Influences of Low-Level Winds and Topography on Nocturnal Convection Initiation over Cangshan Mountain in Southwest China

During the boreal summer, the westerlies of the South Asian monsoon prevail over Southwest China, flowing perpendicularly to the north–south-oriented mountains. However, the initiation mechanism of nocturnal convectionunder weak synoptic forcing in this region remains unclear. This study combines radar observations, convection-resolving Weather Research and Forecasting (WRF) simulations, and terrain sensitivity experiments to investigate the influence of low-level winds and complex terrain on nocturnal convection initiation (CI) around Cangshan Mountain, a typical north–south-oriented range in Southwest China. Diagnostic analysis of vertical velocity acceleration indicates that CI is associated with persistent dynamic forcing and rapid enhancement of thermal buoyancy forcing, and the terrain oriented perpendicular to the flow plays a crucial role in this process. Thermodynamically, the topography of Cangshan Mountain depresses the level of free convection (LFC) and lifting condensation level (LCL) to below 700 hPa via moisture accumulation and enhanced instability in the western valleys, resulting in pronounced thermal asymmetry between the eastern and western flanks of the mountain. Dynamically, low-level winds influenced by Cangshan Mountain and a mountain range to its west generate persistent convergence within the thermally favorable environment, which induces ascending motion above both the LFC and LCL. During the hour before CI, the LFC within the western valleys gradually decreases, while the convergence above the LFC and LCL persistently intensifies, forcing the maintenance and enhancement of ascending motion. This process simultaneously promotes water vapor condensation, thereby enhancing thermal buoyancy forcing and ultimately leading to CI. Terrain sensitivity experiments reveal that scaling down the Cangshan Mountain elevation weakens its topographic barrier effect, raising the LFC and reducing low-level convergence at the CI location, thereby suppressing CI. This study identifies a terrain-elevated convection mechanism, where parallel mountains lower the LFC thermodynamically while raising the convergence layer dynamically. The proposed mechanism provides a reference for improving convection forecasting in complex terrain under weak synoptic forcing.