Improving Arctic Polar Low Forecasting Through FY-3D MWHS-II Radiance Assimilation During the Winter of 2020-2021

During the winter of 2020-2021, the Arctic's extreme cold waves significantly affected East Asia and North America, leading to the occurrence of Polar Low (PL) events in the Norwegian Sea. The Polar Weather Research and Forecasting Model (WRF) and the three-dimensional variational (3D-Var) data assimilation system are used to investigate the effects of assimilating clear-sky radiance data from the Microwave Humidity Sounder-II (MWHS-II) onboard the Fengyun-3C (FY-3C) and Fengyun-3D (FY-3D) satellites on two PL cases. A series of simulation experiments at 9 km resolution with one-way nesting for dynamical downscaling to 3 km resolution over the Norwegian Sea during the forecast period are conducted. The results reveal that assimilating MWHS-II data from FY-3D exhibits smaller observation minus background (OmB) values during one-month statistical averages, leading to accurate thermal and dynamic analysis variations with lower biases with variational bias correction (VarBC). The direct assimilation of FY-3D MWHS-II radiance data enhances forecasts of ocean surface winds, pressure, PL tracks and coastal precipitation along western Norway. Furthermore, it provides added details on the stronger 850 hPa relative vorticity, differences between the sea surface temperature and 500 hPa temperature (SST-T500), and accurate vertical potential vorticity and static instability, consistent with the observed development of the PL in Case 1. For Case 2, benefits are still observed from assimilating FY-3D data. However, the simultaneous assimilation of both FY-3C and FY-3D data does not demonstrate superior performance. This study highlights the importance of assimilating FY-3D MWHS-II radiance data in analyzing and forecasting PL events.