Multi-frequency altimetry snow depth estimates over heterogeneous snow-covered Antarctic summer sea ice – Part 2: Comparing airborne estimates with near-coincident CryoSat-2 and ICESat-2 (CRYO2ICE)

For the first time, a comparison of altimetry-derived snow depth estimates between dual-frequency spaceborne and near-coincident multi-frequency airborne estimates is conducted using data from the recent under-flight of a CryoSat-2 and ICESat-2 (CRYO2ICE) orbit by a simultaneous airborne campaign over the Weddell Sea in December 2022 carrying Ka-, Ku-, C/S-band radars and a scanning near-infrared lidar. From this unique combination of airborne sensors, the accuracy of snow depth captured by the near-coincident CRYO2ICE orbits can be evaluated. The CRYO2ICE snow depth achieved along the orbit was, on average, 0.34 m, which is within 0.01 m from passive-microwave-derived observations and 0.12 m from a model-based estimate. The retrieval methodology appears to play a significant role, which we suspect is highly dependent on the classification and filtration schemes applied to remove potentially ambiguous altimetry observations. Comparison with airborne snow depths at 25 km segments showed correlations of 0.51–0.53, a bias of 0.03 m, and root-mean-square deviation of 0.08 m when using the airborne lidar scanner as air–snow interface and C/S-band at maximum amplitude at the snow–ice interface. To understand how comparisons across ground, air, and space shall be conducted, especially in preparation for the upcoming dual-frequency radar altimeter mission Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL), it is critical that we investigate the impact of different scattering mechanisms at varying frequencies for diverging viewing geometries considering dissimilar spatial and range resolutions.