Characterizing orographic clouds and precipitation in Qilian Mountains, northwestern China

Abstract. Orographic precipitation has a critical role in water resource and hydrologic cycle in many arid and semiarid regions of the world. The formation and characteristics for an orographic precipitation event on 16–17 August 2020 in Qilian Mountains of northwestern China are investigated based on observational data and high-resolution (up to 333 m) simulations of WRF model. The results show that the local mountain-valley wind circulation has a critical role in the formation of orographic clouds and precipitation, showing an obvious daily variation. In the afternoon, due to strong solar radiation heating, there is an obvious upslope wind on the sunny side of the mountain, and the windward slope of the mountain was blocked and lifted, and a strong terrain wave was excited, resulting in strong convective clouds and precipitation. In the evening, due to the strong long-wave radiation cooling effect of the mountains, the strong downslope wind generated converges and lifts at the valley bottom, which promotes the development of weak convective and stratiform clouds over the valley. In the early hours of the morning, the downslope wind reaches its strongest level, producing a strong downhill wind circulation (mountain wind), and the downslope wind produces a strong uplift effect at the bottom of the valley, forming a deep layered cloud and precipitation process. In the afternoon, the convective clouds are dominant. The microphysical process is mainly characterized by high content of graupel particles. The sources of rainwater are mainly from the warm rain process and the melting process of graupel particles, accounting for 30.3 % and 23.6 % respectively. In the evening and early morning, the weak convective clouds and stratiform clouds are dominant. The melting of snow is the main source of rainwater, accounting for 92.6 %; The precipitation conversion rate is basically consistent with the change trend of precipitation over time, and with the increase of terrain height, the precipitation conversion rate in this area also increases.