Role of Local Mountain-Valley Wind Circulation in the Formation of an Orographic Cloud and Precipitation Event on the Southeastern Tibetan Plateau

The role of local mountain-valley wind circulation in an orographic cloud and precipitation event during September 17–28, 2019 in the Nyingchi region of southeastern Tibetan Plateau is investigated using the data of the Second Tibetan Plateau Scientific Expedition and numerical simulation results with the five-nested domain and high resolution (333 m) of the Weather Research and Forecasting (WRF) model, The results show that the precipitation event was caused by a passage of a westerly trough, and Nyingchi station is located at the base of the westerly trough with weak and unstable stratification. The orographic clouds and precipitation event consisted of clear three-stage variations in the afternoon, evening, and night, with the local mountain-valley wind circulation playing an important role in the variation. In the afternoon, the strong solar radiation heating first generates apparent upslope winds and produces strong upvalley winds, blocked and lifted in the windward slope, and induces strong mountain waves, as well as strong convective clouds and precipitation. In the evening, the strong longwave radiation cooling forms obvious downslope winds, causing the convergence and lifting of warmer air in the valley, thereby enhancing the formation and development of weak convection and stratiform clouds in the valley. At nighttime, the downslope winds become the strongest, the downvalley winds (mountain winds) are further enhanced, and the strong lifting generates deeper stratiform clouds.