Diagnosing Thermal and Dynamic Mechanisms of Two Meiyu Rainstorm Processes in Zhejiang Province

Liu Shengnan; Lyu Jian; Liu Hanhua; Wang Xiaohui; Xu Yaqin; Fang Taoni

doi:10.16765/j.cnki.1673-7148.2024.06.003

Abstract：Using the conventional meteorological observations,Quzhou S-band dual-polarization ra-dar products,hourly precipitation data from China automatic stations merged with CMORPH precipitation products and NCEP GDAS/FNL 0.25° x0.25° reanalysis data as well as synoptic method and dynamic diagnostic method,this paper diagnoses two Meiyu rainstorm processes that occurred on 30 June 2020 and 7 July 2020 in Zhejiang Province,respectively.The results show that:(1)The interaction between the cold air behind northeast cold vortex and the southwest warm moist airflow generates a cold shear stretc-hing from northeast to southwest.The cold shear gradually moves southward together with the low-level jet moving eastward and finally incurs the 30 June rainstorm.By contrast,the interaction between the weak cold air behind East Asia deep trough and the warm moist airflow from the north side of the subtropical high generates a stable transmeridional cold shear,which moves eastward accompanied by the low-level jet and eventually causes the 7 July rainstorm.(2)Diagnostic analysis of frontogenesis function shows that the frontogenesis is much stronger in the 30 June process and that the interaction between cold and warm airflows as well as the baroclinicity of frontal zone is much stronger.However,more northward location of the rain belt in the 7 July process attributes to the more northward locations of both the moisture flux con-vergence zone and the large-value zone of precipitable water vapor.(3)In the 30 June process,there ex-ists a conditional symmetric unstable stratification in lower troposphere,while convective instability strati-fication exists in lower troposphere in the 7 July process.(4)Correlations between ensemble dynamic pa-rameters and precipitation intensity are as follows:The common point is that both moist thermodynamic advection parameter and generalized Q vector divergence show the highest relevance with precipitation,which means that the large-value zones of the above two parameters have good correspondences with the heavy precipitation regions in the next 6 hours.Their differences lie in the fact that the high correlation between vertical component of generalized convective vorticity vector and the 30 June rainstorm indicates deeper convective systems and more active meso-scale systems,while the good relationship between verti-cal flux of moisture divergence and the 7 July rainstorm can be attributed to strong converged ascending motion caused by synoptic scale systems.