Multi-frequency altimetry snow depth estimates over heterogeneous snow-covered Antarctic summer sea ice – Part 1: C∕S-, Ku-, and Ka-band airborne observations

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) Resonance Campaign, provided an opportunity to validate these satellites over land and sea ice. This was achieved through a simultaneous airborne campaign which involved an under-flight for near-coincident CryoSat-2 and ICESat-2 orbits in December 2022 and carried, amongst other instrumentation, Ka-, Ku-, C/S-band radars as well as 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 snowpack at both Ka-band 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 interfaces were 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 retrackers 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 the 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 for Ku-band (and, to some extent, Ka-band) at airborne scales. The validity of this assumption needs further evaluation using former campaigns covering different sea ice conditions and seasons. With the unique combination of sensors and methods evaluated here, a shortcoming is the limited validation that can take place without strategically placed coincident in situ efforts. We call for coincident field initiatives as part of future validation campaigns considering the observational capabilities of airborne and spaceborne sensors when deciding on appropriate sampling strategies.