Interpreting the Altimetric Radar Cross Section of Young Sea Ice

Ku-band radar altimeters such as the one onboard CryoSat-2 measure the return-time of backscattered radar pulses to derive sea ice freeboard and thickness. This method relies on the two-way penetration of radar waves through the overlying snow. Early research with a surface-based radar over tank-grown sea ice indicated that most radar power is returned from the sea ice surface rather than the overlying snow, but this has not been replicated by surface-based radars in the field. We present a 24-day timeseries of surface-based radar measurements from young Antarctic sea ice (growing from 20 to 36 cm in thickness),
showing a progressive reduction in its normalised radar cross section (σ0) over time. This is associated with weakening of very strong, single-peaked waveforms associated with specular reflection from the ice surface; these initially swamp the signal from the snow surface but rapidly diminish with snow and ice
ageing. Our observations help resolve the apparent contradiction between previous surface-based studies, and are consistent with an early-winter decline in σ0 observed by CryoSat-2. Our results also highlight the suppressing effect of frost flowers on σ0, and the similarity of σ0 evolution between the Ku- and Ka-bands. Our findings are relevant to the capability of the planned Ku/Ka-band CRISTAL altimetry mission, which aims to leverage differential radar penetration through snow on sea ice.