Exploring microwave penetration into snow on Antarctic summer sea ice along CryoSat-2 and ICESat-2 (CRYO2ICE) orbit from multi-frequency air- and spaceborne altimetry

Abstract. The recent alignment of CryoSat-2 to maximise orbital coincidence with the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) over the Southern Ocean and Antarctica in July 2022, known as the CryoSat-2 and ICESat-2 (CRYO2ICE), provided an opportunity to ground truth these satellites over the same land and sea ice. This was achieved through a simultaneous airborne campaign which under-flew the near-coincident CryoSat-2 and ICESat-2 orbits in December 2022 and carried, amongst other instrumentation, Ka-, Ku-, C/S-band radars and a scanning near-infrared lidar. This campaign resulted in the first multi-frequency radar evaluation of snow penetration over sea ice along near-coincident orbits. The airborne observations (at footprints of 5 m) revealed limited penetration of the snow pack at both Ka- and Ku-band, with the primary scattering occurring either at the air-snow interface or inside the snowpack for both frequencies. On average, the Ka- and Ku-band scattering interface was 0.2 to 0.3 m above that for C/S-band's primary scatter, where the average snow depth using C/S-band reached around 0.5 ± 0.05 m depending on re-trackers and combinations used. Interestingly, when the primary peak in the received signal occurs within the snowpack or at the air-snow interface, some scatter contributions are still present from the sea-ice interface at Ku-band. This suggests a potential for snow depth to be derived from Ku-band signals alone by co-identifying these respective peaks in the waveform. Furthermore, it contradicts the assumption of a single scattering interface primarily contributing to the backscatter at Ku-band on airborne scales. The CRYO2ICE snow depths achieved along the orbit were on average 0.34 m, which are within 0.01 m from passive-microwave-derived observations and 0.12 m from model-based estimates. 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 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.