Impact of Thermally Forced Circulations on the Diurnal Cycle of Summer Precipitation Over the Southeastern Tibetan Plateau

The summer rainfall over the southeastern Tibetan Plateau (SETP) is crucial to the hydrological cycle of both the SETP and nearby regions. However, it remains unclear why the rainfall over the SETP mostly occurs at night due to limited observations. This study aims to investigate the role of thermally driven winds in diurnal rainfall during summer (June–August) over the SETP, using high-resolution reanalysis data and surface rain-gauge observations. The results show that the strong upward motions and low-level upslope winds of the thermally forced mountain-valley wind systems (MVWS) between the SETP and the lowlands (Sichuan Basin and Yunnan–Guizhou Plateau) help to trigger the precipitation and transport moisture in the afternoon. After sunset, thermally forced circulations, including the east–west continental-scale thermal circulation between the Asian continent and its adjacent ocean, the north–south large-scale MVWS between the Yunnan–Guizhou Plateau and its southern plain, and the north–south small-scale MVWS between the SETP and its southern slope, help maintain horizontal winds from the rainy lowlands, transporting abundant moisture to facilitate the rainfall over the SETP at night. Furthermore, the diurnal cycle of gauge precipitation over the southeastern Tibetan Plateau varies markedly among stations because of small-scale topographic forcing. This variability is closely linked to local mountain–valley wind systems in north–south-oriented valleys, which suppress daytime rainfall but maintain intense nocturnal rainfall with a peak around sunset. Vertical profile observations show that daytime rainfall is inhibited by compensating subsidence induced by upslope winds perpendicular to the valley axis, despite warm, moist advection by deep up-valley winds. In contrast, nighttime downslope winds favor deep ascent and enhanced rainfall, accompanied by higher water vapor, lower lifting condensation level, and greater convective available potential energy within the valley.