Assimilation of <scp>FY</scp>‐<scp>3G</scp> Ku‐band radar observations with <scp>1D</scp> Bayesian retrieval and <scp>3DVAR</scp> in <scp>CMA</scp>‐<scp>MESO</scp>

Abstract Feng Yun 3G (FY‐3G) spaceborne Ku‐band radar observations were assimilated for the first time using one‐dimensional (1D) Bayesian retrieval and a three‐dimensional variational (3D‐Var) method for a rainfall event on 28 July 2023, in Chongqing, China. Ground‐based radars were used to validate the spaceborne radar observations, and comparisons using the volume match method demonstrate that the FY‐3G Ku‐band radar observations are reliable. In this study, a spaceborne radar forward operator based on the T‐matrix method is used, and the melting and attenuation processes are also taken into account. Pseudo‐observations of relative humidity profiles were retrieved from FY‐3G Ku‐band radar reflectivity, and subsequently assimilated. In the 1D Bayesian retrieval procedure, not only vertical but also horizontal similarity is considered. Comparison with radiosonde observations indicates that this method is capable of retrieving reasonable relative humidity profiles. By assimilating pseudo‐observations, the forecast of the rainfall event by the China Meteorological Administration Mesoscale model (CMA‐MESO) has been improved. The pattern and magnitude of the echo more closely matched the observations after assimilation and the forecasting improvement was maintained for approximately six hours. The Critical Success Index (CSI) scores increased, although the Frequency Bias (FBIAS) scores also increased. Although there was an increase in positional deviation over time, the hourly precipitation patterns closely match the rain gauge observations after assimilation. It was found that the humidity in the east and southeast of the rainfall region increased, accompanied by a cyclonic pattern in the wind increment after assimilation. These changes are beneficial for the development and maintenance of the convective system. This study proves the value of assimilating FY‐3G Ku‐band radar in improving the forecast.