Benefits of assimilating thin sea-ice thickness from SMOS-Ice into the TOPAZ system

Abstract. An observation product for thin sea ice thickness (SMOS-Ice) is derived from the brightness temperature data of the European Space Agency's (ESA) Soil Moisture and Ocean Salinity (SMOS) Mission, and available in real-time at daily frequency during the winter season. In this study, we investigate the benefit of assimilating SMOS-Ice into the TOPAZ system. TOPAZ is a coupled ocean-sea ice forecast system that assimilates SST, altimetry data, temperature and salinity profiles, ice concentration, and ice drift with the Ensemble Kalman Filter (EnKF). The conditions for assimilation of sea ice thickness thinner than 0.4 m are favorable, as observations are reliable below this threshold and their probability distribution is comparable to that of the model. Two paralleled runs of TOPAZ have been performed respectively in March and November 2014, with assimilation of thin sea ice thickness (thinner than 0.4 m) in addition to the standard ice and ocean observational data sets. It is found that the RMSD of thin sea-ice thickness is reduced by 11 % in March and 22 % in November suggesting that SMOS-Ice has a larger impact during the beginning of freezing season. There is a slight improvement of the ice concentration and no degradation of the ocean variables. The Degrees of Freedom for Signal (DFS) indicate that the SMOS-Ice contents important information (> 20 % of the impact of all observations) for some areas in the Arctic. The areas of largest impact are the Kara Sea, the Canadian archipelago, the Baffin Bay, the Beaufort Sea and the Greenland Sea. This study suggests that SMOS-Ice is a good complementary data set that can be safely included in the TOPAZ system as it improves the ice thickness and the ice concentration but does not degrade other quantities. Keywords: SMOS-Ice; EnKF; OSE; thin sea-ice thickness; DFS;