2024及2020年梅雨期新安江相似环流下致洪暴雨差异的成因及预警指征OA

Severe flooding occurred in the Xin'an River Basin during the Meiyu season in both 2024 and 2020.Understanding the dynamical and microphysical differences between these two flood-producing rainfall events is important for improving heavy rainfall forecasting during the Meiyu period.In this study,surface meteorological observations,raindrop size distribution measurements,Fengyun satellite observations,and National Centers for Environmental Prediction(NCEP)reanalysis data were used to comparatively analyze the circulation background and cloud microphysical characteristics of the two events.Results indicate that although both rainstorms occurred under typical Meiyu-season synoptic conditions,characterized by mesoscale cyclonic activity along the Meiyu front and enhanced precipitation due to orographic lifting,their formation mechanisms exhibited notable differences.The intensity and spatial distribution of vertically integrated moisture flux and moisture flux divergence differed substantially between the two cases,leading to distinct rainfall intensity and spatial patterns.Compared with the 2024 event,the Meiyu front in 2020 showed a stronger equivalent potential temperature(θse)gradient.The unstable thermodynamic structure with dry-cold air aloft and warm-moist air in the lower troposphere enhanced thermodynamic lifting,which combined with low-level convergence-induced ascent,contributed to stronger rainfall intensity in 2020.In addition,higher liquid water content and greater raindrop number concentration were important factors responsible for the overall stronger precipitation in 2020.Raindrop size distribution characteristics varied across different rainfall intensity ranges.During weak precipitation,the raindrop spectra in 2024 were relatively more uniform and stable than those in 2020,whereas during heavy precipitation the 2024 event exhibited a more variable large-drop-dominated pattern.The termination mechanisms of the two rainfall events were also different.In 2024,rainfall weakened as strengthened warm and moist airflow pushed northward,while in 2020 the event ended mainly due to the southward intrusion of weak cold air.Furthermore,during the peak rainfall periods of both events,short-duration intense rainfall tended to occur during the transition stage when cloud-top brightness temperatures in the FY-4A/B infrared and water vapor channels rapidly decreased and then sharply increased.This feature provides a useful satellite-based indicator for the early warning of intense rainfall.