Assessment of Arctic Sea Ice Thickness Retrieval Ability of the Chinese HY-2B Radar Altimeter

Abstract. In the context of global warming, sea ice changes have received increasing attention as "indicators" and "amplifiers" of climate change. With the development of satellite altimeters, satellite altimeter technologies have been increasingly used to retrieve Arctic sea ice thicknesses and have achieved rapid development and application. At present, the CryoSat-2 radar altimeter and Ice, Cloud and land Elevation Satellite-2 (ICESat-2) laser altimeter are the main data sources used in Arctic sea ice thickness retrievals. With the continuous development of the China Ocean Dynamic Environment Satellite Series (HY-2), it is of great significance to explore the potential application of this dataset in Arctic sea ice thickness retrievals. In this study, we first estimated the Arctic radar freeboard and sea ice thickness values during two sea ice growing cycles (from October 2019 to April 2020 and from October 2020 to April 2021) using the China HY-2B radar altimeter and then compared the results with the Alfred Wegener Institute (AWI) CryoSat-2 sea ice freeboard and sea ice thickness products recorded during the same period. The accuracies of the HY-2B radar freeboard and sea ice thickness were then verified with the Operation IceBridge (OIB) airborne data and ICESat-2 laser altimeter data, and the random uncertainties in the HY-2B sea ice freeboard and sea ice thickness results were finally estimated. Although the spatial distributions of the HY-2B radar freeboard and sea ice thickness results agreed well with those of AWI CryoSat-2, the deviation between the HY-2B radar freeboard and CryoSat-2 radar freeboard data was within 2 cm, while the deviation between the HY-2B sea ice thickness data and CryoSat-2 sea ice thickness data was within 0.2 m. In addition, the growth trends of the HY-2B radar freeboard and sea ice thickness were slower than those of AWI CryoSat-2. This finding was related to the applied sea surface height anomaly (SSHA) extraction method. Comparisons with the OIB sea ice freeboard and sea ice thickness values recorded in April 2019 showed that the correlation between the HY-2B sea ice freeboard retrievals and OIB sea ice freeboard data was 0.58, the root mean square error (RMSE) was 0.17 m, and the mean absolute error (MAE) was 0.14 m. The correlation between the HY-2B sea ice thickness retrieval and OIB sea ice thickness data was 0.41, the RMSE was 2.05 m, and the MAE was 1.91 m. Based on the Gaussian error propagation theory, we estimated the uncertainties of the HY-2B sea ice freeboard and sea ice thickness data: the uncertainty of the former ranged from 8.5 cm to 12.0 cm, while the uncertainty of the latter ranged from 26.8 cm to 37.7 cm. Due to the influence of the SSHA uncertainty (σSSA) and the number of observation points inside the grid, the uncertainties in the HY-2B sea ice freeboard and sea ice thickness data were higher at low latitudes than at high latitudes.