A numerical case study on short-time strong convection initiation of a southwest vortex

The outbreak of heavy rain is always associated with strong convection, accumulation of water vapor and unstable energy. During 27–29 June 2009, heavy rainfall occurs over the Sichuan basin in China. In this study, a numerical simulation is made to investigate the creation and characteristics of this case, by using the WRF model and its 3Dvar analysis system. The results shows that over the heavy rainfall region, the strong convection is associated with a typical midaltitude radial convergence (MRC); a vertical wind shear line (VWSL) occurs near 600 hPa, and a radial wind shear line (RWSL) expands from 800 hPa to 300 hPa over rainfall center. The lower-level convergence (CV) center, north wind increment (NWI) center, positive vorticity (PV) center are located in the north of RWSL. The upper-level divergence (ULDV) center, vertical velocity center, south wind increment (SWI) center, potential pseudo-equivalent temperature (PPET) increment center and specific humidity increment (SHI) center are around the RWSL and over the VWSL. It indicates that the rapid development of the upper-level south wind and lower-level north wind is a major characteristic of the evolution of the VWSL and RWSL, and promotes the development of the positive vorticity and severe vertical ascending motion. The moisture flux convergence (MFC) center is along the RWSL. The upper specific humidity is increased with the MFC along the RWSL. Hence, in the space above VWSL and RWSL, the mass unstable energy and water vapor are accumulated rapidly and the strong vertical ascending motion is promoted and a short-time strong rainfall occurs.