Numerical Simulation And Diagnostic Analysis Of A Warm Sector Rainstorm Occurring In Guangxi Province Area

Based upon observational data and satellite images, feature analysis of the warmsector rainstorm occurring in June 19～21, 2005 in Guangxi is made firstly. Thenusing NCEP data and meso-scale model MM53.6 the simulation of the rainstorm inJune 20～21 is carded out. Finally, using the model output with high spatial andtemporal resolution, the diagnostic study of the structures and the evolutionmechanism of the mesoscale convective complexes (MCC) and rainfall has beenconducted.The results show that the main synoptic scale systems are the upper level jet andSouth Asia High at 200hPa, the 500hPa short-wave trough and low level jet. Thequasi-stationary mesoscale convective complex (MCC) is the mesoscale system thatcauses the rainstorm directly. The 500hPa short-wave trough, the mesoscaleconvergence line to the left of low level jet and vertical secondary circulationproduced by the coupling of low and upper level jets trigger the rainfall and MCCoccurring in warm sector. It is noted that MCC occurs at the area always withconvective instability in lower troposphere and deep saturated layer to the south of thewind shear line at 850hPa. The following features are evident in the initial stages ofthe MCC life-cycle: convergence in the low levels, divergence in the upper levels andvorticity increases slowly. In the mature stages, low-level convergence andupper-level divergence are enhanced and positive vorticity at middle-level increasesrapidly. Vertical secondary circulation exists in the initial and mature stages, while inthe mature stages vertical motion in the ascending branch intensifies strongly.Potential vorticity analysis indicates that cold air intrusion in the middle levels plays acrucial role to the development of rainstorm and MCC. The rainband withcharacteristics of mesoscale U pattern rainband belongs to cold surge rainband aheadof the front. The slantwise vorticity development should be one important mechanismof the rainfall formation occurring in warm sector. In the area where the 500hPatrough overlaps the convergence line to the left of low level jet, the moist isentropesare very stiff. This is the area most favorable for the development of cyclonic vorticity.And the higher rotational helicity might be one of the mechanisms for the rainstormand MCC development.