Snow effects on altimeter waveforms over sea ice in the Weddell Sea - Part II: sea ice and snow spaceborne retrieval

Abstract

Snow atop Antarctic sea ice plays a critical role in modulating sea ice growth, surface energy balance, and ocean–atmosphere interactions. However, it also introduces substantial uncertainty into satellite altimeter-based sea ice thickness (SIT) estimates. Ku-band radar altimeters, such as CryoSat-2 (CS-2), are often processed using threshold-based retrackers that implicitly assume the maximum radar intensity return originates near the snow–ice interface. In practice, layered snowpacks featuring wet snow, brine infiltration, and ice lenses can shift the primary scattering contribution upward, leading to overestimated freeboard and higher SIT estimates. In Part I of this study, we used physically based waveform decomposition to quantify the vertical distribution of radar backscatter under Weddell Sea conditions. Building on these insights, Part II introduces an optimized threshold first-maximum retracker algorithm (TFMRA) for CS-2, tuned using airborne observations from NASA’s Operation IceBridge (OIB) over the Weddell Sea. We identify a 70% retracking threshold that minimizes freeboard bias and improves consistency with independent observations. Applying this snow-aware retracker to 46 CRYO2ICE collocated tracks (2020–2022), we retrieve snow depth from the ICESat-2 and CS-2 freeboard difference and reduce mean SIT by ∼[jls-end-space/]0.1 m relative to the ESA Baseline-E product in the southern Weddell Sea. Monte-Carlo (MC) perturbations of OIB snow retrievals, combined with CS-2 threshold-sensitivity tests, indicate an intrinsic ∼[jls-end-space/]0.2 m uncertainty in OIB snow depth and a similar lower-bound CRYO2ICE snow-depth uncertainty of ∼[jls-end-space/]0.21-0.24 m at 10 km scales. Our results offer practical guidance for altimeter algorithm development and are directly relevant to upcoming dual-frequency radar missions such as ESA’s CRISTAL.