Retrieval of snow freeboard of Antarctic sea ice using waveform fitting of CryoSat-2 returns

Abstract

Abstract. In this paper we develop a CryoSat-2 algorithm to retrieve\nthe surface elevation of the air–snow interface over Antarctic sea ice. This algorithm\nutilizes a two-layer physical model that accounts for scattering from a snow layer atop\nsea ice as well as scattering from below the snow surface. The model produces waveforms\nthat are fit to CryoSat-2 level 1B data through a bounded trust region least-squares\nfitting process. These fit waveforms are then used to track the air–snow interface and\nretrieve the surface elevation at each point along the CryoSat-2 ground track, from which\nthe snow freeboard is computed. To validate this algorithm, we compare retrieved surface\nelevation measurements and snow surface radar return power levels with those from\nOperation IceBridge, which flew along a contemporaneous CryoSat-2 orbit in October\xa02011\nand November\xa02012. Average elevation differences (standard deviations) along the flight\nlines (IceBridge Airborne Topographic Mapper, ATM – CryoSat-2) are found to be 0.016\u2009cm\n(29.24\u2009cm) in 2011 and 2.58\u2009cm (26.65\u2009cm) in 2012. The spatial distribution of monthly\naverage pan-Antarctic snow freeboard found using this method is similar to what was\nobserved from NASA s Ice, Cloud, and land Elevation Satellite (ICESat), where the\ndifference (standard deviation) between October 2011–2017 CryoSat-2 mean snow freeboard\nand spring 2003–2007 mean freeboard from ICESat is 1.92\u2009cm (9.23\u2009cm). While our\nresults suggest that this physical model and waveform fitting method can be used to\nretrieve snow freeboard from CryoSat-2, allowing for the potential to join laser and\nradar altimetry data records in the Antarctic, larger ( ∼30 \u2009cm) regional\ndifferences from ICESat and along-track differences from ATM do exist, suggesting the\nneed for future improvements to the method. Snow–ice interface elevation retrieval is\nalso explored as a potential to obtain snow depth measurements. However, it is found that\nthis retrieval method often tracks a strong scattering layer within the snow layer\ninstead of the actual snow–ice interface, leading to an overestimation of ice freeboard\nand an underestimation of snow depth in much of the Southern Ocean but with promising\nresults in areas such as the East Antarctic sector.