Publication

Abstract : Antarctic sea ice extent hit record lows in 2022 and 2023, having shown a small positive trend until 2015. To estimate sea ice volume and investigate volume trends also requires thickness observations, often from satellite altimetry. The thick and complex snow packs atop Antarctic sea pose a variety of challenges for remote sensing, which relies upon understanding the interaction of electromagnetic radiation with the snow and sea ice. A particular challenge is retrieval of sea ice thickness via satellite radar altimetry due to uncertainty around penetration and scattering of the transmitted radiation, and hence, interpretation of waveforms from Cryosat-2's Ku-band altimeter, and SARAL-AltiKa's Ka-band altimeter. These issues with observations of sea ice thickness introduce additional uncertainty in estimates of Antarctic sea ice volume. Most sea ice around Antarctica is seasonal, but in the Weddell and Ross Seas sea ice can survive the summer melt. Data were collected in the Weddell Sea in March 2022 over floes where the sea ice was overlaid with a thick cover of remnant, heavily modified snow from the previous winter, and atop that, a layer of more recent snow from that winter. The surface-based, fully-polarimetric Ku- and Ka-band 'KuKa' radar was deployed and coincident data were collected on snow characteristics. KuKa's Ka- and Ku- band operating frequencies ~match those of satellite altimeters (CryoSat-2, Altika and CRISTAL), but its 1.5-2.5 cm range resolution permits detailed analyses of the influence of snow properties on radar backscatter. This study therefore provides a unique perspective on how older Antarctic sea ice and its snow cover may backscatter radiation with these frequencies and polarisations. We find that generally radar returns show strong backscatter between that season's snowfall and the higher-density snow remaining from the previous winter, but the interface between the remnant snow and sea ice does shows relatively weaker backscatter. Our findings have key implications for interpretation of radar altimetry data over Antarctic sea ice.