The Research Of Numerical Simulation And Diagnostic Analysis For A Heavy Rainfall Caused By Southwest Vortex

Southwest Vortex is the main synoptic system of causing the large-scale rainstorm inSichuan basin. Using the Necp reanalysis data and MM5 mesoscale numerical model,heavy rainstorm occurring in the Sichuan province and Chongqing in September, 2004is simulated and analyzed. The analysis indicates this is the sustained rainfall caused bysouthwest vortex which was blocked by Subtropical High in the Western Pacific andtyphoon Sangda. The simulated results are as follows: (1) From the distributions of Kindex and field ofθ_se (potential pseudo-equivalent temperature), we can see that theinstability energy is continually transported northward from the low latitude under theaction of low level-jet. It supplements the new unstable energy to eastern Sichuan basinand stimulates the severe convection to be maintained so that the strong precipitationweather could be continuing. (2)The cross-sectional view of the divergence fieldreflects well the four stages of the establishment, development, intensification andextinction of the synoptic system about the low-level convergence and upper airdivergence. (3)There is good fitting relationship between the height, the intensity of thevertical motion and the precipitation intensity. The long time strong ascending motionwould bring about the sustained precipitation, which is consistent with the tendency ofpotential vorticity change. It reflects the dynamic action is very important in theprecipitation. (4)The change of intensity of Southwest Vortex has the close positivecorrelation with typhoon central intensity, so as the convergence intensity of the Sangdavortex center. In addition, we carried out the experimental study through assimilatingthe cloud chart and changing the terrain altitude in the numerical simulation. The results indicate that, after changing moisture field by adding the cloud chart to revise the initialfield, there is the certain improvement in precipitation simulation. The terrain sensitivitytest research finds the terrain has the obvious influence to the rainstorm. The terrainfriction action would diminish the wind velocity of low-level atmosphere as the terrainheight elevating. Thus the cyclonic vorticity would reduce and the ascending movementwould become weaken, so the precipitation would be decreased. On the contrary, withthe terrain height reducing, the north wind component would strengthen due to lackingthe landform block so that the cyclonic vorticity would increase, as a result theprecipitation would raise.