Numerical studies on microphysical properties of clouds and precipitation in the summer of 2014 over the Tibetan Plateau

In order to improve the understanding of microphysical properties of clouds and precipitation over the Tibetan Plateau, six cloud and precipitation processes with different intensities during the Third Tibetan Plateau Atmospheric Scientific Experiment from 3 to 25 July 2014 in the Naqu region of the Tibetan Plateau are investigated using the mesoscale numerical prediction model (WRF) with high resolution. The results indicate that the summer clouds and precipitation processes over the TP have some unique properties. The initiation process of clouds is closely associated with strong solar radiation heating in the daytime and the summer clouds and precipitation show an obvious diurnal variation. Generally, convective clouds would transform into stratiform-like clouds with an obvious bright band and often produce strong rainfall in the midnight. The maximum cloud top can reach more than 15 km above the sea level (ASL) and the velocity of updraft ranges from 10 m/s to 40 m/s. The simulations show high amount of supercooled water content primarily located between 0℃ and -20℃ layer in all the six cases. Ice crystals mainly form above -20℃ layer and even appear above -40℃ layer in the strong convective clouds. Rain water mostly appears below the melting layer, indicating that its formation mainly depends on the melting process of precipitating ice particles. Snow and graupel particles have the characteristics of high content and deep vertical distribution, showing that the ice process is very active in the development of clouds and precipitation. The transformation of hydrometeors and formation of precipitation over the plateau exhibit some obvious characteristics. Surface precipitation is mainly formed by the melting of graupel particles. Although the warm cloud microphysical process makes small direct contribution to the formation of surface precipitation, it makes important contribution to the formation of supercooled raindrops, which are essential for the formation of graupel embryos through heterogeneous freezing process. The growth of graupel particles mainly relies on the riming process with supercooled cloud water and aggregation of snow particles.