Synergetic Dynamical and Thermal Effects of Complex Terrain in Convection Initiation of a Heavy Rainfall Event in Ya'an, Southwest China

Abstract Ya'an is located in the transition zone between the eastern Tibetan Plateau and the Sichuan Basin (SB), featuring a distinct trumpet‐shaped terrain. This complex topography provides unique thermodynamic and dynamic conditions for convective initiation (CI), with highly intricate mechanisms. Based on radar observations, National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis and forecast data, and high‐resolution simulations using the Weather Research and Forecasting (WRF) model, this study investigates the terrain‐related triggering mechanisms of a heavy rainfall event that occurred on the southwestern slope of Ya'an on 19 July 2024. Results indicate that convection was initiated over the southwestern slope between 19:00 and 20:00 BJT, with the initiation process jointly modulated by local thermal and dynamic effects. In the daytime, the northwestern slope exhibited a pronounced “heat‐pump” effect due to solar short‐wave radiative heating, enhancing the upslope flow. In the evening, downslope winds developed at the foot of the northwestern slope and extended toward the central SB. This thermally‐forced downslope flow acted to strengthen the dynamical blocking effect of the northwestern slope for the low‐level southeasterly flow from the southeastern SB, which was thus forced to turn leftward, forming barrier‐jet‐like northeasterly winds. In consequence, the upslope lifting on the southwestern slope was enhanced, thereby triggering the convection of the heavy rainfall event. This study elucidates the key mechanisms governing nocturnal CI in this complex terrain region, highlighting the importance of coupling between orographic thermal and dynamical processes.