Analysis Of Precipitation Structure In Summer On The Eastern Slope Of Tibetan Plateau Based On TRMM PR

The east slope of the Tibetan Plateau is a typical place with high incidence of extreme precipitation.Meanwhile,the terrain is complex,and the heavy precipitation is easy to cause various secondary disasters and disturb people's lives.Therefore,it is of great scientific and practical significance to study the spatial and temporal variation characteristics of precipitation over the eastern slope of the Tibetan Plateau.According to the topographic height,the eastern slope of the Tibetan Plateau(28°N-32°N,100°E-106.5°E)is divided into four regions.The characteristics of precipitation structure in different regions are studied by using Tropical Rainfall Measuring Mission(TRMM)satellite observation data and European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis data.The horizontal distribution and vertical structure characteristics of near-surface precipitation parameters of the heavy precipitation system on July 6,2006 and July 30,2011 over the eastern slope of the Tibetan Plateau were studied and compared.On this basis,this paper analyzed the statistical characteristics of precipitation in different regions and different types on the eastern slope of the Tibetan Plateau in summer(June-August)from 1998 to 2012.The similarities and differences of precipitation structure characteristics in different regions on the eastern slope of the Plateau are summarized.And combined with ERA5 reanalysis data,the background field of atmospheric circulation in this region is studied.The main conclusions are as follows:(1)The comparative analysis of the precipitation processes on July 6,2006 and July 30,2011 shows that both of the two precipitation systems have a northeast to southwest trend.The precipitation processes all occurred under the circulation situation of low level convergence and high level divergence,and there was obvious water vapor convergence at 700hPa.The number of stratified precipitation samples detected by precipitation radar in the precipitation systems are more than that of convective precipitation.But the average precipitation rate of stratified precipitation is lower than that of convective precipitation.The case detected by PR in 2006 were in the stage of precipitation enhancement,and stratified precipitation contributed a great deal to the total precipitation.But the average precipitation rate of convective precipitation was more than 4 times that of stratified precipitation.The case in 2011 were in the stage of weakening precipitation.The convective precipitation contributed greatly to the total precipitation.And the average precipitation rate of convective precipitation was more than 3 times that of stratified precipitation.(2)The vertical structure of precipitation systems on July 6,2006 and July 30,2011 shows that the vertical structure of convective precipitation is significantly different from that of stratified precipitation.As the particles in the convective precipitation cloud collide and grow during the descent,the radar reflectivity in the high frequency region of convective precipitation gradually increases with the decreases of the height.But in stratified precipitation,due to the existence of frozen layer,there are obvious bright band characteristics in radar echo.In 2006,the top height of echo was more than 15 km,and the strong echo was more than 40 dBz at 6 km,which indicated that the convective development was more vigorous in this precipitation process.In 2011,the precipitation system is shallow,the echo top height is only 10 km,the near surface radar echo reflectivity is less than 45 dBz,and the strong echo is only located in the lower layer near the surface.(3)The background field of atmospheric circulation and the distribution characteristics of precipitation in different regions on the eastern slope of the Tibetan Plateau in summer from 1998 to 2012 were analyzed statistically.The results show that the area A and B has low-level convergence and high-level divergence,and in the middle-lower level the warm moist air from the Bay of Bengal flows to these areas to transport water vapor.In area C,there is a place which is located in part of the 3000m altitude of the Tibetan Autonomous Prefecture of Garze and the Tibetan Qiang Autonomous Prefecture of Ngawa has low-level divergence and in the high-level is convergence.Moreover,the blocking effect of Western Sichuan Plateau reduces the water vapor from the bay of Bengal.As a result,the seasonal a mean daily precipitation and precipitation frequency in this region are very small.There is a good correspondence between the average intensity and frequency of total precipitation,which increases first and then decreases with the increase of terrain height.The intensity and frequency of precipitation in area C are higher than those in other areas.There is a high frequency characteristic area of precipitation frequency in area B.Since the stratified precipitation is the majority of summer precipitation on the eastern slope of the plateau,the horizontal distribution of the intensity and frequency of stratified precipitation is basically the same as the total precipitation.(4)The vertical structure characteristics of precipitation over the eastern slope of the Tibetan Plateau in summer from 1998 to 2012 were analyzed statistically.The results show that there are significant differences in the precipitation structure of the four regions.The precipitation system in area A is the thinnest,while the precipitation system in area D is deep intense.The precipitation echo reflectivity factors in other regions all have a high frequency distribution of less than 35dBz at the height of 4-10km except for area D.The probability density distribution with height(PDDH)of convective precipitation reflectivity factors in area A and area B are basically the same,such as the center frequency and height are similar.However,the vertical development of precipitation in area C and area D area are deep intense,so the PDDH in the vertical direction is relatively dispersed.The stratified precipitation's PDDH in four regions are basically the same with that of the total precipitation,which may be due to the larger proportion of stratified precipitation.In comparison,the echo intensity and echo top height of convective precipitation are greater than that of stratified precipitation.