The quasi-stationary feature of nocturnal precipitation in the Sichuan Basin and the role of the Tibetan Plateau

The nocturnal precipitation in the Sichuan Basin in summer has been studied in many previous works. This paper expands the study on the diurnal cycle of precipitation in the Sichuan Basin to the whole year. Results show that the nocturnal precipitation has a specific quasi-stationary feature in the basin. It occurs not only in summer but also in other three seasons, even more remarkable in spring and autumn than in summer. There is a prominent eastward timing delay in the nocturnal precipitation, that is, the diurnal peak of precipitation occurs at early-night in the western basin whereas at late-night in the center and east of the basin. The Tibetan Plateau plays an essential role in the formation of this quasi-stationary nocturnal precipitation. The early-night peak of precipitation in the western basin is largely due to strong ascending over the plateau and its eastern lee side. In the central and eastern basin, three coexisting factors contribute to the late-night peak of precipitation. One is the lower-tropospheric southwesterly flow around the southeastern edge of the Tibetan Plateau, which creates a strong cyclonic rotation and ascendance in the basin at late-night, as well as brings abundant water vapor. The second is the descending motion downslope along the eastern lee side of the plateau, together with an air mass accumulation caused by the warmer air mass transport from the southeast of the Yunnan-Guizhou Plateau, creating a diabatic warming at low level of the troposphere in the central basin. The third is a cold advection from the plateau to the basin at late-night, which leads to a cooling in the middle troposphere over the central basin. All these factors are responsible for precipitation to occur at late-night in the central to eastern basin.