Analysis of the characteristics and diagnosis of extreme heavy precipitation associated with two Jiang-huai cyclones with similar paths

The central and northern parts of Jiangsu have encountered reginal heavy to extreme heavy precipitation under the influence of two Jiang-huai cyclones with similar paths that occurred on August 27 to 28 and September 19 to 20 (referred to as the “8.27” and “9.19” process) in 2023. The characteristics of heavy precipitation, weather background, and occurrence causes of these two processes have been compared and analyzed in this study based on the ground observation data from national and reginal stations, radiosonde, ERA5 reanalysis data and FY-4A’s TBB product, and by using synoptic diagnostic analysis methods. The results are as follows. (1) Both processes were regional extreme precipitation events with strong extremity, with the “8.27” process being more severe. (2) During the two processes, the upper-level trough was stable in the early stage, while the surface low-pressure inverted trough has evolved in to Jiang-huai cyclone. The low-pressure shear line, the low-level jet stream, and the upper-level diversion zone, all together provided the circulation background conducive to the generation of heavy precipitation. (3) Before the center of cyclones moved to Anhui during the two processes, the convergence and ascending motion produced by the baroclinic frontogenesis was weak, and the rainfall intensity was weak. After the center of cyclone moved to Anhui, the geostrophic deviation effect of the baroclinic frontogenesis was strengthened which led to the development of secondary circulation, and the coupling effect of the low-level and upper-level jet streams was enhanced. The rainfall intensity in the northern and central Jiangsu, which located in the first and fourth quadrants of the cyclone, was significantly increased.(4) In the "8.27" process, the upper-level positive vorticity advection was stronger, the duration of the strong ascending motion was longer and the ascending branch of the secondary circulation was inclined. Combined with stronger water vapor conditions, the range, intensity and extreme value of heavy rainfall were larger. In the "9.19" process, the low-level warm advection was dominant, the ascending motion and convective conditions were stronger, but the weaker tilted component of the secondary circulation limited the spatial extent of heavy rainfall.