Analysis of the characteristics and causes of the "6·20" localized extreme precipitation event during the Meiyu season in northwestern Zhejiang

The Meiyu season rainstorm event in northwestern Zhejiang on June 20, 2024 (hereinafter referred to as "6·20"), was characterized by its long duration, large cumulative precipitation, distinct localized nature, and extreme intensity. To investigate the characteristics and mechanisms of this extreme rainstorm, multiple datasets were employed, including meteorological observation station data from Zhejiang and surrounding areas, ERA5 reanalysis data, dual-polarization radar observations, and mesoscale numerical forecast analysis fields. By combining these datasets with synoptic diagnostic methods, this study analyzed the extreme precipitation characteristics, circulation background, physical quantity features, topographic enhancement, and radar echo characteristics of the rainstorm, and established a conceptual model for the weather process. The results are as follows. (1) The rainstorm process consisted of two phases. In phase I, convection was triggered by convergence in the exit region of the boundary layer jet and sustained by energy release from a conditionally unstable layer in the lower troposphere. In phase II, dynamic forcing resulting from the coupling of upper-level northwesterly divergence and lower-level southwesterly jet convergence created a pumping effect that sustained the precipitation. (2) The extremeness of the rainstorm was significantly influenced by notable climatic anomalies in water vapor flux and the low-level jet. (3) Topography exerted a dynamic forcing effect on the low-level wind field, with a pronounced increase in precipitation in front of mountains, where the average accumulated precipitation amount was 1.78 times that in other areas. (4) Radar precipitation echoes displayed phased differences. Phase I was dominated by convective clouds with low-level southwesterly radial velocity convergence, while phase II featured mixed stratiform-convective precipitation, with convective clouds corresponding to the coupling of radial convergence and divergence at high and low levels. This study reveals that the extreme Meiyu rainstorm resulted from sustained precipitation driven by the phased evolution of the synoptic situation, and that the combined influence of the synoptic situation and topographic forcing was key to the occurrence of this extreme rainstorm.