Based on a high resolution numerical simulation of the flash-flooding rainstorm process in Guangxi in June 2008,the evolution of the inner structure and dynamical features of the mesoscale vortex,which caused the heavy rainfall,have been investigated in terms of movement and continued development process.
By means of synoptic analysis,it is found that the mesoscale vortex was the main system leading to the rainstorm in Guangxi.The core of the rain band at different times mainly lied in the eastern part of the vortex within the range of 400 km,and the track and intensity of the rain band coincided with those of the vortex.The structure of the mesoscale vortex was especially distinct on 700 hPa,with the horizontal size ranging from 600 to 900 km.In addition,its move path could be divided into two stages:the southward moving stage and the eastward shifting stage.
Using the high-resolution model output data to analyze the inner subtle dynamic and thermodynamic structures of the mesoscale vortex,it is shown that strong ascending motion existed in the vortex center,which was quite beneficial to the formation of organized deep and moist convection systems.When the vortex reached its intensity peak,the vertical size of these convection systems could run through the entire troposphere,with the rotational motion and potential motion reaching the same magnitude.During the vortex's development process,the release of latent heat showed that there was an obvious warm core at the middle and high levels of the vortex center,and the stronger the vortex grew,the more distinct it became.Meanwhile,upward vertical transportation of warm wet flow,which was caused by strong convergence at low levels,signified that the vortex center attained high humidity.Influenced by the environment in the vortex's movement and development process,the redistribution of momentum,heat and moisture in the vortex center area took place as follows:(1)As the vortex moved southward along the Yunnan-GuizhouPlateau,its central pressure decreased and a uniform horizontal circulation was formed,with multiple strong convective systems evolving at the vortex center and on both sides.(2)As the vortex moved southward and approached the low-level jet(LLJ),the coupling effects of positive vorticity disturbance in the vortex center and positive vorticity shear on the left of the LLJ caused the vorticity to strengthen greatly,thus generating the maximum of vorticity and upward motion near the ground layer,which caused the moist convection and rainfall to both become the strongest in the vortex's center.(3)During the vortex's eastward movement stage,the influence of the LLJ became more important.In the eastern side of the vortex,the air was warm and wet,and possessed more convective instability energy,while stronger vertical movement and convective activity also appeared.Meanwhile,in the west,the air became significantly colder and drier,with almost no vertical movement or convective activity.Analysis of the three-dimensional flow field structure of the mesoscale vortex indicates that there were typical quasi-balanced dynamical features present in the mesoscale vortex,which could be described in detail by successively occurring long-lived organized deep moist convection systems,strong super-geostrophic flow with a large ageostrophic component,as well as intensive divergence which coexisted and had the same magnitude as the strong rotation during its lifetime.