A., Favier, V., Gallee, H., et al. (2019). Estimation of the Antarctic surface mass balance using the regional climate model MAR (1979-2015) and identification of dominant processes. Cryosphere, 13(1), 281–296.
Abstract: The Antarctic ice sheet mass balance is a major component of the sea level budget and results from the difference of two fluxes of a similar magnitude: ice flow discharging in the ocean and net snow accumulation on the ice sheet surface, i.e. the surface mass balance (SMB). Separately modelling ice dynamics and SMB is the only way to project future trends. In addition, mass balance studies frequently use regional climate models (RCMs) outputs as an alternative to observed fields because SMB observations are particularly scarce on the ice sheet. Here we evaluate new simulations of the polar RCM MAR forced by three reanalyses, ERA-Interim, JRA-55, and MERRA-2, for the period 1979-2015, and we compare MAR results to the last outputs of the RCM RACMO2 forced by ERA-Interim. We show that MAR and RACMO2 perform similarly well in simulating coast-to-plateau SMB gradients, and we find no significant differences in their simulated SMB when integrated over the ice sheet or its major basins. More importantly, we outline and quantify missing or underestimated processes in both RCMs. Along stake transects, we show that both models accumulate too much snow on crests, and not enough snow in valleys, as a result of drifting snow transport fluxes not included in MAR and probably underestimated in RACMO2 by a factor of 3. Our results tend to confirm that drifting snow transport and sublimation fluxes are much larger than previous model-based estimates and need to be better resolved and constrained in climate models. Sublimation of precipitating particles in low-level atmospheric layers is responsible for the significantly lower snowfall rates in MAR than in RACMO2 in katabatic channels at the ice sheet margins. Atmospheric sublimation in MAR represents 363 Gt yr(-1) over the grounded ice sheet for the year 2015, which is 16% of the simulated snowfall loaded at the ground. This estimate is consistent with a recent study based on precipitation radar observations and is more than twice as much as simulated in RACMO2 because of different time residence of precipitating particles in the atmosphere. The remaining spatial differences in snowfall between MAR and RACMO2 are attributed to differences in advection of precipitation with snowfall particles being likely advected too far inland in MAR.
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2017 |
Amory, C., Gallee, H., Naaim-Bouvet, F., Favier, V., Vignon, E., Picard, G., et al. (2017). Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica. Boundary-Layer Meteorology, 164(1), 107–133.
Abstract: The surface of windy Antarctic snowfields is subject to drifting snow, which leads to the formation of sastrugi. In turn, sastrugi contribute to the drag exerted by the snowsurface on the atmosphere and hence influence drifting snow. Although the surface drag over rough sastrugi fields has been estimated for individual locations in Antarctica, its variation over time and with respect to drifting snow has received little attention. Using year-round data from a meteorological mast, seasonal variations in the neutral drag coefficient at a height of 10m (C-DN10) in coastal Adelie Land are presented and discussed in light of the formation and behaviour of sastrugi based on observed aeolian erosion patterns. The measurements revealed high C-DN10 values (>= 2 x 10(-3)) and limited drifting snow (35% of the time) in summer (December-February) versus lower C-DN10 values (approximate to 1.5 x 10(-3)) associated with more frequent drifting snow (70% of the time) in winter (March-November). Without the seasonal distinction, there was no clear dependence of C-DN10 on friction velocity or wind direction, but observations revealed a general increase in C-DN10 with rising air temperature. Themain hypothesis defended here is that higher temperatures increase snowcohesion and the development of sastrugi just after snow deposition while inhibiting the sastrugi streamlining process by raising the erosion threshold. This increases the contribution of the sastrugi form drag to the total surface drag in summer when winds are lighter and more variable. The analysis also showed that, in the absence of erosion, single snowfall events can reduce C-DN10 to 1 x 10(-3) due to the burying of pre-existing microrelief under newly deposited snow. The results suggest that polar atmospheric models should account for spatial and temporal variations in snow surface roughness through a dynamic representation of the sastrugi form drag.
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Favier, V., Krinner, G., Amory, C., Gallee, H., Beaumet, J., & Agosta, C. (2017). Antarctica-Regional Climate and Surface Mass Budget. Current Climate Change Reports, 3(4), 303–315.
