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Sentia Goursaud, Valérie Masson-Delmotte, Vincent Favier, Suzanne Preunkert, Michel Legrand, Bénédicte Minster, Martin Werner. (2019). Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica (Vol. 13). Bachelor's thesis, , .
Abstract: A New 21.3 M Firn Core Was Drilled In 2015 At A Coastal Antarctic High-accumulation Site In Adélie Land (66.78∘ S; 139.56∘ E, 602 M A.s.l.), Named Terre Adélie 192a (Ta192a). The Mean Isotopic Values (-19.3‰±3.1 ‰ For δ18o And 5.4 ‰±2.2 ‰ For Deuterium Excess) Are Consistent With Other Coastal Antarctic Values. No Significant Isotope–temperature Relationship Can Be Evidenced At Any Timescale. This Rules Out A Simple Interpretation In Terms Of Local Temperature. An Observed Asymmetry In The δ18o Seasonal Cycle May Be Explained By The Precipitation Of Air Masses Coming From The Eastern And Western Sectors In Autumn And Winter, Recorded In The D-excess Signal Showing Outstanding Values In Austral Spring Versus Autumn. Significant Positive Trends Are Observed In The Annual D-excess Record And Local Sea Ice Extent (135–145∘ E) Over The Period 1998–2014. However, Process Studies Focusing On Resulting Isotopic Compositions And Particularly The Deuterium Excess–δ18o Relationship, Evidenced As A Potential Fingerprint Of Moisture Origins, As Well As The Collection Of More Isotopic Measurements In Adélie Land Are Needed For An Accurate Interpretation Of Our Signals.
Programme: 411,414
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Sentia Goursaud, Valérie Masson-delmotte, Vincent Favier, Susanne Preunkert, Michel Fily, Hubert Gallée, Bruno Jourdain, Michel Legrand, Olivier Magand, Bénédicte Minster, Martin Werner. (2017). A 60-year ice-core record of regional climate from Adélie Land, coastal Antarctica (Vol. 11).
Abstract: Abstract. A 22.4 M-long Shallow Firn Core Was Extracted During The 2006/2007 Field Season From Coastal Adélie Land. Annual Layer Counting Based On Subannual Analyses Of δ18o And Major Chemical Components Was Combined With 5 Reference Years Associated With Nuclear Tests And Non-retreat Of Summer Sea Ice To Build The Initial Ice-core Chronology (1946–2006), Stressing Uncertain Counting For 8 Years. We Focus Here On The Resulting δ18o And Accumulation Records. With An Average Value Of 21.8 ± 6.9 Cm W.e. Yr−1, Local Accumulation Shows Multi-decadal Variations Peaking In The 1980s, But No Long-term Trend. Similar Results Are Obtained For δ18o, Also Characterised By A Remarkably Low And Variable Amplitude Of The Seasonal Cycle. The Ice-core Records Are Compared With Regional Records Of Temperature, Stake Area Accumulation Measurements And Variations In Sea-ice Extent, And Outputs From Two Models Nudged To Era (European Reanalysis) Atmospheric Reanalyses: The High-resolution Atmospheric General Circulation Model (Agcm), Including Stable Water Isotopes Echam5-wiso (European Centre Hamburg Model), And The Regional Atmospheric Model Modèle Atmosphérique Régional (Ar). A Significant Linear Correlation Is Identified Between Decadal Variations In δ18o And Regional Temperature. No Significant Relationship Appears With Regional Sea-ice Extent. A Weak And Significant Correlation Appears With Dumont D'urville Wind Speed, Increasing After 1979. The Model-data Comparison Highlights The Inadequacy Of Echam5-wiso Simulations Prior To 1979, Possibly Due To The Lack Of Data Assimilation To Constrain Atmospheric Reanalyses. Systematic Biases Are Identified In The Echam5-wiso Simulation, Such As An Overestimation Of The Mean Accumulation Rate And Its Interannual Variability, A Strong Cold Bias And An Underestimation Of The Mean δ18o Value And Its Interannual Variability. As A Result, Relationships Between Simulated δ18o And Temperature Are Weaker Than Observed. Such Systematic Precipitation And Temperature Biases Are Not Displayed By Mar, Suggesting That The Model Resolution Plays A Key Role Along The Antarctic Ice Sheet Coastal Topography. Interannual Variations In Echam5-wiso Temperature And Precipitation Accurately Capture Signals From Meteorological Data And Stake Observations And Are Used To Refine The Initial Ice-core Chronology Within 2 Years. After This Adjustment, Remarkable Positive (Negative) δ18o Anomalies Are Identified In The Ice-core Record And The Echam5-wiso Simulation In 1986 And 2002 (1998–1999), Respectively. Despite Uncertainties Associated With Post-deposition Processes And Signal-to-noise Issues, In One Single Coastal Ice-core Record, We Conclude That The S1c1 Core Can Correctly Capture Major Annual Anomalies In δ18o As Well As Multi-decadal Variations. These Findings Highlight The Importance Of Improving The Network Of Coastal High-resolution Ice-core Records, And Stress The Skills And Limitations Of Atmospheric Models For Accumulation And δ18o In Coastal Antarctic Areas. This Is Particularly Important For The Overall East Antarctic Ice Sheet Mass Balance.
