Exploring the Assimilation of All-Sky FY-4A GIIRS Radiances and Its Forecasts for Binary Typhoons

Cloud interference significantly affects infrared (IR) satellite observations, posing substantial challenges in data assimilation. The geostationary interferometric infrared sounder (GIIRS) onboard Fengyun-4A (FY-4A), the first IR hyperspectral instrument carried on a geostationary satellite, has been extensively evaluated through direct clear-sky assimilation. However, its utility in cloud-affected areas has yet to be thoroughly evaluated. This study investigates the impact of all-sky assimilating long-wave temperature channels from GIIRS on forecasts of binary typhoons Maysak and Haishen (2020) using the Weather Research Forecast model. Quality control procedures, observation error settings, and variational bias corrections are incorporated into the three-dimensional variational data assimilation system for both clear-sky and all-sky scenarios. These approaches mitigate negative observation-minus-background statistics, producing a more symmetric distribution, along with a better brightness temperature simulation under all-sky conditions. Furthermore, assimilating all-sky GIIRS observations enhances the depiction of detailed typhoon structures, such as the upper level warmer distribution in Maysak, accompanied by larger analysis variations. This process also improves subsequent typhoon track and landfall precipitation forecasts. This research highlights the significance of employing high-temporal IR data in cloudy regions for the all-sky assimilation of FY-4A GIIRS data.