All‐Sky Assimilation of the MWHS‐2 Observations and Evaluation the Impacts on the Analyses and Forecasts of Binary Typhoons

Satellite data assimilation is transitioning from clear‐sky to all‐sky approach at some operational forecasting centers; the all‐sky approach directly assimilates observations under clear, cloudy, and precipitating conditions and shows a positive impact on medium‐range forecasts. Although there has been progress in the all‐sky technique for the forecasting of high‐impact weather processes, such as typhoon, these studies mainly focused on a single typhoon case, and the influence of all‐sky assimilation on the forecasts of binary typhoons, which have tracks and intensities that are more complex, is less investigated. In this study, a binary typhoons case (i.e., Typhoons Haitang and Nesat), which occurred in the western Pacific in July 2017, is selected, and the Weather Research and Forecasting data assimilation system is applied to examine the impact of all‐sky assimilation of the Microwave Humidity Sounder‐2 data from FengYun‐3C on the analysis and forecasting of the binary typhoons for the first time. The results show that with more cloud‐ and precipitation‐affected data being assimilated, the vertical structures of temperature and humidity around the core areas of the two typhoons and the forecasts of wind, temperature, and humidity are improved significantly in the all‐sky assimilation. These improvements result in the simulated tracks being more similar to the observations, and an error reduction in intensity is achieved in the all‐sky assimilation. The heavy rainfall (>250 mm) caused by Nesat over Taiwan is also improved. These encouraging results suggest that all‐sky assimilation is able to improve the forecasts of binary typhoons.