Programme: 411
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Nicolas Champollion, Ghislain Picard, Laurent Arnaud, Éric Lefebvre, Giovanni Macelloni, Frédérique Rémy, Michel Fily. (2019). Marked decrease in the near-surface snow density retrieved by AMSR-E satellite at Dome C, Antarctica, between 2002 and 2011 (Vol. 13).
Abstract:
Abstract. Surface snow density is an important variable for the surface mass balance and energy budget. It evolves according to meteorological conditions, in particular, snowfall, wind, and temperature, but the physical processes governing atmospheric influence on snow are not fully understood. A reason is that no systematic observation is available on a continental scale. Here, we use the passive microwave observations from AMSR-E satellite to retrieve the surface snow density at Dome C on the East Antarctic Plateau. The retrieval method is based on the difference of surface reflections between horizontally and vertically polarized brightness temperatures at 37 GHz, highlighted by the computation of the polarization ratio, which is related to surface snow density. The relationship has been obtained with a microwave emission radiative transfer model (DMRT-ML). The retrieved density, approximately representative of the topmost 3 cm of the snowpack, compares well with in situ measurements. The difference between mean in situ measurements and mean retrieved density is 26.2 kg m−3, which is within typical in situ measurement uncertainties. We apply the retrieval method to derive the time series over the period 2002–2011. The results show a marked and persistent pluri-annual decrease of about 10 kg m−3 yr−1, in addition to atmosphere-related seasonal, weekly, and daily density variations. This trend is confirmed by independent active microwave observations from the ENVISAT and QuikSCAT satellites, though the link to the density is more difficult to establish. However, no related pluri-annual change in meteorological conditions has been found to explain such a trend in snow density. Further work will concern the extension of the method to the continental scale. Programme: 1110
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Ghislain Picard, Laurent Arnaud, Romain Caneill, Eric Lefebvre, Maxim Lamare. (2019). Observation of the process of snow accumulation on the Antarctic Plateau by time lapse laser scanning (Vol. 13).
Abstract:
Abstract. Snow accumulation is the main positive component of the mass balance in Antarctica. In contrast to the major efforts deployed to estimate its overall value on a continental scale – to assess the contribution of the ice sheet to sea level rise – knowledge about the accumulation process itself is relatively poor, although many complex phenomena occur between snowfall and the definitive settling of the snow particles on the snowpack. Here we exploit a dataset of near-daily surface elevation maps recorded over 3 years at Dome C using an automatic laser scanner sampling 40–100 m2 in area. We find that the averaged accumulation is relatively regular over the 3 years at a rate of +8.7 cm yr−1. Despite this overall regularity, the surface changes very frequently (every 3 d on average) due to snow erosion and heterogeneous snow deposition that we call accumulation by “patches”. Most of these patches (60 %–85 %) are ephemeral but can survive a few weeks before being eroded. As a result, the surface is continuously rough (6–8 cm root-mean-square height) featuring meter-scale dunes aligned along the wind and larger, decameter-scale undulations. Additionally, we deduce the age of the snow present at a given time on the surface from elevation time series and find that snow age spans over more than a year. Some of the patches ultimately settle, leading to a heterogeneous internal structure which reflects the surface heterogeneity, with many snowfall events missing at a given point, whilst many others are overrepresented. These findings have important consequences for several research topics including surface mass balance, surface energy budget, photochemistry, snowpack evolution, and the interpretation of the signals archived in ice cores. Programme: 1110
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Claudio Durán-Alarcón, Brice Boudevillain, Christophe Genthon, Jacopo Grazioli, Niels Souverijns, Nicole P. M. van Lipzig, Irina V. Gorodetskaya, Alexis Berne. (2018). The vertical structure of precipitation at two stations in East Antarctica derived from micro rain radars (Vol. 13).
