Improving the Accuracy of the SUrface Mass balance of Antarctica – ASUMA

The ASUMA project proposes to assess the SMB value of Antarctica, by filling the spatial gap that exists in the coast to plateau transition zone. For this task, we propose to conduct a scientific traverse in unexplored regions of Adelie Land in order to 1) get data from new ice cores, 2) make field observations on snow physics, and 3) deduce the relationship existing between field data, remote sensing data and regional climate model outputs. These three points are developed hereafter:
1) The spatial and temporal variability of SMB will be obtained through the cross-comparison of signals stored in water stable isotopes and snow chemistry. Besides their use for dating the ice core, they will provide key information to detect changes in moisture origin and post-depositional effects. Both factors are important to understand the key drivers of changes in SMB, and provide independent constraints for evaluating the performance of atmospheric models with respect to atmospheric transport.
2) ASUMA aims to determine the existing relationship between surface snow physics linked with SMB variations and remote sensing signals, with a focus on radar altimetry. Our strategy is therefore to obtain key information on surface roughness, which controls surface echo, and on density and temperature depth profiles, which control volume echo. Interpretation of these snow physical characteristics requires to understand the metamorphism of surface snow.
3) ASUMA proposes to implement a combined analysis based on the SMB modeling and results from the distributed SMB maps produced by field and satellite data interpretation. Information on air masses will be necessary to test model capability to correctly reproduce the processes observed in the field. Modeling approach will focus on bias-corrected regional scale modeling with LMDZ model, and on the accurate representation of SMB processes using a regional circulation model (MAR).

1) The signal to noise ratio is critical for climatic interpretation of firn cores. This ratio was analyzed in different cores from the coastal region. A 22.4 m-long shallow core extracted at 5 km from the coast offered to jointly study the d18O, the chemical contents and accumulation over 60 years (1947-2007), showing that a single core correctly captures major annual anomalies and the multi-decadal variations in d18O (Goursaud et al., Submitted). Moreover, multiple year-round records of bulk and size-segregated compositions of aerosol were analyzed at Dumont d’Urville and Concordia sites, to document the sea-salt aerosol load and composition (Legrand et al., 2015). This information is crucial to retrieve the origin of air masses in cores.
2) Moreover, we developed and tested in Antarctica the instruments used to study the surface snow physics during the long traverse. This concerns sensors designed to assess the surface roughness and albedo, and the solar radiation extinction in snow. All these sensors will be used to get information for a radiative transfer model used to interpret altimeter radar backscatter and passive micro-wave data from satellite.
3) Except for the positive phase in the Southern Annular Mode (SAM+), the recent climatic trends in Antarctica are within the internal climatic variability and are incorrectly reproduced by climate models (Jones et al., 2016). In the southern Indian Ocean, the SAM+ trend induced a shift in the storm track from Kerguelen region before 1975 to a region located near Law Dome afterward (Favier et al., 2016). The quality of the regional circulation model MAR and of the snow flux transport routine were tested using measurements in the coastal region. High temperature controls the sintering of the surface snow and the variations of the surface drag coefficients over sastrugi. For the first time, it is now possible to reproduce the observed transported snow flux at one site in Antarctica (Amory et al., 2016).

The overarching goal of this project will be the realization of a long distance traverse in the coastal region of Adelie Land in order to perform original observations in an unexplored region of Antarctica. This field campaign has not been performed yet.

The most important results will thus be obtained after the 2016-17 summer field campaign (December 2016). The main communication media will be used during this traverse to inform the scientific and non scientific audiences. This includes: information on the website of the project, interaction with school where children will interact with the traverse team, intervention in radio programs, and movies.

However, one outstanding objective was already reached. Indeed, we developed several sensors to assess the surface roguhness length (laserscan), the Specific Surface Area (SSA - POSSSUM instrument), the surface albedo (Multiband and solalb instruments) and the solar radiation extinction in the sub-surface (Solex instrument). We also tested a system for continuous flow analysis of water isotopes in firn cores. This equipment will be applied to the shallow cores collected along the transect in order to significantly reduce preparation time and analytical load on our instrument.

