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Jan Melchior van Wessem, Willem Jan van de Berg, Brice P. Y. Noël, Erik van Meijgaard, Charles Amory, Gerit Birnbaum, Constantijn L. Jakobs, Konstantin Krüger, Jan T. M. Lenaerts, Stef Lhermitte, Stefan R. M. Ligtenberg, Brooke Medley, Carleen H. Reijmer, Kristof van Tricht, Luke D. Trusel, Lambertus H. van Ulft, Bert Wouters, Jan Wuite, Michiel R. van den Broeke. (2018). Modelling the climate and surface mass balance of polar ice sheets using RACMO2 – Part 2: Antarctica (1979–2016) (Vol. 12).
Abstract: Abstract. We evaluate modelled Antarctic ice sheet (AIS) near-surface climate, surface mass balance (SMB) and surface energy balance (SEB) from the updated polar version of the regional atmospheric climate model, RACMO2 (1979–2016). The updated model, referred to as RACMO2.3p2, incorporates upper-air relaxation, a revised topography, tuned parameters in the cloud scheme to generate more precipitation towards the AIS interior and modified snow properties reducing drifting snow sublimation and increasing surface snowmelt. Comparisons of RACMO2 model output with several independent observational data show that the existing biases in AIS temperature, radiative fluxes and SMB components are further reduced with respect to the previous model version. The model-integrated annual average SMB for the ice sheet including ice shelves (minus the Antarctic Peninsula, AP) now amounts to 2229Gty−1, with an interannual variability of 109Gty−1. The largest improvement is found in modelled surface snowmelt, which now compares well with satellite and weather station observations. For the high-resolution (∼5.5km) AP simulation, results remain comparable to earlier studies. The updated model provides a new, high-resolution data set of the contemporary near-surface climate and SMB of the AIS; this model version will be used for future climate scenario projections in a forthcoming study.
Programme: 1013
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E. Berthier, C. Vincent, E. Magnússon, Á Þ Gunnlaugsson, P. Pitte, E. Le Meur, M. Masiokas, L. Ruiz, F. Pálsson, J. M. C. Belart, P. Wagnon. (2014). Glacier topography and elevation changes derived from Pléiades sub-meter stereo images (Vol. 8).
Abstract: Abstract. In response to climate change, most glaciers are losing mass and hence contribute to sea-level rise. Repeated and accurate mapping of their surface topography is required to estimate their mass balance and to extrapolate/calibrate sparse field glaciological measurements. In this study we evaluate the potential of sub-meter stereo imagery from the recently launched Pléiades satellites to derive digital elevation models (DEMs) of glaciers and their elevation changes. Our five evaluation sites, where nearly simultaneous field measurements were collected, are located in Iceland, the European Alps, the central Andes, Nepal and Antarctica. For Iceland, the Pléiades DEM is also compared to a lidar DEM. The vertical biases of the Pléiades DEMs are less than 1 m if ground control points (GCPs) are used, but reach up to 7 m without GCPs. Even without GCPs, vertical biases can be reduced to a few decimetres by horizontal and vertical co-registration of the DEMs to reference altimetric data on ice-free terrain. Around these biases, the vertical precision of the Pléiades DEMs is ±1 m and even ±0.5 m on the flat glacier tongues (1σ confidence level). Similar precision levels are obtained in the accumulation areas of glaciers and in Antarctica. We also demonstrate the high potential of Pléiades DEMs for measuring seasonal, annual and multi-annual elevation changes with an accuracy of 1 m or better if cloud-free images are available. The negative region-wide mass balances of glaciers in the Mont-Blanc area (−1.04 ± 0.23 m a−1 water equivalent, w.e.) are revealed by differencing Satellite pour l'Observation de la Terre 5 (SPOT 5) and Pléiades DEMs acquired in August 2003 and 2012, confirming the accelerated glacial wastage in the European Alps.
Programme: 1053
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E. Le Meur, M. Sacchettini, S. Garambois, E. Berthier, A. S. Drouet, G. Durand, D. Young, J. S. Greenbaum, J. W. Holt, D. D. Blankenship, E. Rignot, J. Mouginot, Y. Gim, D. Kirchner, B. de Fleurian, O. Gagliardini, F. Gillet-Chaulet. (2014). Two independent methods for mapping the grounding line of an outlet glacier – an example from the Astrolabe Glacier, Terre Adélie, Antarctica (Vol. 8).