Abstract: We review recent literature on atmospheric, surface ocean and sea-ice observations and modeling results in the Antarctic sector and relate the observed climatic trends with the potential changes in the surface mass balance (SMB) of the ice sheet since 1900. Estimates of regional scale SMB distribution and trends remain subject to large uncertainties. Approaches combining and comparing multiple satellite and model-based assessments of ice sheet mass balance aim at reducing these knowledge gaps. During the last decades, significant changes in atmospheric circulation occurred around Antarctica, due to the exceptional positive trend in the Southern Annular Mode and to the climate variability observed in the tropical Pacific at the end of the twentieth century. Even though climate over the East Antarctic Ice-Sheet remained quite stable, a warming and precipitation increase was observed over the West Antarctic Ice-Sheet and over the West Antarctic Peninsula (AP) during the twentieth century. However, the high regional climate variability overwhelms climate changes associated to human drivers of global temperature changes, as reflected by a slight recent decadal cooling trend over the AP. Climate models still fail to accurately reproduce the multi-decadal SMB trends at a regional scale, and progress has to be achieved in reproducing atmospheric circulation changes related to complex ocean/ice/atmosphere interactions. Complex processes are also still insufficiently considered, such as (1) specific polar atmospheric processes (clouds, drifting snow, and stable boundary layer physics), (2) surface firn physics involved in the surface drag variations, or in firn air depletion and albedo feedbacks. Finally, progress in reducing the uncertainties relative to projections of the future SMB of Antarctica will largely depend on climate model capability to correctly consider teleconnections with low and mid-latitudes, and on the ability to correct them for biases, taking into account the coupling between ocean, ice, and atmosphere in high southern latitudes.
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Fettweis, X., Box, J. E., Agosta, C., Amory, C., Kittel, C., Lang, C., et al. (2017). Reconstructions of the 1900-2015 Greenland ice sheet surface mass balance using the regional climate MAR model. Cryosphere, 11(2), 1015–1033.
Abstract: 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 (ModSle Atmospherique Regional; 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; 19482015), NCEP-NCARv2 (1979-2015), JRA-55 (Japanese 55year Reanalysis; 1958-2014), 20CRv2(c) (Twentieth Century Reanalysis version 2; 1900-2014) and ERA-20C (19002010) 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 C 1 ffi C (respectively 1 degrees C) was, for example, needed at the MAR boundaries with ERA-20C (20CRv2) reanalysis, given that ERA-20C (20CRv2) is similar to 1 degrees C colder (warmer) than ERAInterim 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 (C 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 20CRv2forced 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-2
0Cforced 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.
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Vignon, E., Genthon, C., Barral, H., Amory, C., Picard, G., Gallee, H., et al. (2017). Momentum- and Heat-Flux Parametrization at Dome C, Antarctica: A Sensitivity Study. Boundary-Layer Meteorology, 162(2), 341–367.
Abstract: An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant of 1 mm leads to a mean friction velocity bias of in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat to on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached when ; (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.
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2016 |
Amory, C. (2016). Erosion éolienne et rugosité de la surface neigeuse en Terre Adélie. Ph.D. thesis, Université Grenoble Alpes, Grenoble.
Abstract: The Antarctic ice sheet surface mass balance (SMB; result of the balance between accumulation and ablation terms) has a direct influence on variations in the global mean sea level. In the context of climate change, atmospheric models are nedded to is necessary to refine its current and future estimation.
Intense surface winds over the coastal slopes of East Antarctica are responsible for aerodynamic entrainment of snow at the surface, which influences significantly local accumulation. Transport of snow by the wind also produces the development of aeolian erosion features aligned parallel to the prevailing winds at the time of their formation. The spatial distribution of these features is a major determinant of surface roughness. On the other hand, surface roughness is an obstacle to flow and directly affects the surface wind field and, by extension, aeolian snow transport.
