Causes of the “8·20” extraordinary rainstorm in Huludao and exploration of topographic effects

This study investigates the causes of the extreme rainstorm in Huludao,Liaoning Province during 19-20 August 2024 and the role of terrain,utilizing dense precipitation observations and ERA5 reanalysis data.Results show that the "east-high-west-low" circulation pattern formed by the Northeast Cold Vortex and the western Pacific subtropical high facilitated moisture transport via southerly flows along the subtropical high's western flank.The convergence between cold air from the vortex and warm-moist air from the subtropical high promoted Frontogenesis over western Liaoning.Terrain-front interactions intensified precipitation,as terrain-blocked flows in the western/northern frontal zone enhanced vertical motion.This uplift further amplified Frontogenesis through its effect on Tilted Items in the frontogenetic function,constituting a key dynamic mechanism.During peak rainfall,maximum precipitation occurred in a horn-shaped mountainous area where east/southeast flow over terrain converged with northeastward ascending flows,triggering mesoscale convective systems.The southward flow over slopes generated localized convergence through flow separation-reunion mechanisms,providing additional dynamic forcing.Prolonged frontal zone stagnation caused by orographically-induced ascent contributed to the event's exceptional duration.