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

The central and northern parts of Jiangsu have encountered reginal heavy to extreme heavy precipitation under the influence of two similar paths of Jiang-huai cyclones from August 27 to 28 (referred to as the “8.27” process) and from September 19 to 20 (referred to as the “9.19” process) in 2023. 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 characteristics of heavy precipitation, weather background, and causes of these two processes have been compared and analyzed. The results are as follows. (1) Both processes were regional extremely heavy precipitation events. The range, rainfall intensity and extreme value of the heavy rainfall were greater in the “8.27” process. (2) During the two processes, the upper-level trough was stable in the early stage and the surface low-pressure inverted trough has enchanted to Jiang-huai cyclone, combined with the low-pressure shear line, the low-level jet stream, and the upper-level diversion zone, the circulation background was conducive to the generation of heavy precipitation. Under these background, the source and path of the two cyclones were similar. (3) Before the center of cyclone moved to Anhui, 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 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 ascending inclined component of the secondary circulation was weaker, which resulted in a relatively smaller range of heavy rainfall.