Evolution and Cause Analysis of a Heavy Precipitation Process of Meiyu Along Yangtze River

Based on the observational and forecast datasets from precipitation merging product, radiosonde, Doppler radar, wind profiler radar and ECMWF product, the evolution and causes of the heavy precipitation process of Meiyu in the middle and lower reaches of the Yangtze River in China from June 21 to 22, 2024 were analyzed. The results show as the followings. (1) The heavy precipitation was mainly distributed in the northern part of Hunan Province, the southeastern part of Hubei Province and the western part of Anhui Province, with the main period from 15:00 on June 21 to 15:00 on June 22, especially in the early morning of June 22. The rain belt was located to the north of the subtropical high, in the north of the low-level jet, and at the front side of the moving trough line. (2) The K index exceeded 38℃ in all areas, and the CAPE before and after this heavy precipitation process was over 800 J/kg and less than 100 J/kg, respectively, indicating the evolution characteristics of unstable atmospheric stratification as well as the energy accumulation and release. (3) In the early stage of this process, the surface high temperature was distributed to the south of Wuhan, and the near-surface convergence line extended from the eastern part of Henan Province to the central part of Hubei Province. In the middle stage of this process, the convergence line moved eastward. In the later stage of this process, there was a significant cold pool over the land surface along the Yangtze River. The near-surface high temperature and convergence line were the triggering mechanisms of the heavy precipitation, while the cold pool led to the gradual weakening of the precipitation. (4) The water vapor flux was mainly located in the northern part of Hunan Province, the eastern part of Hubei Province as well as the southern part of Anhui Province, and gradually moved eastward. The flux values in the middle and lower layers were relatively high in the early morning of June 22. There were two water vapor transport belts in the lower layer, corresponding to different heavy precipitation centers. (5) The approximately east-west oriented echo band moved from west to east through the forms of merging, strengthening and dissipating. The south side of the echo band was the mesoscale linear or hook-shaped strong echo accompanied by high echo top and strong VIL. The meso-β scale convective system was composed of several meso-γ scale convective cells, and the meso-γ scale convective cells caused strong cumulative precipitation through the ‘train effect’.