Assimilation of FY-3D MWHS-2 Radiances with WRF Hybrid-3DVAR System for the Forecast of Heavy Rainfall Evolution Associated with Typhoon Ampil

A detailed investigation about the effects of the Microwave Humidity Sounder-2 (MWHS-2) radiances on board the Fengyun-3D (FY-3D) satellite is combined with developments within the Weather Research and Forecasting Data Assimilation (WRFDA) system and analyses on the evolution of the heavy rainfall associated with Typhoon Ampil during 23-24 July 2018. In the analysis field, the position of Typhoon Ampil is found out to be distinctly affected by the MWHS-2 assimilation. The experiment that assimilates MWHS-2 radiances through hybrid-3DVAR generates the best analysis with large increments around the typhoon, which contributes to the typhoon moving inland to the southwest. In the forecast fields, the MWHS-2 assimilation improves the rainfall in not only the accumulated amount, but also the evolution characteristics. The hybrid-3DVAR experiment reduces the RMSE of the rainfall amount, and enhances the spatial correlation and the fractions skill score of the rainfall evolution to the greatest extent, followed by the 3DVAR MWHS-2 experiment. As for the cause of the rainfall improvements, analyses suggest that it could be closely connected with the characteristics of the circulation structures related to the typhoon evolution. On one hand, the increases of the rainfall amount and intensities in the MWHS-2 assimilation experiments (previously underestimated) correspond to the strengthened typhoon structures with strong anomalies in both the upper-layer temperature and the lower-layer geopotential height. On the other hand, the better rainfall evolution in the hybrid-3DVAR experiment could be explained by its clearer evolution of the structure of typhoon under the effects of an approaching upper trough, and its smallest typhoon track errors around the middle time period.