Abstract: Abstract. Precipitation Over Antarctica Is The Main Term In The Surface Mass Balance Of The Antarctic Ice Sheet, Which Is Crucial For The Future Evolution Of The Sea Level Worldwide. Precipitation, However, Remains Poorly Documented And Understood Mainly Because Of A Lack Of Observations In This Extreme Environment. Two Observatories Dedicated To Precipitation Have Been Set Up At The Belgian Station Princess Elisabeth (Pe) And At The French Station Dumont D'urville (Ddu) In East Antarctica. Among Other Instruments, Both Sites Have A Vertically Pointing Micro Rain Radar (Mrr) Working At The K Band. Measurements Have Been Continuously Collected At Ddu Since The Austral Summer Of 2015–2016, While They Have Been Collected Mostly During Summer Seasons At Pe Since 2010, With A Full Year Of Observation During 2012. In This Study, The Statistics Of The Vertical Profiles Of Reflectivity, Vertical Velocity, And Spectral Width Are Analyzed For All Seasons. Vertical Profiles Were Separated Into Surface Precipitation And Virga To Evaluate The Impact Of Virga On The Structure Of The Vertical Profiles. The Climatology Of The Study Area Plays An Important Role In The Structure Of The Precipitation: Warmer And Moister Atmospheric Conditions At Ddu Favor The Occurrence Of More Intense Precipitation Compared With Pe, With A Difference Of 8 Dbz Between Both Stations. The Strong Katabatic Winds Blowing At Ddu Induce A Decrease In Reflectivity Close To The Ground Due To The Sublimation Of The Snowfall Particles. The Vertical Profiles Of Precipitation Velocity Show Significant Differences Between The Two Stations. In General, At Ddu The Vertical Velocity Increases As The Height Decreases, While At Pe The Vertical Velocity Decreases As The Height Decreases. These Features Of The Vertical Profiles Of Reflectivity And Vertical Velocity Could Be Explained By The More Frequent Occurrence Of Aggregation And Riming At Ddu Compared To Pe Because Of The Lower Temperature And Relative Humidity At The Latter, Located Further In The Interior. Robust And Reliable Statistics About The Vertical Profile Of Precipitation In Antarctica, As Derived From Mrrs For Instance, Are Necessary And Valuable For The Evaluation Of Precipitation Estimates Derived From Satellite Measurements And From Numerical Atmospheric Models.
Programme: 1013
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Charles Amory, Christoph Kittel. (2019). Brief communication: Rare ambient saturation during drifting snow occurrences at a coastal location of East Antarctica (Vol. 13).
Abstract: Sublimation of snow particles during transport has been recognized as an important ablation process on the Antarctic ice sheet. The resulting increase in moisture content and cooling of the ambient air are thermodynamic negative feedbacks that both contribute to increase the relative humidity of the air, inhibiting further sublimation when saturation is reached. This self-limiting effect and the associated development of saturated near-surface air layers in drifting snow conditions have mainly been described through modelling studies and a few field observations. A set of meteorological data, including drifting snow mass fluxes and vertical profiles of relative humidity, collected at site D17 in coastal Adélie Land (East Antarctica) during 2013 is used to study the relationship between saturation of the near-surface atmosphere and the occurrence of drifting snow in a katabatic wind region that is among the most prone to snow transport by wind. Atmospheric moistening by the sublimation of the windborne snow particles generally results in a strong increase in relative humidity with the magnitude of drifting snow and a decrease in its vertical gradient, suggesting that windborne-snow sublimation can be an important contributor to the local near-surface moisture budget. Despite a high incidence of drifting snow at the measurement location (60.1 % of the time), saturation, when attained, is however most often limited to a thin air layer below 1 m above ground. The development of a near-surface saturated air layer up to the highest measurement level of 5.5 m is observed in only 8.2 % of the drifting snow occurrences or 6.3 % of the time and mainly occurs in strong wind speed and drift conditions. This relatively rare occurrence of ambient saturation is explained by the likely existence of moisture-removal mechanisms inherent to the katabatic and turbulent nature of the boundary-layer flow that weaken the negative feedback of windborne-snow sublimation. Such mechanisms, potentially quite active in katabatic-generated windborne-snow layers all over Antarctica, may be very important in understanding the surface mass and atmospheric moisture budgets of the ice sheet by enhancing windborne-snow sublimation.
Programme: 1013
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Marion Donat-Magnin, Nicolas C. Jourdain, Hubert Gallée, Charles Amory, Christoph Kittel, Xavier Fettweis, Jonathan D. Wille, Vincent Favier, Amine Drira, Cécile Agosta. (2020). Interannual variability of summer surface mass balance and surface melting in the Amundsen sector, West Antarctica (Vol. 14).
Abstract:
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 ∘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 Niño–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. Programme: 411
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Marion Leduc-Leballeur, Ghislain Picard, Giovanni Macelloni, Arnaud Mialon, Yann H. Kerr. (2020). Melt in Antarctica derived from Soil Moisture and Ocean Salinity (SMOS) observations at L band (Vol. 14).
