Study of the characteristics and causes of rainstorm in Jiangdu, Nanjing under the impact of Typhoon "Co-may"

Typhoon “Co-May” (No. 8 of 2025) brought persistent torrential rainfall to Jiangdu District, Jiangsu Province. Event-total rainfall exceeded 100 mm at all stations and reached 208.2 mm at Daqiao Kunping Station. The analysis used typhoon-track data, observed precipitation and composite radar reflectivity data, ERA5 and FNL (Final Operational Global Analysis) reanalysis data, and WRF simulations. Synoptic diagnosis, radar echo analysis, and numerical simulation were combined. Based on rainfall evolution and changes in the dominant weather systems, the event was divided into two stages. Stage 1 lasted from 08:00 BT on 30 July to 08:00 BT on 31 July. It was dominated by the circulation ahead of the typhoon and the typhoon circulation itself. Stage 2 lasted from 08:00 BT on 1 August to 08:00 BT on 2 August. It was jointly affected by cold air and the outer circulation of the remnant vortex. The study examined large-scale circulation and moisture conditions, radar echoes and hydrometeor characteristics during the two stages, and the dynamic and thermodynamic mechanisms of local heavy rainfall. The results are as follows. (1) The Western Pacific Subtropical High and the continental high formed a saddle pattern that sustained a weak low-pressure area at 500 hPa. Airflows east of the typhoon and west of the Western Pacific Subtropical High combined to strengthen southeasterly warm-moist transport. Weak cold air from the north provided additional dynamic forcing. (2) Stage 1 had abundant and concentrated hydrometeors, and spiral rainbands produced persistent widespread precipitation. Stage 2 had fewer hydrometeors concentrated aloft, while cold-air intrusion produced stronger convective echoes. (3) During Stage 2, the high-CAPE area moved from Taizhou toward Jiangdu, and the pseudo-equivalent potential temperature gradient increased in the lower and middle troposphere. MPV1 was negative, whereas MPV2 along the Yangtze River was positive. Low-level convergence, upper-level divergence, and condensational heating enhanced ascent through positive feedback. Persistent moisture transport and local surface conditions along the Yangtze River further intensified the rainfall. These findings provide a reference for diagnosing rainfall mechanisms and improving monitoring and early warning during the typhoon and remnant-vortex stages along the Yangtze River in Jiangsu.