The work presented here is based on observations and numerical modeling of aeolian snow erosion in a coastal stretch of Adélie Land, East Antarctica. First, the regional atmospheric model MAR, which includes a detailed representation of aeolian transport processes, was run at a spatial resolution of 5 km over a zone including Adélie Land and model results were compared with meteorological observations made over one month in summer, including continuous measurements of the wind and horizontal aeolian snow mass flux. Aeolian fluxes modeled by the MAR were highly sensitive to parameterization of surface roughness, and a single calibration of this parameter was not enough to simulate the surface wind field at two measurement points located only 100 km apart with the same accuracy. Consequently, roughness-erosion interactions were analyzed at the scale of individual aeolian erosion events using observations. The results of this analysis underlined that (i) the barrier effect generated by roughness has an inhibiting impact on the aeolian flux and that (ii) this barrier effect can be strongly reduced by the ability of aeolian erosion features to realign with the dominant wind during a transport event. Examination of observations made over a period of one year revealed that this adjustment process is prone to temporal variations mainly linked to the temperature history. Finally, it was shown that reconfiguring the model, including introducing a temperature dependence in the parameterization of surface roughness considerably improved the representation of aeolian fluxes by the MAR model for the year concerned. These results suggest that a spatial and temporal distribution of surface roughness should be included in atmospheric models for realistic simulations of aeolian snow transport over Antarctica.
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Amory, C., Naaim-Bouvet, F., Gallee, H., & Vignon, E. (2016). Brief communication: Two well-marked cases of aerodynamic adjustment of sastrugi. Cryosphere, 10(2), 743–750.
Abstract: In polar regions, sastrugi are a direct manifestation of drifting snow and form the main surface roughness elements. In turn, sastrugi alter the generation of atmospheric turbulence and thus modify the wind field and the aeolian snow mass fluxes. Little attention has been paid to these feedback processes, mainly because of experimental difficulties. As a result, most polar atmospheric models currently ignore sastrugi over snow-covered regions. This paper aims at quantifying the potential influence of sastrugi on the local wind field and on snow erosion over a sastrugi-covered snowfield in coastal Ad,lie Land, East Antarctica. We focus on two erosion events during which sastrugi responses to shifts in wind direction have been interpreted from temporal variations in drag and aeolian snow mass flux measurements during austral winter 2013. Using this data set, it is shown that (i) neutral stability, 10aEuro-m drag coefficient (C-DN10) values are in the range of 1.3-1.5 x 10(-3) when the wind is well aligned with the sastrugi, (ii) as the wind shifts by only 20-30A degrees away from the streamlined direction, C-DN10 increases (by 30-120aEuro-%) and the aeolian snow mass flux decreases (by 30-80aEuro-%), thereby reflecting the growing contribution of the sastrugi form drag to the total surface drag and its inhibiting effect on snow erosion, (iii) the timescale of sastrugi aerodynamic adjustment can be as short as 3aEuro-h for friction velocities greater than 1aEuro-maEuro-s(-1) and during strong drifting snow conditions and (iv) knowing C-DN10 is not sufficient to estimate the snow erosion flux that results from drag partitioning at the surface because C-DN10 includes the contribution of the sastrugi form drag.
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2015 |
Amory, C., Trouvilliez, A., Gallee, H., Favier, V., Naaim-Bouvet, F., Genthon, C., et al. (2015). Comparison between observed and simulated aeolian snow mass fluxes in Adelie Land, East Antarctica. Cryosphere, 9(4), 1373–1383.
Abstract: Using the original setup described in Gallee et al. (2013), the MAR regional climate model including a coupled snowpack/aeolian snow transport parameterization, was run at a fine spatial (5 km horizontal and 2m vertical) resolution over 1 summer month in coastal Adelie Land. Different types of feedback were taken into account in MAR including drag partitioning caused by surface roughness elements. Model outputs are compared with observations made at two coastal locations, D17 and D47, situated respectively 10 and 100 km inland. Wind speed was correctly simulated with positive values of the Nash test (0.60 for D17 and 0.37 for D47) but wind velocities above 10 m s(-1) were underestimated at both D17 and D47; at D47, the model consistently underestimated wind velocity by 2 m s(-1). Aeolian snow transport events were correctly reproduced with the right timing and a good temporal resolution at both locations except when the maximum particle height was less than 1 m. The threshold friction velocity, evaluated only at D17 for a 7-day period without snowfall, was overestimated. The simulated aeolian snow mass fluxes between 0 and 2m at D47 displayed the same variations but were underestimated compared to the second-generation FlowCapt (TM) values, as was the simulated relative humidity at 2m above the surface. As a result, MAR underestimated the total aeolian horizontal snow transport for the first 2 m above the ground by a factor of 10 compared to estimations by the second-generation FlowCapt (TM). The simulation was significantly improved at D47 if a 1-order decrease in the magnitude of z(0) was accounted for, but agreement with observations was reduced at D17. Our results suggest that z(0) may vary regionally depending on snowpack properties, which are involved in different types of feedback between aeolian transport of snow and z(0).
