Study on Circulation Precursors and Complex Topographic Effects of the torrential rain events in the South of Changbai Mountains

Based on the observed precipitation data from 2010 to 2019 in the complex terrain region of southeastern Liaoning, this study identified 44 torrential rain events (TRECT).By employing composite analysis methods and decomposing the lower-level wind field using flow-over and flow-around equations, the circulation precursors, common characteristics of circulation fields, and the dynamic impact mechanism·s of topography on regional torrential rain were systematically investigated.The results indicate that: Six days prior to the occurrence of TRECT events, the mid-troposphere over the Eurasian continent exhibited a blocking high pattern, followed by the eastward movement and development of a shallow trough near Lake Baikal.On the day of the event, the subtropical high moved northward and intensified, forming a slanting trough over the western part of the key area, with a closed cyclonic circulation appearing in the lower troposphere.The North China low-pressure system developed and moved eastward, positioning the key area at the top of the inverted trough, providing a favorable circulation background for widespread precipitation.During the peak period of the event, the flow over and around the terrain in the key area showed stronger southern and weaker northern characteristics, with high-value areas concentrated in the trumpet-shaped terrain region of the Changbai Mountain's foothills.Areas with flow-over speeds exceeding 4 m⋅s-1 were located on the southern slopes of the low-altitude hills in the trumpet-shaped area, while flow-around was observed around the high-altitude mountains, with meridional branches of both flows playing a dominant role.The flow-over and flow-around entered the trumpet-shaped area by crossing and bypassing the mountains, respectively, primarily from two air stream·s originating from the Yellow Sea and the Korean Peninsula.In the trumpet-shaped area, the upward motion contributed by the flow-over accounted for more than 80% of the total system's upward motion, serving as the main dynamic source for precipitation enhancement.The flow-around, under the influence of topography, formed local positive vorticity, further enhancing the dynamic conditions for precipitation.This study systematically investigates the evolutionary characteristics of precursor large-scale circulation system·s associated with torrential rain events in the complex topography of the Changbai Mountain foothills, exploring atmospheric circulation precursors preceding extreme precipitation.Through mesoscale topographic dynamic forcing mechanism·s, it specifically elucidates the orographic lifting-convergence effect and flow-around vorticity development within the trumpet-shaped terrain of southeastern Liaoning Province.It provides a theoretical basis for improving the accuracy of regional mountain type rainstorm forecast.