Peer reviewed journals:

1. Legrand, M., X. Yang, S. Preunkert, and N. Theys (2016) Year-round records of sea salt, gaseous, and particulate inorganic bromine in the atmospheric boundary layer at coastal (Dumont d'Urville) and central (Concordia) East Antarctic sites, Journal of geophysical research-atmospheres, 121, 997 – 1023. [doi:10.1002/2015JD024066].
2. V. Favier, et al. (2016) Atmospheric drying as the main driver of dramatic glacier wastage in the southern Indian Ocean, Scientific reports, 6, 32396. [doi:10.1038/srep32396]
3. Amory, C., et al. (2016) Seasonal variations in drag coefficients over a sastrugi-covered snowfield of coastal East Antarctica, Boundary Layer Meteorology, in press.
4. Jones, J.M., et al. (2016) Assessing recent trends in high-latitude Southern Hemisphere surface climate, Nature Climate Change, 6, 917–926, doi:10.1038/nclimate3103
5. Goursaud, S., et al., A sixty year ice-core record of regional climate from Adélie Land, coastal Antarctica, The Cryosphere Discussion, submitted

International conferences:

1. V. Favier, et al. (2015) Climatic information from unexplored areas of East Antarctica: The French ITASE Contribution, “Our Common Future under Climate Change” Conference, 7-10 July 2015, Paris, France.
2. V. Favier, et al. (2016) The French ITASE Contribution - Recent results, XXXIV SCAR Biennial Meetings and the 2016 Open Science Conference, 19-28 August 2016, Kuala Lumpur, Malaysia
3. Amory, C. et al.(2016) Modelling aeolian transport of snow at a coastal location of East Antarctica using a temperature-dependent parameterization for surface roughness, XXXIV SCAR Biennial Meetings and the 2016 Open Science Conference, 19-28 August 2016, Kuala Lumpur, Malaysia

Antarctica represents the largest ice reservoir on the Earth. In the context of climate change, this amount of may play a major role for the evolution of sea level. The knowledge of current variations of the Antarctic surface mass balance (SMB) is thus a global challenge. However, the integrated value of SMB still lacks from solid constraints for Antarctica and the fifth IPCC assessment report (AR5) highlighted this uncertainty as one of the main scientific challenges in climate science.

The ASUMA project proposes to assess the integrated SMB value over Antarctica, by filling the gap that exists in the coast to central plateau transition zone, where large variations of SMB are observed within small distances. In the plateau, satellite data have been calibrated for interpretation in SMB. However, snow metamorphism occurring in the transition region induces distortions in surface characteristics preventing interpretation of satellite data in terms of SMB. In this project, we propose an alternative method for quantifying the recent variations of SMB and their relationship with climate, atmospheric circulation and moisture origin. We will perform original field measurements of SMB and snow physics and robustly link them to satellite data. We will combine this information with the use of back-trajectories and regional to global modeling.
Three field trips are planned during successive austral summers. Two field trips are planned in the first 50 km from the coast to study melting areas, and a long distance traverse is proposed for the 2016-17 summer, to collect 8 firn cores (25-100m each) and samples in about 10 snow pits. To retrieve robust estimate of the mean level and variability of SMB during last decades, we will accurately date firn cores from radiochemistry analyses, especially in the region where melting and erosion are identified. Interpolation of SMB data will be done with ground penetrating radar and satellite data. For this task, understanding how to assess the SMB at a given point with the help of satellite will be crucial.

Thus, we will 1) perform an accurate analysis of surface snow physics in the study area, 2) define the physical relationships between the surface physical characteristics and the remote sensing signal, 3) validate SMB outputs form regional atmospheric circulation models.
This will allow to answer to four overarching questions:
1. How does the SMB vary in the coast-to-plateau transition zone?
2. What are the processes responsible for these differences and how does the origin and transport of moisture affect the signal stored in firn and ice?
3. How are snow physical characteristics related to the local SMB?
4. How can remote sensing data be best used to infer snow physical characteristics and SMB?
5. What are recent and future SMB distributions over Antarctica?

The ASUMA project proposes to combine numerous field data and original techniques to better constrain remote sensing data and modeling results in the transition zone from the coast to the Antarctic plateau. Innovative methodologies will be deployed for grains size measurements, for continuous roughness estimates and for reflectance measurements. The combined information of snow physics, chemistry and isotopic content will be used to constrain output of general circulation models that will in turn be used for estimating future Antarctic SMB. The project will also train two PhD students which will be co-supervised within three project partners providing complementary expertise, and reinforcing strong collaborations between three laboratories.

The ASUMA project represents the glaciological part of a long term target of the French climate and cryosphere community, which aims to reduce uncertainties of the mass balance estimates of major ice caps, in order to better understand recent and future changes in sea level rise and Antarctic climate and water cycle.