Climatology and structures of southwest vortices in the NCEP climate forecast system reanalysis

The southwest vortex (SWV) is a meso-a-scale cyclonic low pressure system originating in southwest China. It is a high-impact precipitation-generating weather system in this region and a large downstream area. A climatology of the SWV over 1979-2010 is presented here. Results indicate that the SWV is a common regional weather system with about 73 annual occurrences. Two primary genesis regions are identified, one in the Sichuan basin and another one in the southeast flank of the Tibetan Plateau. SWV genesis displays seasonality with a spring-summer preference and diurnal variations. The average life cycle, horizontal dimension, and translation speed are 15.1 h, 435 km, and 8.6 m s^-1, respectively. SWVs are classified into four types that show regional and seasonal contrasts in structure. In type I, the winter-spring elevated dry vortex in the basin is vertically confined to a shallow layer between 850 and 600 hPa and tilts northeastward. The low-level vortex has a cold center, and the middle to upper levels feature baroclinicity. In type II, the nighttime warm-season precipitating vortex system in the basin has a deep structure with the cyclonic vorticity extending from the surface into the upper troposphere. The nonsevere precipitating vortex (type IIa) is weakly baroclinic and tilts northward with height, whereas the severe precipitating vortex (type IIb) is vertically aligned. For type III, in the southern mountains, the shallow surface-based vortex develops in a well-mixed boundary layer and vertically tilts in the upslope direction and has a warm and low-humidity core. For type IV, the heavy precipitating vortex in the mountainous region is large, deep, and nearly upright with a fairly barotropic environment.