Characteristics Analysis of Heavy Rainfall Induced by the Residual Vortex of Typhoon Co-may (2025) in Southern Jiangsu

Using conventional observations, S-band dual-polarization radar data from Changzhou, the fifth-generation reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), and tropical cyclone best-track data, this study investigates the extreme rainstorm event over southern Jiangsu induced by the residual vortex of Typhoon Co-May on 12 August 2025, with emphasis on the similarities and differences among three severe convective events occurring on the evening of 1 August, the early morning of 2 August, and the afternoon of 2 August in terms of synoptic background, environmental conditions, mesoscale triggering mechanisms, and radar-derived microphysical characteristics. The results show that, although all three severe convective events developed under a favorable large-scale circulation background, with the upper- and lower-level circulation configuration providing persistent dynamical forcing and moisture supply for repeated convection, and were triggered by surface convergence lines and cold-pool outflow boundaries, they differed markedly in circulation coupling, the degree of dry-cold air intrusion, moisture transport structure, and thermodynamic conditions. During the first event, boundary-layer convergence and low-level warm-moist advection were most pronounced, resulting in relatively favorable convective initiation conditions. During the second and third events, the convergence center shifted upward to the lower and middle troposphere. Although the cold pool and drag-induced downdrafts produced by earlier precipitation made near-surface convective initiation less favorable, the midlevel high-energy region was maintained and released under persistent warm-moist transport and lower- to midlevel lifting. The role of dry-cold air exhibited clear stage dependence. In the initial stage, the intrusion of midlevel dry-cold air did not simply enhance atmospheric instability; instead, it increased the threshold for near-surface convective initiation to some extent. However, through evaporative cooling, it enhanced downdrafts and cold-pool development, thereby favoring the maintenance of boundary-layer convergence lines and the initiation of new convection, and playing an important modulating role in the organization and redevelopment of heavy precipitation. Mesoscale analysis indicates that boundary-layer convergence lines and cold-pool outflow boundaries were the direct triggers of the three severe convective events. Convective cells continuously formed on the upshear side of the system or near the convergence zone and merged into the main echo band, producing pronounced back-building, which was an important mechanism for sustaining convective regeneration and slowing system movement. On this basis, the training effect was the key mechanism responsible for the repeated superposition of local precipitation and the eventual formation of extreme accumulated rainfall. Dual-polarization radar and disdrometer observations further show that the first event was dominated by deep warm-cloud, low-centroid precipitation, with warm-rain collision-coalescence processes prevailing; the second event exhibited the deepest convective development and the most active mixed-phase processes; and the third event was characterized by the combined effects of warm-rain processes and relatively weak ice-phase processes, producing the strongest instantaneous rainfall under the joint influence of back-building and the training effect. Overall, the extreme rainfall over southern Jiangsu resulted from the combined effects of the long-lived stagnation of the typhoon residual vortex, persistent warm-moist transport, favorable upper- and lower-level circulation configuration, stage-dependent modulation by dry-cold air, the maintenance of mesoscale convective systems through back-building and the training effect, and efficient warm-rain and mixed-phase microphysical processes.