| There were 20 rainstorms taken place in Jianghuai area in the Meiyu period of 2003. Using observational data, ξmpv , ξmpvl and ξmpv2 in the rainfall centers have been calculated. It has been found that: (1) rainstorms can be classified into 4 categories according to convective instability and rain property at the same time. If the rain is shower or thundershower but the stratification is stable, this kind of rainstorm falls into the first kind of rainstorm. The second kind of rainstorm is that the stratification is unstable and the rain is shower or thundershower. The third kind of rainstorm is that the stratification is stable and the rain is steady. The fourth kind of rainstorm is that the stratification is unstable but the rain is steady. (2) The first kind of rainstorm is related to symmetry instability(SI), the second to convective stability(CI), and there was heavier rainfall in these two kinds of rainstorm. Relatively there was less rainfall in the latter two kinds of rainstorms. Chances of heavier rainstorms in the first kind of rainstorm were much bigger than that in other 3 kinds. So when stratification is stable but the rain is unsteady we should be alert whether it is going to rain heavier or more even. (3) There were often low ξ mpvl areas over rainfall centers and up in the southern ranges, this is mostly because of inertial instability(II) in the upper layers. \ When large negative ξ mpvl appear in the upper layer up and over rainfall center, it usually predict a strong rainstorm. (4) baroclinity, moist convective instability(MCI), moist symmetry instability(MSI), and II in upper layers are all causes which led to Meiyu rainstorms, and when II in upper layers joins with MCI, MSI in the middle and lower levels, the rain is sure to become larger.By using daily observational and NCEP re-analyses data, a very heavy rain taken place in Jianghuai area in Meiyu period of 2003 has been simulated with the MM5 model. On the basis of truly simulating both the scope and magnitude of heavy rain, the modeled results have been examined in detail to deduce the reason of the heavy rain. It turns out that, (1) in the low levels of the heavy rain, there was no vortex, so the rain belonged to a shear-induced heavy rain. Factor θ_e(equivalent potential temperature) in 850hPa level shows that, there existing warm and moist air and energy front. During the heavy rain, there existed distinct dry-lines in 500hPa level. (2) in the middle and low levels of the rain spot, through the heavy rain, θ_e remained high and the stratification kept unstable. The heavy rain was in favor of II in the upper levels. Analyses of equivalent potential vorticity( ξ epv) and its components demonstrates that during the heavy rain, in both the high and low levels, there existed SI and CI. When SI strengthened, the rain became larger. And vice versa. (3) by observing the changes of physical features in the center of the rainfall with time and altitude, one can see that, when the rain got stronger negative ξ npv, negative ξ npv1 and negative ξ npv2 got larger, and vice versa. At the rainfall center, ξ npv and ξ npv1 varied within a quasi-horizontal level, they did not exchange vertically overtly. While ξ npv2 exchanged vertically obviously. The changes of features in the boundary layer were related to the rain very tightly. The strengthening of negative ξ npv, negative ξ npvl and negative ξ npv2 was 3 hours ahead of the getting larger of rainfall. (4) The occurrence of the heavy rain was obviously because of baroclinity, SI, II in the upper levels and CI in the low levels. The scales of the direct influencing weather systems were obviously meso-scale, with time scale of several to decadal hours, and the horizontal scale several hundred kilometers.
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