Structure And Mechnism Of The Meso-β-scale Vortex Along Meiyu Front

The severe rainstorm along Meiyu front is one of the main weather disasters over southern China. In summer 1998, a long-duration catastrophic flooding occurred along the whole middle and lower reaches of the Changjiang River. It is caused by numbers of long-lasting violent rainstorms and heavy rainfall over large areas along this region. Among the rainstorms, extensive attention is focused on the severe rainstorm happened over the vicinity of Wuhan province during 21 July this year (978 Wuhan rainstorm) for its abruptness and the bad situation of the disaster. In this paper, three different methods (statistical analysis, numerical simulation and dynamical diagnosis) from three different viewpoints are applied to study the case, several findings are achieved as following:First, a synthetical numerical-filter method (LDFCM) based on the window function is put forward. Comparing the ideal statistical analysis with the general diagnosis, the LDFCM method can capture the conformation and collocation more integrallty and clearly than the traditionary arithmetical average method.LDFCM method is also combined with the GAME reanalysis data in a resolution of 0.5 degree to diagnose the weather patterns throughout 15days during with 987 Wuhan rainstorm occurred. A meso-scale saddle-backed field is founded to form over the middle reaches of the Changjiang river from 13 July to 28 July (a second Meiyu front period and before), with subtropical high east, southwest vortex west and the cold vortex northeast and the blocked high over Qinling mountainous area. Frontogenesis is located along the expended axis of the saddle-backed field and the frontogenesis function is diagnosed, the convergent term and the horizontal deformation term have direct contribution to the frontogenesis function while the latter have more supply. It is over the east part of the expended axis where 987 Wuhan rainstorm formed, so the heavy rainfall is caused by large-scale weather systems.Further diagnostic study shown that a mesos-α-scale convective system passed Wuhai during two heavy rainfall processes(the night of 20 and 21, July), respectively. After a few hours, a mesos-β-scale vortex strengthened rapidly at the bottom of the mesos-α-scale convective system. The mesos-β-scale vortex, which formed, developed, matured and died out in the lower and middle tropospheric levels, was closely related to the "98,7" Wuhan heavy rainfall. The formation and evolution of the mesos-β-scale vortex should be regarded.Through the detailed numeric simulation and diagnosis of the vorticity, the effect of different scale interactions between the meso-β-scale vortex and synoptic-scale environments was investigated. The convergence of the synoptic-scale vorticity in in boundary layer was the vorticity source of the meso-β-scale vortex. Then, the upward motion, which was enlarged by associated latent heat release, led to the upward transformation of the vorticity. So the mesos-β-scale vortex, which located only in the boundary layer in early stage, developed up to about 700 hPa. Through such mesosacle convergence and upward transformation, the meso-β-scale vortex achieve mature stage and maintained for a few of hours, which resulted in the heavy rainfall near Wuhan.Trough 2km high resolution data simulation via MM5 model, several meso-y-scale convective cells are found in the meso-β-scale vortex metioned above. It is these cells that induced instant precipitaition burst on 21th July in Wuhan with extreme value 88.4mm/h directly. A potive-negative vortex tube couple formed the convective cell.This voetex couple will twist along with the horizontal envirnoment wind shearing.In the center of the cell,it is high ascend area and warm core and it is also extreme high rainfall area.To describe the complex scheme like these convective cells, supervorticiy and Shearing SuperHelicity is advocated. Through academic analysis, it is proved that SSH is equal to the general titlting of supervorticity equation and can describe the vortex tube. And then SSH can forecast distribution and intensity of the regional heavy rainfall induced by the convective cells very well through contrast test of Meiyu rainstorm that contrasted to the traditional helicity.