Comprehensive Evaluation of Atmospheric Water Vapor Products Retrieved From FY‐4B/GIIRS Using the OSPRS Algorithm

Abstract

Water vapor plays a critical role in weather and climate, and high‐temporal resolution satellite observations are essential for characterizing its variability. The geostationary meteorological satellite Fengyun‐4B (FY‐4B) carries the Geostationary Interferometric Infrared Sounder (GIIRS), the first operational geostationary hyperspectral thermal infrared sounder, which provides continuous observations over East Asia. We evaluate water vapor profiles and total column water vapor (TCWV) retrieved from FY‐4B/GIIRS using the Optimal Sequential Physical Retrieval System (OSPRS) against ERA5 reanalysis, polar‐orbiting JPSS‐1/CrIS TROPESS retrievals, and IGRA radiosonde and AERONET sun‐photometer measurements. OSPRS TCWV reproduces the spatial patterns and seasonal cycle of ERA5 and TROPESS, with differences concentrated in warm, moist monsoon and oceanic regions, while absolute differences remain small over cold, dry mid–high latitudes and the Tibetan Plateau. OSPRS TCWV is highly correlated with IGRA and AERONET in all seasons, with correlation coefficients of 0.94–0.99 and 0.93–0.98, respectively, and mean absolute errors of 1.69–3.33 and 1.65–3.85 kg m ⁻² , respectively. Vertically, OSPRS water vapor profiles show smaller differences with the averaging kernel‐smoothed IGRA than TROPESS, especially in the lower troposphere and in summer. The Super Typhoon Doksuri case further shows that OSPRS produces sharper local moisture gradients than ERA5, while localized 2‐hr fluctuations in cloud‐affected regions require cautious interpretation, as variable cloud cover and associated retrieval gaps may also contribute to these fluctuations. These results demonstrate that OSPRS provides a reliable FY‐4B/GIIRS water vapor product for characterizing spatial, temporal, and vertical variations and is valuable for monitoring extreme events, though further optimization is needed.