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Gallee, H., Preunkert, S., Argentini, S., Frey, M. M., Genthon, C., Jourdain, B., et al. (2015). Characterization of the boundary layer at Dome C (East Antarctica) during the OPALE summer campaign. Atmospheric Chemistry And Physics, 15(11), 6225–6236.
Abstract: Regional climate model MAR (Modele Atmospherique Regional) was run for the region of Dome C located on the East Antarctic plateau, during Antarctic summer 2011-2012, in order to refine our understanding of meteorological conditions during the OPALE tropospheric chemistry campaign. A very high vertical resolution is set up in the lower troposphere, with a grid spacing of roughly 2 m. Model output is compared with temperatures and winds observed near the surface and from a 45m high tower as well as sodar and radiation data. MAR is generally in very good agreement with the observations, but sometimes underestimates cloud formation, leading to an underestimation of the simulated downward long-wave radiation. Absorbed short-wave radiation may also be slightly overestimated due to an underestimation of the snow albedo, and this influences the surface energy budget and atmospheric turbulence. Nevertheless, the model provides sufficiently reliable information about surface turbulent fluxes, vertical profiles of vertical diffusion coefficients and boundary layer height when discussing the representativeness of chemical measurements made nearby the ground surface during field campaigns conducted at Concordia station located at Dome C (3233m above sea level).
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2014 |
Barral, H., Genthon, C., Trouvilliez, A., Brun, C., & Amory, C. (2014). Blowing snow in coastal Adelie Land, Antarctica: three atmospheric-moisture issues. Cryosphere, 8(5), 1905–1919.
Abstract: A total of 3 years of blowing-snow observations and associated meteorology along a 7 m mast at site D17 in coastal Adelie Land are presented. The observations are used to address three atmospheric-moisture issues related to the occurrence of blowing snow, a feature which largely affects many regions of Antarctica: ( 1) blowing-snow sublimation raises the moisture content of the surface atmosphere close to saturation, and atmospheric models and meteorological analyses that do not carry blowing-snow parameterizations are affected by a systematic dry bias; ( 2) while snowpack modelling with a parameterization of surface-snow erosion by wind can reproduce the variability of snow accumulation and ablation, ignoring the high levels of atmospheric-moisture content associated with blowing snow results in overestimating surface sublimation, affecting the energy budget of the snowpack; ( 3) the well-known profile method of calculating turbulent moisture fluxes is not applicable when blowing snow occurs, because moisture gradients are weak due to blowing-snow sublimation, and the impact of measurement uncertainties are strongly amplified in the case of strong winds.
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Trouvilliez, A., Naaim-Bouvet, F., Genthon, C., Piard, L., Favier, V., Bellot, H., et al. (2014). A novel experimental study of aeolian snow transport in Adelie Land (Antarctica). Cold Regions Science And Technology, 108, 125–138.
Abstract: None of the previous aeolian snow transport campaigns in Antarctica meet the requirements in terms of temporal 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 Adelie 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: FlowCapt (TM) acoustic sensors. Three automatic weather stations were deployed with FlowCapts (TM) 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 Adelie Land was very high with seasonal variation of transport occurring with minima during the austral summer. Seven percent of the aeolian snow transport events were drifting snow (maximum particle's height, < 1 m above the surface). The snow quantity transported was above 1 kiloton per year in the first meter above the surface. (C) 2014 Elsevier B.V. All rights reserved.
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