Assimilation Experiments of Microwave and Infrared Radiance Data in JMA Global Numrical Weather Prediction System

The assimilation of microwave and infrared radiance data is essential to obtain accurate initial fields for numerical weather prediction (NWP). To obtain temperature and water vapor information under cloudy conditions, cloud- and precipitation- affected radiance observations from polar-orbit satellites should be assimilated in a data assimilation system. An all-sky microwave radiance assimilation system was developed for future operational use in the Japan Meteorological Agency global NWP system. The developed system employs 4D-Var data assimilation method with single outer-loop iteration and an update of first guess departure during a minimization of a cost function. By using the developed system, several data denial experiments are performed to investigate impacts of assimilating microwave radiance data under all-sky conditions and infrared radiance data under clear sky condition. Experimental results demonstrate significant positive impacts brought by the assimilation of all-sky microwave radiance and clear sky infrared radiance. Synergy effects from a simultaneous use of both microwave and infrared radiance data were found as improved upper and middle tropospheric water vapor fields in analyses and forecasts.