Abstract: Abstract. The grounding line is a key element of coastal outlet glaciers, acting on their dynamics. Accurately knowing its position is fundamental for both modelling the glacier dynamics and establishing a benchmark for later change detection. Here we map the grounding line of the Astrolabe Glacier in East Antarctica (66°41' S, 140°05' E), using both hydrostatic and tidal methods. The first method is based on new surface and ice thickness data from which the line of buoyant floatation is found. The second method uses kinematic GPS measurements of the tidal response of the ice surface. By detecting the transitions where the ice starts to move vertically in response to the tidal forcing we determine control points for the grounding line position along GPS profiles. Employing a two-dimensional elastic plate model, we compute the rigid short-term behaviour of the ice plate and estimate the correction required to compare the kinematic GPS control points with the previously determined line of floatation. These two approaches show consistency and lead us to propose a grounding line for the Astrolabe Glacier that significantly deviates from the lines obtained so far from satellite imagery.
Programme: 1053
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Alexandra Touzeau, Amaëlle Landais, Barbara Stenni, Ryu Uemura, Kotaro Fukui, Shuji Fujita, Sarah Guilbaud, Alexey Ekaykin, Mathieu Casado, Eugeni Barkan, Boaz Luz, Olivier Magand, Grégory Teste, Emmanuel Le Meur, Mélanie Baroni, Joël Savarino, Ilann Bourgeois, Camille Risi. (2016). Acquisition of isotopic composition for surface snow in East Antarctica and the links to climatic parameters (Vol. 10).
Abstract: Abstract. The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica. First, we look at the relationship between isotopes and temperature from a geographical point of view, using results from three traverses across Antarctica, to see how the relationship is built up through the distillation process. We also take advantage of these measures to see how second-order parameters (d-excess and 17O-excess) are related to δ18O and how they are controlled. d-excess increases in the interior of the continent (i.e., when δ18O decreases), due to the distillation process, whereas 17O-excess decreases in remote areas, due to kinetic fractionation at low temperature. In both cases, these changes are associated with the loss of original information regarding the source. Then, we look at the same relationships in precipitation samples collected over 1 year at Dome C and Vostok, as well as in surface snow at Dome C. We note that the slope of the δ18O vs. temperature (T) relationship decreases in these samples compared to those from the traverses, and thus caution is advocated when using spatial slopes for past climate reconstruction. The second-order parameters behave in the same way in the precipitation as in the surface snow from traverses, indicating that similar processes are active and that their interpretation in terms of source climatic parameters is strongly complicated by local temperature effects in East Antarctica. Finally we check if the same relationships between δ18O and second-order parameters are also found in the snow from four snow pits. While the d-excess remains opposed to δ18O in most snow pits, the 17O-excess is no longer positively correlated to δ18O and even shows anti-correlation to δ18O at Vostok. This may be due to a stratospheric influence at this site and/or to post-deposition processes.
Programme: 1177
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Fifi Ibrahime Adodo, Frédérique Remy, Ghislain Picard. (2018). Seasonal variations of the backscattering coefficient measured by radar altimeters over the Antarctic Ice Sheet (Vol. 12).
Abstract: Abstract. Spaceborne radar altimeters are a valuable tool for observing the Antarctic Ice Sheet. The radar wave interaction with the snow provides information on both the surface and the subsurface of the snowpack due to its dependence on the snow properties. However, the penetration of the radar wave within the snowpack also induces a negative bias on the estimated surface elevation. Empirical corrections of this space- and time-varying bias are usually based on the backscattering coefficient variability. We investigate the spatial and seasonal variations of the backscattering coefficient at the S (3.2GHz∼9.4cm), Ku (13.6GHz∼2.3cm) and Ka (37GHz∼0.8cm) bands. We identified that the backscattering coefficient at Ku band reaches a maximum in winter in part of the continent (Region 1) and in the summer in the remaining (Region 2), while the evolution at other frequencies is relatively uniform over the whole continent. To explain this contrasting behavior between frequencies and between regions, we studied the sensitivity of the backscattering coefficient at three frequencies to several parameters (surface snow density, snow temperature and snow grain size) using an electromagnetic model. The results show that the seasonal cycle of the backscattering coefficient at Ka frequency is dominated by the volume echo and is mainly driven by snow temperature evolution everywhere. In contrast, at S band, the cycle is dominated by the surface echo. At Ku band, the seasonal cycle is dominated by the volume echo in Region 1 and by the surface echo in Region 2. This investigation provides new information on the seasonal dynamics of the Antarctic Ice Sheet surface and provides new clues to build more accurate corrections of the radar altimeter surface elevation signal in the future.
