A Case Analysis Of Mesoscale Heavy Rainfall In Sichuan Province And Chongqing City In Sep 2004

Because the mesoscale convective system, developing with vortex and shearing line system, is the chief system causeing to torrential rainfall, so based on the routine, non-conventional data and model data, circumfluence position, mesoscale convective systems and backgrounds causing heavy rain in Sichuan and Chongqing during 3—6 September 2004 are analyzed, and then discuss the mechanism of mesoscale vortex ,convective system and mesoscale rainfall system, that has a great value.By using global reanalysis grid data of NCEP/NCAR, routine data, TBB data,and automatic data, large-scale synoptic situation and weather system causing heavy rain in Sichuan and Chongqing during 3—6 September 2004 are analyzed, It is found that: there existed a close relationship between the "049" torrential rainfall and the change of the large-scale synoptic situation in the early September and the intensification of cross-equatorial flow which bring a lot of vapor and meet with cold masses at the eastern of Sichuan province and Chongqing city on 2^ed Sep causing extremely heavy rainfall. The chief synoptic system causing to the "049" rainfall are the Southwest vortex and shearing line which accruing and stagnating at Sichuan Basin. The reason for heavy rainfall is meso-β scale convective systems, occurring and developing frequently on the low vortex and shear line, with local and explosive features occurring and developed in the Sichuan Province and Chongqing City.Using the NCEP/NCARR reanalysized data to make analysis of the "049" rainstorm's qusi-geostrophic vector Q and vorticity and the distribution of the frontogenesis function as well as the space and time scheme relation.It found out that verctor Q had benifit on the rainstorm's begining. Conditional symmetry instability and convective instability play important role on the rainstorm.MM5V3 mesoacale model is used to similar from 00BT of 3^th to 00BT of 5^th Sep in 2004. By analyzing the background of the development of the mesoscale convective system through the model data from shorter time-scale, the results show that (1) strong convergence of water vapor flux and the conditional convective instability in the lower troposphere probably provided the suitable thermodynamic environment for the generation and development of the mesoscale convective system;(2) the lower-level convergence, upper-level divergence, lower level positive vorticity and upper-level minus vorticity and vertical ascent motion probably provided thesuitable dynamic environment for the generation and development of the mesoscale convective system;(3)the maintain positive vorticity advection in the middle troposphere and the warm advection in the lower troposphere take advantage of the vortex development and maintain;(4)the scheme of the high-level jet at 200hPa and low-level jet is benefited to the form and development and maintain of the heavy rainfall;(5)the analyse of the potential vorticity indicate that the low-level positive potential vorticity has impotent effect on the heavy rainfall;(6)by analyzing the results of the sensitivity experiments about condensation heating and surface sensible and latent flux which is important for extremely heavy rainfall ,bying enhance convergence and vertical ascent motionand the development of the Southwest vortex.