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CASSIS (Climate Altimetric Studies with Sea Ice and Snow)



Since the 1970s, spatial imagery has allowed to witness dramatic changes in sea ice extension and distribution. However, the sea ice thickness (SIT), which is a fundamental variable to understand and predict sea ice dynamics, is still insufficiently observed. So far, the only possibility to observe the sea ice freeboard (the sea ice emerged part which is measured by altimetry) at a global scale is to use spatial altimetry. However, despite recent advances mainly coming from the development of SAR altimeters on-board the CryoSat-2 and Sentinel-3A&B satellites, sea ice freeboard measurements can not provide reliable climate time series. As a consequence, sea ice thickness satellite data can hardly be integrated into sea ice models for climate studies. In order to improve sea ice freeboard and thickness measurements from altimetry, this project will address the following points:

  • The use of radar echo processing methods (also called ‘retrackers’) based on physical models instead of current heuristic models.
  • The use of new snow depth observation products (dual-frequency altimetry, Advanced Microwave Scanning Radiometer, AMSR),
  • A better quantification of uncertainties in sea ice products.
  • As a first step, we will compute sea ice freeboard and thickness in Antarctica using a classical processing chain previously validated in Arctic (e.g., Guerreiro et al 2017, Paul et al 2018).This will provide a first sea ice data reference based on the state of the art covering the Envisat and CryoSat-2 time period (2002-2019). Note that the bi-frequency Ka/Ku snow depth data, developed at the LEGOS within the ESA CryoSat-2 +Antarctica, will be used. These results will then support the launch of the future Copernicus CRISTAL satellite mission
  • The second step is to substitute the heuristic TFMRA retracker (Helm et al, 2014) by the SAMOSA+ physical retracker outputs provided through the ESA GPOD   cluster in the processing chain. The relevancy of the methodology used at the LEGOS to derive freeboard estimates has already been demonstrated in Arctic within the Cryo-seaNice ESA project (Laforge et al 2019). We will apply the same procedure in the southern ocean. In addition, a SLA product aiming to ocean circulation studies will be provided.
  • On a third step these new sea ice altimetric products will be compared with the LIM-3 CMEMS sea ice model. This work will be carried out in cooperation with MERCATOR-Ocean. The analyses will allow to identify the possible biases of the model and to re-adjust the level of uncertainty of sea ice observation products. The objective is to perform a preliminary work in the perspective of data assimilation and OSE’s experiments.
  • Finally, we plan to use this synergy to provide a new estimation of global sea ice volume variability and changes. Our scientific objective is to re-evaluate the ocean global freshwater budget to provide, for the first time, a new constraint to the land ice contribution to sea level rise. Note that this exciting challenge was considered impossible in Monk, 2003.


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