During the mei-yu period,the east edge of the Tibetan Plateau and the Dabie Mountain are two main sources of eastward-moving mesoscale vortices along the mei-yu front(MYF).In this study,an eastward-moving southwest vortex(SWV) and an eastward-moving Dabie vortex(DBV) during the mei-yu period of 2010 have been investigated to clarify the main similarities and differences between them.The synoptic analyses reveal that the SWV and DBV were both located at the lower troposphere;however,the SWV developed in a "from top down" trend,whereas the DBV developed in an opposite way.There were obvious surface closed low centers corresponding to the DBV during its life span,whereas for the SWV,the closed low center only appeared at the mature stage.Cold and warm air intersected intensely after the formation of both the vortices,and the cold advection in the SWV case was stronger than that in the DBV case,whereas the warm advection in the DBV case was more intense than that in the SWV case.The Bay of Bengal and the South China Sea were main moisture sources for the SWV,whereas for the DBV,in addition to the above two moisture sources,the East China Sea was also an important moisture source.The vorticity budget indicates that the convergence was the most important common factor conducive to the formation,development,and maintenance of the SWV and DBV,whereas the conversion from the vertical vorticity to the horizontal one(tilting) was the most important common factor caused the dissipation of both of the vortices.The kinetic energy(KE) budget reveals that the KE generation by the rotational wind was the dominant factor for the enhancement of KE associated with the SWV,whereas for the DBV,the KE transport by the rotational wind was more important than the KE generation.The KE associated with the SWV and the DBV weakened with different mechanisms during the decaying stage.Furthermore,the characteristics of baroclinic and barotropic energy conversions during the life spans of both vortices indicate that the SWV and DBV both belong
Derechos occur frequently in Europe and the United States, but reports of derechos in China are scarce. In this paper, radar, satellite, and surface observation data are used to analyze a derecho event in South China on 17 April 2011. A derecho-producing mesoscale convective system formed in an environment with medium convective available energy~ strong vertical wind shear, and a dry layer in the middle troposphere, and progressed southward in tandem with a front and a surface wind convergence line. The windstorm can be divided into two stages according to differences in the characteristics of the radar echo and the causes of the gale. One stage was a supercell stage, in which the sinking rear inflow of a high-precipitation supercell with a bow-shaped radar echo induced a Fujita F0 class gale. The other stage was a non-supercell stage （the echo was sequentially kidney-shaped, foot-shaped, and an ordinary single cell）, in which downbursts induced a gale in Fujita F1 class. This derecho event had many similarities with derechos observed in western countries. For example, the windstorm was perpendicular to the mean flow, the gale was located in the bulging portion of the bow echo, and the derecho moved southward along with the surface front. Some differences were observed as well. The synoptic-scale forcing was weak in the absence of an advancing high-amplitude midlevel trough and an accompanying strong surface cyclone; however, the vertical wind shear was very strong, a characteristic typical of derechos associated with strong synoptic-scale forcing. Extremely high values of convective availa- ble potential energy and downdraft convective available potential energy have previously been considered necessary to the formation of weak-forcing archetype and hybrid derechos; however, these values were much less than 2000 J during this derecho event.
