Diagnostic Analysis And Numerical Simulation Of A Rainstorm Process In Sichuan Province

This paper uses the precipitation data of the China Meteorological Administration,the NCEP/NCAR reanalysis data(FNL)and the temperature of black body(TBB)from Japan Meteorological Agency's Himawari-8 satellite to analysis the weather of a heavy rainstorm that occurred in Sichuan region on July 2,2018.The diagnostic methods moist potential vortices(MPV)and Weather Research Forecast(WRF)were used to conduct in-depth research on the mechanism of this storm.In particular,the terrain sensitivity test was used to explore the effect of the Qinling-Daba Mountain terrain on the precipitation in Sichuan.The main conclusions are as follows:(1)The rainstorm occurred in the gradient area where the positive and negative values of 700 h Pa MPV1 and MPV2 intersect.The vertical distribution of MPV1 shows that it basically coincides with the distribution of convection stability,and the form of“superposition of positive and negative values” of the upper and lower layers of MPV1 is conducive to heavy rain.The positive value zone of MPV2 is closely related to the baroclinity of the atmosphere.The configuration of MPV1 lower than zero and MPV2 higher than zero in the lower layer of the troposphere can be used as the basis for judging the precipitation fall zone,because the cold and warm air converge here,it is also a large convection instability and strong baroclinic region can enhance the development of water vapor convergence and cyclonic vorticity,which is conducive to the occurrence of heavy rain.(2)The WRF model reproduced the rainstorm process well,and further analysis using the high-resolution data output by the model revealed: The positive value of the vertical helicity at 700 h Pa can be a good indicator of the location and movement of the precipitation zone.The vertical helicity of the high and low altitude configuration plays a vital role in revealing the evolution of the convection system.At the initial stage of convection,there is a positive vertical helicity zone here because of the cyclonic convergence and the ascending motion is initially established in the lower layer of the troposphere.As convection continues to develop,the negative value of the upper vertical helicity closely matches the positive value of the lower vertical helicity,forming a closed circulation loop in the vertical direction.The suction effect caused by the divergent airflow of the high-level anticyclone will strengthen the development of the positive helicity of the low-level layer,and continually transport the vertical vorticity upward,which is conducive to the strong development of the convection system.In the convection extinction stage,the positive value of the vertical helicity in the lower layer is gradually replaced by negative values,the favorable configuration of the upper and lower layers is broken,the strength of the convection system is weakened,and precipitation gradually stops.(3)The heavy rain occurred under the background of favorable stratification conditions and vertical circulation.At the beginning of the heavy rain,a large amount of warm and moist air gathered in the eastern side of the basin.The eastward wind in the lower layer cooperated with the upward movement to transport the warm and moist air to the heavy rain area,the unstable stratification in the middle and lower layers of the troposphere has been maintained for a long time and continues to strengthen with the occurrence of heavy rains.The main expression is that the vertical gradient of pseudo-equivalent temperature increases and the isoline becomes steep and straight.The middle stable stratification weakens and retreats to the east with the development of heavy rain,which makes the convection instability significantly strengthen.In the prime of rainstorm,the entire troposphere is ascending to help the lower layers continue to deliver warm and moist air to the basin continuously accumulates unstable energy,which makes the rainstorm continue to develop.When the range of convective unstable stratification in the lower atmosphere gradually shrinks until it is completely destroyed,the intensity and range of the ascending movement are reduced,and the warm and moist airflow is gradually weakened,corresponding to the precipitation is also weakened.(4)After reducing the terrain height of Qinling-Daba Mountain,significant changes in the airflow field,(1)the intensity of the center of the 700 h Pa height field weakened,and the southwest wind in the eastern part of the basin transformed into a southern wind,which weakened the range and intensity of the cyclonic vortex.The northeast wind flowing around Longnan area blocked by the mountain,turned to the east,and the wind shear in the Gan-Chuan-Shan border region will weaken.(2)Warm and moist airflows from the sea cannot gather in the Sichuan Basin,all flow to the southern Shanxi region and branch here into east and west branches.The large area of water vapor convergence in the basin disappears made the low-level water vapor conditions in the northeastern Sichuan-South Shanxi region insufficient.(3)The convergence field in the lower layer of the mountain area turns into divergence,and the ascending motion induced by the terrain is weakened,which reduces the height of the development of positive vorticity,and the intensity and range are also weakened,resulting in insufficient dynamic conditions in the eastern part of the precipitation area.The combined effect of these three factors has significantly reduced the precipitation range and precipitation from the northeast of Sichuan to the south of Shanxi,but has little effect on the precipitation in the southeast of the basin.