FY-4A/AGRI Sea Surface Temperature Product and Quality Validation

Fengyun-4A (FY-4A) is the first satellite of China's second-generation geostationary orbit meteorological satellites. The advanced geostationary radiation imager (AGRI), a multiple channel radiation imager, is one of the primary payloads onboard FY-4A. As one basic quantitative remote sensing product, the operational sea surface temperature (SST) is derived with the split-window nonlinear SST (NLSST) algorithm in real time. The operational NLSST procedures and products are described in detail. The FY-4A/AGRI SST products provide full-disk SST with spatial resolution of 4 km at the nadir. The quality level information is delivered with each pixel to provide information about the conditions of the processing. Quality level for each pixel information is defined as follows: Excellent, good, bad and unprocessed (cloud, land, no satellite data etc.). The users can select the SST data with certain quality level according to their application purposes (e.g., for climate-related studies, only the SST data with the excellent quality level in the time series are used, and for identifying and tracking specific ocean features, users may be more tolerant of lower-quality SST data). The accuracy of FY-4A/AGRI SST algorithm is assessed by determining the standard deviation and bias errors from the regression procedure of the matchup database between satellite data and quality controlled SST data from NOAA in situ SST quality monitor, from July 2021 to June 2022. The validation methods and results are described in detail. The matchup database combining FY-4A/AGRI measurements and in situ SST have been built on a routine basis. At the stage of the matchup database in association with the drifter temperatures, the matchup database is composed of pixels under clear sky conditions. The FY-4A/AGRI SST data with excellent quality level are compared with drifting and tropical moored buoy data. The matchup space-time window is 4 km and 30 min from the buoy location to the center of the SST pixel. The comparison shows a bias of -0.45 to -0.42℃ and a standard deviation of 0.81-0.88℃ for FY-4A/AGRI SST with excellent quality. The correlation coefficients between FY-4A/AGRI SST and buoy SST are better than 0.985. The FY-4A/AGRI SST are also compared with the ACSPO SST produced at NOAA/STAR from the Himawari-8/AHI (advanced Himawari imager). The comparison shows a bias of -0.26 to -0.07℃ and a standard deviation of 0.68-0.82℃ with excellent quality level. The correlation coefficients between FY-4A/AGRI SST and Himawari-8/AHI are better than 0.985. The correlation coefficients shows that there is a good correlation between FY-4A/AGRI SST and Himawari-8/AHI SST.