Anticipating CRISTAL: an exploration of multi-frequency satellite altimeter snow depth estimates over Arctic sea ice, 2018–2023

The EU and ESA plan to launch a dual-frequency Ku- and Ka-band polar-orbiting synthetic aperture radar (SAR) altimeter, the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL), by 2027 to monitor polar sea ice thickness (SIT) and its overlying snow depth, among other applications. However, the interactions of Ku- and Ka-band radar waves with snow and sea ice are not fully understood, demanding further research effort before we can take full advantage of the CRISTAL observations. Here, we use three ongoing altimetry missions to mimic the sensing configuration of CRISTAL over Arctic sea ice and investigate the derived snow depth estimates obtained from dual-frequency altimetry. We apply a physical model for the backscattered radar altimeter echo over sea ice to CryoSat-2's (CS2's) Ku-band altimeter in SAR mode and to the SARAL mission's AltiKa (AK) Ka-band altimeter in low-resolution mode (LRM), and then we compare it to reference laser altimetry observations from ICESat-2 (IS2). ICESat-2 snow freeboards (snow + sea ice) are representative of the air–snow interface, whereas the radar freeboards of AltiKa are expected to represent a height at or close to the air–snow interface, and CryoSat-2 radar freeboards are expected to represent a height at or close to the snow–ice interface. The freeboards from AltiKa and ICESat-2 show similar patterns and distributions; however, the AltiKa freeboards do not thicken at the same rate over winter, implying that Ka-band height estimates can be biased low by 10 cm relative to the snow surface due to uncertain penetration over first-year ice in spring. Previously observed mismatches between radar freeboards and independent airborne reference data have frequently been attributed to radar penetration biases, but they can be significantly reduced by accounting for surface topography when retracking the radar waveforms. Waveform simulations of CRISTAL in its expected sea ice mode reveal that the heights of the detected snow and ice interfaces are more sensitive to multi-scale surface roughness than to snow properties. For late-winter conditions, the simulations suggest that the CRISTAL Ku-band radar retrievals will track a median elevation 3 % of the snow depth above the snow–ice interface because the radar return is dominated by surface scattering from the snow–ice interface which has a consistently smoother footprint-scale slope distribution than the air–snow interface. Significantly more backscatter is simulated to return from the air–snow interface and snow volume at Ka band, with the radar retrievals tracking a median elevation 10 % of the snow depth below the air–snow interface. These model results generally agree with the derived satellite radar freeboards, which are consistently thicker for AltiKa than CryoSat-2, across all measured snow and sea ice conditions.