Analysis of the echo structure and its evolution as shown in a severe precipitation event caused by the plateau vortex and the southwest vortex

On 30 and 31 July 2009, a lasting severe rainfall occurred in the eastern lee of the Tibetan Plateau. There were three meso-scale convective systems in the Sichuan Basin, under the synoptic background that a plateau vortex and a southwest vortex had interacted at this region. The radar mosaic reflectivity data, NCEP reanalysis data and TRMM PR data are used to analyse its evolution and three-dimensional structure. By contrasting with TRMM PR, it is found that the radar mosaic has the same results and could describe the evolution of this event. In the plateau and the mountains with no mosaic data, TRMM PR could be used for supplement. The analysis results show that: at the precipitation area, positive vorticity at the middle and low levels and the moisture convergence upward motion matched high-level negative vorticity and water vapor divergence, which was a favorable condition to trigger severe rain. The first precipitation system is located southeast of the plateau vortex, moved toward the east with it and then dissipated. The precipitation has been found to lag the plateau vortex in terms of the intensity correlation, while its speed changed simultaneously with the vortex. The second and third precipitation systems have developed strongly during the existing stage of the southwest vortex, and then mixed into an echo belt throughout the basin along the mountains, causing the most severe rainfall in the southern basin. The intensity and movement of the precipitation system changed almost in the same time as those of the vortex. Under the complex terrain conditions with the mountains adjacent to the basin, the zero layers did not change significantly, but the cloud melted partially from ice to liquid when entering the basin and being under zero layers. This resulted precipitation in the change from convective cloud to straitform cloud with a stratified structure.