Assimilation of Clear- and All-Sky Himawari-AHI Radiances for Convective-Scale Forecasting

The Advanced Himawari Imager (AHI) on board Himawari-8 provides continuous high-resolution observations of severe weather phenomena in space and time. The capability to assimilate AHI radiances under both clear-sky and cloudy situations has been developed within the WRF data assimilation (WRFDA) system. The added value of assimilating hourly clear-sky AHI radiances from three water vapor channels was firstly demonstrated for the convection-permitting (3 km) analyses and forecasts of the “7.19” severe rainstorm that occurred over north China during 18–21 July 2016. Analyses were produced hourly, and 24 h forecasts were produced every 6 h. The results showed that improved wind and humidity fields were obtained in analyses and forecasts verified against conventional observations after assimilating AHI water vapor radiances when compared to the control experiment which assimilated only conventional observations. It was also found that the assimilation of AHI water vapor radiances had a clearly positive impact on the rainfall forecast for the first 6 h lead time, especially for heavy rainfall exceeding 100 mm when verified against the observed rainfall. To allow effective assimilation of all-sky AHI radiances, the so-called “symmetric error model”, in which the observation errors are varied with the “cloud amount” derived from both the radiance observations and the background, was implemented for different AHI channels. Analysis variables and the associated background error covariances were also extended for hydrometeors. Preliminary results of assimilating all-sky AHI radiances are encouraging for a severe storm case over Northeastern China with a setting of 3 km model resolution.