Impact of the Low-Pressure System of Qinghai-Xizang Plateau on the Occurrence and Maintenance of Extreme Heavy Rainfall Event

An analysis of the mesoscale convective system in a persistent heavy rain event during 8 to 11 July 2013 in Sichuan basin is performed by using of the routine observational data、ground intensive observational data、FY-2E satellite observational data and NCEP/NCAR 1°×1° reanalysis data.It is found that: (1) The heavy rainfall occurs in the stable circulation of two trough and a ridge, then caused by active plateau systems、the exceptionally stable subtropical anticyclone with a southerly jet and the low-level cold air. (2) This extreme precipitation process is composed by a warm-area mesoscale-feature precipitation stage and a relatively stable frontal precipitation stage. Adequate energy and moisture is delivered by a southerly jet to the rainfall area in the stage of warm-area precipitation, and it provides a high-energy and high humidity condition for the mesoscale convective systems happen and development; The energy dissipate and the convective systems subside in the stage of frontal precipitation, but enough water vapor is delivered to the basin make the rainfall continued. (3) The south-west vortex is induced by the positive vorticity advection of 500 h Pa upper trough, it will make the warm-humid air continues to rise in the western basin that provides for the maintenance heavy rainfall a good dynamic condition, and the characteristic of low convergence-high divergence is found in the two precipitation stage and vorticity develop more stronger in the stage of warm-area precipitation. (4) The terrain of the Tibetan Plateau force airflow strongly convergence to rise in the western basin that makes warm water vapor more efficient formats precipitation is an important factor in the extreme precipitation process.