Factors Influencing Diurnal Variations of Cloud and Precipitation in the Yushu Area of the Tibetan Plateau

Abstract

Using data from cloud radar, ground observations and ERA5 reanalysis data, the factors influencing nighttime precipitation during summer in the Yushu area of the Tibetan Plateau (TP) were investigated. The cloud top height (CTH), cloud base height (CBH) and liquid water content (LWC) were compared between non-precipitating days and precipitating days. The results showed that the average CTH on precipitating days in Yushu was below 10 km above ground level (AGL) in the daytime, whereas it exceeded 10 km AGL at night, with the maximum at 2300 Beijing Standard Time (BST = Coordinated Universal Time + 8 h). The CBH was in-phase with the dewpoint spread. The precipitation intensity and CTH were in-phase with the LWC. The hourly averaged precipitation intensity and convective available potential energy in ERA5 reached their maximums at 2100 BST, which was 3 h ahead of their observed counterparts. There was descending motion in the middle of the day on non-precipitating days, whereas there was ascending motion at night on precipitating days. In addition, the horizontal wind direction in the lower level (below 5000 m) showed clockwise rotation from morning to night. Wind shear occurred in the middle level of the atmosphere, accompanied by a subtropical westerly jet in the upper level. The difference in horizontal wind speed between 200 hPa and 500 hPa was positively related to the LWC, thereby contributing to the formation of upper-level cloud.