A Study on the Enhancement Mechanism of the Eastward Moving Convective Clouds over the Tibetan Plateau and their Enhancement in the Secondary Terrain

Using the cloud top blackbody brightness temperature (TBB) data retrieved from the FY-2B satellite observations in June-August 2010-2020, which is 1 ° × 1 ° per hour, the criterion is that the area of the cold cloud (TBB ≤ -32 ° C) is greater than or equal to 5000 km², there were 114 cases of enhanced eastward moving convective systems over the Tibetan plateau（ECSs-TP） and 56 cases of weakened ECSs-TP in the areas of secondary topography. Six types of major circulation patterns are obtained from the enhanced case study, of which the southwest vortex in the middle troposphere is the most significant. The favorable conditions are: At 500 hPa, there is a large gradient area of the meridional distribution of the potential height difference, i. e. , there is an obvious meridional disturbance in the middle troposphere. At 700 hPa, the southwesterly wind increment in the south of the secondary topography and the northward wind disturbance increment in the east of the plateau form an obvious cyclonic anomalous circulation.The whole-layer water vapor flux is the large value center of anomalous convergence, and the vertical wind shear and the equivalent potential temperature from the surface to 700 hpa are the large value regions of positive anomaly.

The heavy precipitation in the middle reaches of the Yangtze River caused by ECSs-TP with the development of the vortex system is analyzed. The vertical distribution of the non-adiabatic heating of convective clouds during the enhancement of the second-order precipitation is studied. It was found that the wet snow particles increased significantly near the melting layer during the enhancement of local precipitation, indicating that the falling and melting of snow particles contributed to the increase of precipitation. In addition, when the content of large raindrops under the melting layer increased, the content of graupel particles near the melting layer also showed a significant increase trend, indicating that the falling and melting of graupel also contributed to the increase of precipitation. The main source of non-adiabatic heating consists of two parts, one is the latent heat released by the condensation of water vapor into liquid phase particles, the other is the latent heat released by the condensation of water vapor into ice phase particles. The maximum value of the former appeared near 600hPa height, the maximum value of the latter appeared near 400hPa height, and the maximum value of the sum of the two appeared near 550hPa. For the latent heat released by the ice phase particles, the proportion of latent heat released by the ice phase particles is the largest.