Dual-frequency radar observations of snowmelt processes on Antarctic perennial sea ice by CFOSCAT and ASCAT

Abstract. Since 2017, Antarctic sea ice coverage has shown significant reductions, and therefore observations of its surface melting behavior are of utmost importance. Here we study the capability of the Ku-band Chinese-French Oceanography Satellite Scatterometer (CFOSCAT) launched in 2018 to detect surface melting and compare it with more established observations of the C-band Advanced Scatterometer (ASCAT) in orbit since 2007. Both CFOSCAT and ASCAT observations show increases in radar backscatter of more than 2 dB over perennial ice once the ice surface warms and destructive snow metamorphism commences, defined as pre-melt onset (PMO). Backscatter increases by more than 3 dB once prominent thaw–freeze cycles commence, defined as snowmelt onset (SMO). Scatterometer data are compared with drifting buoy and ERA5 reanalysis air temperature data to support the interpretation of melt-related snow processes. Between 2019 and 2022, the average CFOSCAT pre-melt and snowmelt onset dates for 12 perennial ice study regions are 9 November (±23 d) and 1 December (±22 d), earlier than those of ASCAT on 21 November (±22 d) and 11 December (±25 d), respectively. Sensitivity tests show that results depend slightly on chosen backscatter thresholds but little on sea ice concentration. The derived SMOs are in good agreement with previous studies, but the SMO difference between dual-frequency radar observations is smaller than that reported by previous studies due to the sensor differences and different spatiotemporal resolutions. SMO differences between dual-frequency radar observations were also found to be potentially related to regional differences in snow metamorphism. With regard to the long-term changes in SMO, there are strong interannual and regional variabilities in SMO changes, and no consistent changes could be detected among different sub-regions with the beginning of Antarctic sea ice decline after 2015. Dual-frequency CFOSCAT and ASCAT observations hold strong promise for a better understanding of snowmelt processes on Antarctic sea ice, and it is necessary to extend the observation of Antarctic snowmelt based on dual-frequency scatterometers.