| Based on the 6-hourly,0.5°×0.5° CFSR (climate forecast system reanalysis) dataset during the summer (June-August) of 2000-2013, the southwest mesoscale vortex (SWV) generated around the Sichuan Basin and the Dabie Mountain mesoscale vortex (DBV) originated around the Dabie Mountain over the Yangtze River Basin were detected. Statistical analyses were applied to these two types of vortices. According to the 3-hourly surface observation, the SWVs and DBVs were classified into four types, respectively:the precipitation and surface-low type (PL); the precipitation-only type (PN); the surface-low-only type (NL); and the no-precipitation and no-surface-low type (NN). In addition, in order to investigate the main evolution mechanisms and energy features of these types of vortices, composite studies including the Euler composite and Lagrange composite under the normalized polar coordinate were conducted. Main results of this study are as follows:(1) SWVs occurred most frequent in early July whereas DBVs are most frequently in early June. The early morning is the most frequent occurring time for both types of vortices. Most of the SWVs/DBVs maintain for less than 12 hours. Most of the SWVs are quasi-stationary; in contrast, most of the DBVs can move out, mainly along the eastward and northeastward tracks.(2) Both the SWVs and DBVs are mainly located in the middle and lower troposphere. Overall, the DBVs are located at lower levels than the SWVs. Compared with SWVs, the precipitation asscociated with the DBVs are generally heavier, and the stronger latent heat release associated with heavier rainfall events provide more favorable conditions for the persistence of DBVs.(3) Compared with those SWVs/DBVs with no precipitation before their formation, the SWVs/DBVs with obvious precipitation before their initiation are featured by more intense divergence in the upper troposphere, more favorable configuration relative to the 500 hPa trough, stronger convergence and shear in the lower troposphere and more intense ascending motions in the middle and lower troposphere. The vortex types with obvious precipitation before its formation are conducive to the longer life spanning, larger horizontal radii and heavier precipitation.(4) Results of the vorticity budget indicate that, during the developing stage (DVS) of the PN-SWV and the PL-SWV, vorticity enhanced rapidly mainly due to the stretching (STR) effects associated with convergence. During the maintaining stage (MTS), both PN-SWV and PL-SWV varied slightly, VAV was the most favorable factor for the persistence. During the decaying stage (DCS), PN-SWV weakened mainly due to the STR associated with divergence and the TIL; wheras for the PL-DBV, the horizontal advection of vorticity (HAV) and TIL were the dominant factors. For the long-lived DBVs, the convergence-related STR dominated the rapid intensifying of the PN-and PL-DBVs during the DVS. During the MTS, the PL-DBV kept on intensifying, whereas the PN-DBV varied slightly. VAV and STR were the most favorable factors for the maintenance of the PN-and PL-DBVs. During the DCS, for the PN-SWV, TIL dominated the attenuation, whereas the divergence-related STR dominated the PL-DBV’s dissipation.(5) The energy conversion processes of the long-lived SWVs (DBVs) showed that, during the DVS, both positive barotropic energy conversion (BTC) and baroclinic energy conversion (BCC) were very strong, which provided very favorable energy conditions for the vortex’s development. During the MTS, the BTC and BCC weakened significantly, implying the vortex enter a stable maintaining stage. During the DCS, BTC and BCC weakened remarkably, implying the energy condition were no longer favorable for the persistence of the vortices. The distribution of the energy conversion was characterized by significant unevenness during the different stages, and the difference among each type was rather significant. |
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