Assessing CryoSat‐2 Antarctic Snow Freeboard Retrievals Using Data From ICESat‐2.

NASA's Ice, Cloud, and land Elevation Satellite‐2 (ICESat‐2) laser altimeter launched in Fall 2018, providing an invaluable addition to the polar altimetry record generated by ESA's CryoSat‐2 radar altimeter. The simultaneous operation of these two satellite altimeters enables unique comparison studies of sea ice altimetry, utilizing the different frequencies and profiling strategies of the two instruments. Here, we use freeboard data from ICESat‐2 to assess Antarctic snow freeboard retrievals from CryoSat‐2. We first discuss updates made to a previously published CryoSat‐2 retrieval process and show how this Version 2 algorithm improves upon the original method by comparing the new retrievals to ICESat‐2 in specific along‐track profiles as well as on the basin‐scale. In two near‐coincident along‐track profiles, we find mean snow freeboard differences (standard deviations of differences) of 0.3 (9.3) and 7.6 cm (9.6 cm) with 25 km binned correlation coefficients of 0.77 and 0.89. Monthly mean freeboard differences range between −2.9 (10.8) and 6.6 cm (16.8 cm) basin wide, with the largest differences typically occurring in Austral fall months that is hypothesized to be related to new ice growth and the use of static snow backscatter coefficients in the retrieval. Monthly mean correlation coefficients range between 0.57 and 0.80. While coincident data show good agreement between the two sensors, they highlight issues related to geometric and frequency sampling differences that can impact the freeboard distributions. Plain Language Summary: Measuring sea ice freeboard from space is an important first step in estimating its thickness. A previous study had developed a new method of measuring freeboard over Antarctic sea ice using ESA's CryoSat‐2 altimeter, however, few validation data existed at the time to determine how well it performed. In this paper, we improve the CryoSat‐2 processing and make use of data from NASA's ICESat‐2 altimeter for comparisons with the CryoSat‐2 data. While agreement is strong overall, there are still differences between the measurements that we hypothesize come from the different footprint sizes and wavelengths of the two instruments. Key Points: We present an updated CryoSat‐2 Antarctic snow freeboard retrieval methodThese improved CryoSat‐2 snow freeboard retrievals show strong agreement with ICESat‐2 data both along‐track and basin‐wideThis assessment highlights difficulties in laser‐radar comparisons brought on by frequency and geometric sampling discrepancies [ABSTRACT FROM AUTHOR]