First results of joint sea surface salinity, temperature, and wind speed retrieval from MICAP Simultaneous L/C/K-band active and passive observations

The Microwave Imager Combined Active and Passive (MICAP) onboard the Chinese Ocean Salinity Satellite (HY-4A) is a novel spaceborne payload developed for the simultaneous retrieval of Sea Surface Salinity (SSS), Sea Surface Temperature (SST), and Sea Surface Wind Speed (WS). MICAP uniquely combines L-band (1.4 GHz), C-band (6.9 GHz), and K-band (18.7 GHz) one-dimensional (1-D) interferometric synthetic aperture radiometers with an L-band Digital Beam-Forming (DBF) scatterometer. This paper presents the Level-2 retrieval algorithms and preliminary validation results obtained during the early in-orbit verification phase of the MICAP ocean products. The retrieval framework adopts a two-step strategy, including an L-band only SSS retrieval scheme (Mode-1) and a combined multi-parameter retrieval scheme (Mode-2). In Mode-2, the simultaneous C/K-band measurements are first used to derive estimates of SST and WS. These auxiliary parameters are subsequently combined with the L-band radiometer and scatterometer measurements to perform the final SSS and WS retrieval. Preliminary validation against Argo observations shows an SSS RMSE of approximately 0.58 psu for the Mode-2 retrieval over the screened open ocean validation sample. The retrieved SST and WS products show RMSEs of 0.80 K and 1.17 m/s, respectively, when compared with iQuam and TAO buoy data. These results provide an initial in-orbit demonstration of simultaneous multi-parameter retrieval using the MICAP multi-frequency active and passive observations.