Direct Assimilation of FY-3E Hyperspectral Infrared Atmospheric Sounder-II Radiance Data in CMA-GFS System

The assimilation of satellite-borne hyperspectral infrared data shows an increasingly important impact on the improvement of numerical weather prediction. FengYun-3E (FY-3E) is China's first early-morning-orbit satellite, which was launched on 5 July 2021. The second-generation Hyperspectral Infrared Atmospheric Sounder-II (HIRAS-II) on board FY-3E can provide continuous high-spectral-resolution radiation data. In this study, the FY-3E HIRAS-II data assimilation system was established within the framework of the China Meteorological Administration global forecast system (CMA-GFS). Data pre-processing involves apodization, preliminary channel selection, data splicing and thinning, etc. Key modules for assimilation include the clear channel cloud detection based on the differences between observed and simulated brightness temperature (O-Bs), the quality control module according to the characteristics of the data error, and the bias correction module considering the satellite scanning bias and real-time atmospheric state. The impact of HIRAS-II data on the analysis fields and model forecasts was evaluated through four sets of 40-day cycling experiments. The results show that single instrument assimilation significantly reduced the analysis errors, particularly the root-mean-square error (RMSE) of the geopotential height and temperature fields above 500 hPa in the Southern Hemisphere. HIRAS-II data assimilation in operational settings reduces the analysis RMSE of geopotential height by 0-2% in the Southern Hemisphere and approximately 1% in the Northern Hemisphere. For forecasting, the assimilation of HIRAS-II data has a neutral to slightly positive impact. The improvement is mainly reflected in the Southern Hemisphere, where it reduces the RMSE of 72-120 h forecasts with statistical significance at the 95% level.