Characteristics and cause analysis of summer precipitation conversion rate in eastern Southwest China

Using CN05.1 and ECMWF Reanalysis v5 (ERA5) data, the characteristics of precipitation, water vapor content and precipitation conversion rate in eastern Southwest China (ESWC) during the summer of 1961-2020 were analyzed, and the influence of terrain distribution on the spatial distribution difference of precipitation conversion rate was preliminarily explored by means of synoptic analysis. Finally, the mesoscale numerical Weather Research and Forecasting Model (WRF4.0) was used to design terrain sensitivity tests to verify the effect of terrain on summer precipitation in the ESWC. The results are as follow. (1) In the summer of 1961-2020, the precipitation in the ESWC shows the characteristics of more precipitation in the east and less precipitation in the west, but there are two large value areas of water vapor content in the southeast and northwest of ESWC. The precipitation conversion rate in the large value area of water vapor is low, and the distribution of the strong precipitation area and the large value area of water vapor content are significantly different. By analyzing the situation of heavy precipitation areas in conjunction with horizontal wind fields and vertical velocity fields, it is found that topographic distribution is an important factor leading to this difference. (2) WRF model can well reflect the characteristics of summer precipitation in the ESWC. The terrain sensitivity test shows that the southwest to northeast mountain terrain distribution consisting of Dalou Mountain, Fangdou Mountain and Daba Mountain has a significant impact on the intensity of precipitation, and the decrease of terrain height will lead to a significant decrease of precipitation in the southeastern part of the region. (3) In particular, after reducing the topographic height of the region by half and to 0, respectively, the precipitation in the southeast of the region will decrease by 9.89% and 19.90% respectively on the monthly time scale. The change of topographic height will also cause the change of vertical velocity, horizontal wind field, water vapor transport and water vapor convergence, which will lead to the change of precipitation intensity. When the terrain height decreases, the upward motion and the southwest wind will weaken obviously, and the intensity of water vapor transport and water vapor convergence will decrease, which is not conducive to the formation of precipitation.