A case study on the role of water vapor from Southwest China in downstream heavy rainfall

Based on the observation data analysis and numerical simulation, the development of an eastward- moving vortex generated in Southwest China during the period 25-27 June 2003 is studied. The water vapor budget analysis indicates that water vapor in the lower troposphere over Southwest China is transported downstream to the Yangtze and Huaihe River valleys by the southwesterly winds south of the vortex center. A potential vorticity (PV) budget analysis reveals that a positive feedback between latent heat release and low-level positive vorticity plays a vital role in the sudden development and eastward movement of the vortex. Numerical simulations are consistent with these results.