Abstract: Melt occurrence in Antarctica is derived from L-band observations from the Soil Moisture and Ocean Salinity (SMOS) satellite between the austral summer 2010–2011 and 2017–2018. The detection algorithm is adapted from a threshold method previously developed for 19 GHz passive microwave measurements from the special sensor microwave imager (SSM/I) and special sensor microwave imager sounder (SSMIS). The comparison of daily melt occurrence retrieved from 1.4 and 19 GHz observations shows an overall close agreement, but a lag of few days is usually observed by SMOS at the beginning of the melt season. To understand the difference, a theoretical analysis is performed using a microwave emission radiative transfer model. It shows that the sensitivity of 1.4 GHz signal to liquid water is significantly weaker than at 19 GHz if the water is only present in the uppermost tens of centimetres of the snowpack. Conversely, 1.4 GHz measurements are sensitive to water when spread over at least 1 m and when present in depths up to hundreds of metres. This is explained by the large penetration depth in dry snow and by the long wavelength (21 cm). We conclude that SMOS and higher-frequency radiometers provide interesting complementary information on melt occurrence and on the location of the water in the snowpack.
Programme: 1110
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Dominic Saunderson, Andrew Mackintosh, Felicity McCormack, Richard Selwyn Jones, Ghislain Picard. (2022). Surface melt on the Shackleton Ice Shelf, East Antarctica (2003–2021) (Vol. 16).
Abstract: Melt on the surface of Antarctic ice shelves can potentially lead to their disintegration, accelerating the flow of grounded ice to the ocean and raising global sea levels. However, the current understanding of the processes driving surface melt is incomplete, increasing uncertainty in predictions of ice shelf stability and thus of Antarctica's contribution to sea-level rise. Previous studies of surface melt in Antarctica have usually focused on either a process-level understanding of melt through energy-balance investigations or used metrics such as the annual number of melt days to quantify spatiotemporal variability in satellite observations of surface melt. Here, we help bridge the gap between work at these two scales. Using daily passive microwave observations from the AMSR-E and AMSR-2 sensors and the machine learning approach of a self-organising map, we identify nine representative spatial distributions (“patterns”) of surface melt on the Shackleton Ice Shelf in East Antarctica from 2002/03–2020/21. Combined with output from the RACMO2.3p3 regional climate model and surface topography from the REMA digital elevation model, our results point to a significant role for surface air temperatures in controlling the interannual variability in summer melt and also reveal the influence of localised controls on melt. In particular, prolonged melt along the grounding line shows the importance of katabatic winds and surface albedo. Our approach highlights the necessity of understanding both local and large-scale controls on surface melt and demonstrates that self-organising maps can be used to investigate the variability in surface melt on Antarctic ice shelves.
Programme: 1110
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Kévin Fourteau, Patricia Martinerie, Xavier Faïn, Christoph F. Schaller, Rebecca J. Tuckwell, Henning Löwe, Laurent Arnaud, Olivier Magand, Elizabeth R. Thomas, Johannes Freitag, Robert Mulvaney, Martin Schneebeli, Vladimir Ya Lipenkov. (2019). Multi-tracer study of gas trapping in an East Antarctic ice core (Vol. 13).
Abstract:
Abstract. We study a firn and ice core drilled at the new “Lock-In” site in East Antarctica, located 136 km away from Concordia station towards Dumont d'Urville. High-resolution chemical and physical measurements were performed on the core, with a particular focus on the trapping zone of the firn where air bubbles are formed. We measured the air content in the ice, closed and open porous volumes in the firn, firn density, firn liquid conductivity, major ion concentrations, and methane concentrations in the ice. The closed and open porosity volumes of firn samples were obtained using the two independent methods of pycnometry and tomography, which yield similar results. The measured increase in the closed porosity with density is used to estimate the air content trapped in the ice with the aid of a simple gas-trapping model. Results show a discrepancy, with the model trapping too much air. Experimental errors have been considered but do not explain the discrepancy between the model and the observations. The model and data can be reconciled with the introduction of a reduced compression of the closed porosity compared to the open porosity. Yet, it is not clear if this limited compression of closed pores is the actual mechanism responsible for the low amount of air in the ice. High-resolution density measurements reveal the presence of strong layering, manifesting itself as centimeter-scale variations. Despite this heterogeneous stratification, all layers, including the ones that are especially dense or less dense compared to their surroundings, display similar pore morphology and closed porosity as a function of density. This implies that all layers close in a similar way, even though some close in advance or later compared to the bulk firn. Investigation of the chemistry data suggests that in the trapping zone, the observed stratification is partly related to the presence of chemical impurities. Programme: 1028
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