Programme: 1110
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Mathieu Casado, Amaelle Landais, Ghislain Picard, Thomas Münch, Thomas Laepple, Barbara Stenni, Giuliano Dreossi, Alexey Ekaykin, Laurent Arnaud, Christophe Genthon, Alexandra Touzeau, Valerie Masson-Delmotte, Jean Jouzel. (2018). Archival processes of the water stable isotope signal in East Antarctic ice cores (Vol. 12).
Abstract: Abstract. The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotope-enabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation–condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas.
Programme: 1110
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Emmanuel Le Meur, Olivier Magand, Laurent Arnaud, Michel Fily, Massimo Frezzotti, Marie Cavitte, Robert Mulvaney, Stefano Urbini. (2018). Spatial and temporal distributions of surface mass balance between Concordia and Vostok stations, Antarctica, from combined radar and ice core data: first results and detailed error analysis (Vol. 12). Bachelor's thesis, , .
Abstract: Abstract. Results from ground-penetrating radar (GPR) measurements and shallow ice cores carried out during a scientific traverse between Dome Concordia (DC) and Vostok stations are presented in order to infer both spatial and temporal characteristics of snow accumulation over the East Antarctic Plateau. Spatially continuous accumulation rates along the traverse are computed from the identification of three equally spaced radar reflections spanning about the last
Programme: 1028,1053,1110
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Thomas Laepple, Thomas Münch, Mathieu Casado, Maria Hoerhold, Amaelle Landais, Sepp Kipfstuhl. (2018). On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits (Vol. 12).
Abstract: Stable isotope ratios δ18O and δD in polar ice provide a wealth of information about past climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured isotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are formed and stored in firn and ice. As δ18O and δD in the snowfall are strongly correlated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given site. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of snow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in the δ18O and δD profiles with typical wavelengths of ∼ 20cm. These observations are unexpected as the accumulation rates strongly differ between the sites, ranging from 20 to 80mm w. e. yr−1 ( ∼ 6–21cm of snow per year). Various mechanisms have been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with the similarity of the different profiles independent of the local accumulation conditions.Here, we systematically analyse the properties and origins of δ18O and δD variations in high-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance between peaks) of ∼ 20cm by counting the isotopic maxima. Spectral analysis further shows a strong similarity between the sites but indicates no dominant periodic features. Furthermore, the apparent cycle length increases with depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal cycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of the isotopic variations. This in turn implies a similar distance between isotopic maxima in the firn profiles.Our results explain a large set of observations discussed in the literature, providing a simple explanation for the interpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results underline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small signal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations.
Programme: 1110
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Christoph Kittel, Charles Amory, Cécile Agosta, Alison Delhasse, Sébastien Doutreloup, Pierre-Vincent Huot, Coraline Wyard, Thierry Fichefet, Xavier Fettweis. (2018). Sensitivity of the current Antarctic surface mass balance to sea surface conditions using MAR (Vol. 12).
Abstract: Abstract. Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields prescribed by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSCs, namely sea ice concentration, SIC, and sea surface temperature, SST) over the Southern Ocean is not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5), RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM “Modèle Atmosphérique Régional” (MAR), different sensitivity experiments are performed over 1979–2015 by modifying the ERA-Interim SSCs with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (decreased) precipitation due to perturbations inducing warmer, i.e. higher SST and lower SIC (colder, i.e. lower SST and higher SIC), SSCs than ERA-Interim, significantly affecting the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e greater than the interannual variability) are found for the largest combined SST/SIC perturbations. This is notably due to moisture anomalies above the ocean, reaching sufficiently high atmospheric levels to influence accumulation rates further inland. Sensitivity experiments with warmer SSCs based on the CMIP5 biases reveal integrated SMB anomalies (+5% to +13%) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century.
Programme: 411
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Cécile Agosta, Charles Amory, Christoph Kittel, Anais Orsi, Vincent Favier, Hubert Gallée, Michiel R. van den Broeke, Jan T. M. Lenaerts, Jan Melchior van Wessem, Willem Jan van de Berg, Xavier Fettweis. (2019). Estimation of the Antarctic surface mass balance using the regional climate model MAR (1979–2015) and identification of dominant processes (Vol. 13). Bachelor's thesis, , .
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.
Programme: 411,1169
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