An orographically generated low‐level jet (OGLJ) and asymmetric mountain profile triggered rare and severe downslope windstorms (DSWSs) over the southern slope of the Tianshan Mountains in northwest China in August 2019. Based on the Final Operational Global Analysis (FNL) data, observations from ground‐based meteorological stations, and high‐resolution Weather Research and Forecasting Model (WRF) simulation, this study aimed to understand the mesoscale features and triggering mechanisms of this event. The results show that during the intrusion of a cold air mass into China, Tianshan acted as a topographic barrier, causing significant damming on its northern flank, which in turn led to the formation of a pressure gradient extending from the surface to the mid–lower troposphere across the mountain range. Driven by the strong pressure gradient, air flow was funneled southeastward through the Tianshan Canyon, forming OGLJs due to funneling. As the airflow passed through the canyon, it encountered the asymmetric mountain, which has a gentle northern slope and a steep southern slope, which further excited gravity waves. The jet stream propagated with gravity waves and descended on the leeward slope of the mountains. This process facilitated the downward transport of momentum to the surface and triggered the DSWSs. Stronger wind speeds were observed in regions with more pronounced topographic asymmetry. The state of atmospheric stratification stability during strong winds is manifested as follows: on the windward slope, a stable layer persisted at 2500–3000 m AGL, while on the leeward slope, a stronger and lower‐level stable layer persisted 500–1000 m AGL. Such a state is conducive to the development and enhancement of downslope storms on the leeward slope. The research results clarify the mesoscale characteristics and triggering mechanisms of downslope storms on the leeward slope under the combined action of topographic canyon and asymmetric mountain profile in the context of synoptic‐scale circulation. In particular, the vertical structure of air flow over mountains clearly depicts the three‐dimensional structure of the low‐level jet formed and accelerated by the funnel effect and the sinking of gravity waves on the leeward slope. These research results are helpful for deepening the understanding of the mechanism of DSWSs and contribute to improve weather warnings of these events.
Tang et al. (Thu,) studied this question.