Intercomparing Arctic winter radar and laser freeboards obtained from CryoSat-2, AltiKa and ICESat-2 over the common mission period October 2018 - April 2021

A growing number of satellite altimeters with high orbital inclination are now operating concurrently over the polar regions. The range of sensing configurations and frequencies afforded by these missions offers the potential for detecting geophysical properties of sea ice beyond the ice freeboard. Here, we apply a physical model for the backscattered radar altimeter echo over sea ice to the ESA Ku-band mission CryoSat-2 operating in SAR mode and to the ISRO/CNES Ka-band mission AltiKa SARAL operating in pulse-limited low-resolution mode (LRM). Radar scattering theory indicates that the Ku-band sensor may detect the snow-sea ice interface and the Ka-band sensor may detect the air-snow interface under certain snow conditions. However, previous studies have discovered that the radar's principal scattering horizon at both frequencies can depend on variable snow properties such as the layering and basal salinity. Our physical model confirms the role of an additional variable: the sea ice surface roughness, for identifying the principal radar scattering horizon at both Ku- and Ka-band frequencies, supporting the findings of Lawrence et al. (The Cryosphere, 2018). The surface roughness statistics must be properly represented in the altimeter echo model to accurately identify the CryoSat-2 SAR-mode principal scattering horizon. It is even more important for altimeters such as AltiKa operating in LRM-mode, where our model shows the theoretical difference between the mean footprint surface level and principal scattering horizon widens significantly with increasing roughness. We fit the model simulations, which aim to account for the roughness bias, to observed AltiKa waveforms over Arctic sea ice during winters 2018-19, 2019-20 and 2020-21 and compare radar freeboards from AltiKa to coinciding laser freeboards from NASA's ICESat-2 (see attached plot for Dec 2019). Differences between AltiKa/ICESat-2 freeboards and CryoSat-2 freeboards give a first approximation for the pan-Arctic snow depth. We will discuss these differences in the context of variable snow penetration depths and consider implications for the upcoming EU dual-frequency polar altimetry mission CRISTAL. This is a contribution to the ESA Polar+ Snow on Sea Ice Project.