Publications

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 Ade&#769;lie 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 Ade&#769;lie 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.

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).

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.