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The mesoscale weather system which affected the Guangxi flash-flood-producing rainstorm of China in June 2008 is a quasi-stationary mesoscale vortex.Its genesis and development is closely related to the coupling effects of weather systems in different scales and different latitudes.On the one hand,the coupling of synoptic scale high- and low-level jets provides the environmental conditions for development of vortices and vertical circulations in the mesoscale vortex; On the other hand,the coupling of waves in mid-latitude westerlies and perturbations in low-latitude warm-moist flow under the influence of complex terrain makes the mesoscale vortex circulations strengthened.With the piecewise potential vorticity (PV) inversion method,PV anomalies in different regions are analyzed; also the vortex-vortex interactions and vortex-background flow interactions are diagnosed.Thus,the reasons why the mesoscale is quasi-stationary at first,while developing and deepening later are indicated.Under the condition of coupling effects,the vertical motions accompanied with the mesoscale vortex can be diagnosed with the PV-ω inversion system based on the analysis of quasi-balanced flow.