Analysis of a heavy rainstorm in the Hai river basin caused by the residual vortex of typhoon Doksuri (2305)

From 28 July to 1 August 2023, the Hai River Basin—North China’s core hydrologic system (318,000 km², covering > 98% of the North China Plain)—experienced the extreme “23.7” heavy rainfall event (HRE), triggered by the combined influence of Typhoon Doksuri’’s (2305) remnant vortex and the West Pacific Subtropical High (WPSH). In this study, we investigated the causes of this rainstorm by using the precipitation data from China Meteorological/Hydrological observation stations and NCEP/NCAR reanalysis data. Our conclusions indicate that the formation of a stable and sustained circulation pattern, resulting from the connection of the westerly high pressure ridge and the WPSH, provides a favorable large-scale background for the initiation of the HRE. The coupling of the upper-level westerly jet and the low-level southerly moisture transport were beneficial to the development of strong convections. The positive vorticity associated with the residual vortex of typhoon “Doksuri”, and the strong dynamic uplift in the eastern foothills of the Taihang Mountains, promoted the maintenance and intensification of convective motion in the rainstorm region. Additionally, there was a lower-tropospheric moisture transport path connecting the tropical oceans and the rainstorm area, providing abundant moisture for the rainstorm. Compared to the rainstorms in North China induced by eight weakened landing tropical cyclones, the “23.7” HRE exhibited further westward, enhanced water vapor transport, stronger vertical upward movement and topographic forcing.