Sensitivity Experiments on Assimilating GIIRS Temperature and Humidity Retrieved Profiles for Three Typhoons Affecting Zhejiang

Assimilating satellite-retrieved profiles is an important approach to improving a numerical model’s initial conditions and typhoon forecasting skill. This study uses temperature and humidity retrieved profile products from the Geostationary Interferometric Infrared Sounder (GIIRS) onboard Fengyun-4B for three typhoons that affected Zhejiang Province in July 2025 with different track types. Sensitivity experiments are conducted under clear-sky, cloudy-sky, and all-sky conditions. Vortex relocation experiments are designed to separate the contributions of typhoon vortex structure and environmental field to track forecasts. Results show that assimilation systematically adjusts temperature and humidity in the typhoon inner core, rainband, and outer environment. All schemes produce lower–middle tropospheric warming and upper tropospheric cooling. Humidity decreases in the low-level outer environment but increases around the typhoon vortex and along the Western Pacific Subtropical High edge. For the three typhoon cases examined in this study, all-sky assimilation is optimal for track forecasts, whereas clear-sky or cloudy-sky assimilation alone can be adverse. For the 24 h accumulated precipitation during the period of severe typhoon impact on Zhejiang, all-sky assimilation increases the threat scores for heavy rain and torrential rain by 0.04 and 0.09, respectively, relative to the control experiment, and increases the extreme torrential rain threat score by 0.09 relative to cloudy-sky assimilation. For the three typhoon cases examined in this study, all-sky GIIRS profile assimilation shows clear advantages for typhoon forecasting and highlights the potential for the operational application of GIIRS-retrieved temperature and humidity profiles.