Variational All-Sky Assimilation Framework for MWHS-II With Hydrometeors Control Variables and Its Impacts on Analysis and Forecast of Typhoon Cases

All-sky radiance assimilation has been extensively developed to provide additional information for numerical weather prediction under cloudy conditions. Microwave radiances are particularly sensitive to hydrometeors, which can be used to initialize hydrometeor directly if the hydrometeor control variables (HCVs) are available. However, the effects of HCVs statistical structure and their multivariate correlation on all-sky radiance assimilation remain unclear. In this study, five HCVs are introduced into the variational assimilation system. The characteristics of hydrometeor background errors are analyzed, and the combined effect with the observation operator is discussed. Then a 3D Variational all-sky assimilation framework with HCVs is modified to assimilate Fengyun-3C/D Microwave Humidity Sounder-II radiance. It is shown that hydrometeors are initialized by radiance directly, and the thermodynamic fields are adjusted accordingly. The characteristics of multi-variables increments are associated with both the characteristics of HCVs in background error and the Jacobians in observation operator. Furthermore, cycle assimilation and forecast experiments for three typhoon cases are conducted. It is found that the difference between observed and analyzed brightness temperatures decreases when HCVs are activated, and the hydrometeors analysis fields are more consistent with observations. Additionally, the typhoon intensity forecasts are improved with enhanced double warm-core and the secondary circulation. This paper analyzes the characteristics of variational all-sky assimilation framework with HCVs, and demonstrates the potential value of HCVs for variational all-sky radiance assimilation.