A Statistical and Compositional Study on the Two Types of Mesoscale Vortices over the Yangtze River Basin

Using the 6-hourly, 0.5°×0.5° CFSR (Climate Forecast System Reanalysis) data for the summers (June-August) of 2000-2013, the climate characteristics of the southwest vortices (SWVs) generated over the Sichuan Basin and Dabie vortices (DBVs) around the Dabie Mountains (e.g. frequency, initial time, moving tracks, three-dimensional shape), are detected and statistically analyzed. Based on the associated surface pressure characteristics and precipitation characteristics before their initiation, the SWVs and DBVs are classified, composited, and compared. The main results are as follows: (1) Both types of vortices occur frequently in summer. SWVs are most frequent in early July and DBVs are most frequent in early June. 0200 LST to 0800 LST is the most frequent time of occurrence for both types of vortices. SWVs are more frequent in the daytime, whereas DBVs are more frequent at night. (2) Most SWVs/DBVs persist for less than 12 hours. Under the influences of the Sichuan Basin, most SWVs are quasi-stationary, while in contrast, most DBVs move significantly along eastward and northeastward tracks. (3) Both types of vortices are mainly located in the middle and lower troposphere, but DBVs are generally located at lower levels than SWVs. Compared with SWVs, the precipitation associated with DBVs is heavier, mainly due to their more favorable moisture conditions. The larger latent heat release associated with these heavier rainfall events provide conditions that are more favorable for the persistence of DBVs. (4) Compared to those without precipitation before their formation, those SWVs/DBVs with obvious precipitation before their initiation are characterized by a stronger South Asia high and more intense divergence in the upper troposphere, a more favorable configuration relative to the 500 hPa trough, stronger convergence and shear in the lower troposphere, and more intense ascending motion in the middle and lower troposphere. The thermodynamical conditions associated with the latent heat release before the formation of both types of vortices are conducive to a longer life span, larger horizontal radius, and heavier precipitation.