Circulation classification and preferred dynamic factors of rainstorm induced by vortexes in Zhejiang

Atmospheric vortexes are important weather systems that cause regional rainstorm in Zhejiang, to investigate the atmospheric circulation patterns and highly dominant ensemble dynamic factors under different circulation types, we classify the circulation of 44 vortexes induced rainstorm days in Zhejiang based on daily precipitation in Zhejiang and surrounding areas, as well as NCEP-FNL reanalysis data, covering from 2016 to 2023. Classification is achieved using Multi-Variable Empirical Orthogonal Functions (MVEOF) combined with an objective typing method that integrates Simulated Annealing and Diversified Randomization (SANDRA). Additionally, we diagnose and select the primary ensemble dynamic factors contributing to the rainstorms under different circulation patterns. Results show that both classification methods categorize circulation of vortexes induced rainstorm days into three types: warm sector type, front type and return flow type, with warm sector type occurring most frequently, followed by the frontal type, and the return flow type being the least common. Under various circulation patterns, vortexes typically induce precipitation in central and western Zhejiang. The warm-sector type, characterized by consistent southwesterly winds throughout the atmosphere, produces the strongest rainstorm intensity and coverage. The diagnosis and selection of ensemble dynamic factors under different circulation patterns reveal that external conditions causing rainstorms in Zhejiang vary among types. For the warm sector type, mesoscale dynamic factors are the preferred factors; in the frontal type, in addition to mesoscale factors, large-scale water vapor and vertical motion induced by the vortexes also play a role; in the return flow type, the influence of mesoscale factors diminishes, while the role of large-scale divergence-related factors increases significantly.