Meiyu rainstorms occurred in the Changjiang-Huaihe River valley in 2003 are classified into four categories according to the convective stability and precipitation property at the center of rainstorm,and the ralationships of moist potential vorticity with inertial,symmetry,and convective instabilities are analyzed.The analyses suggest that the first and second categories of rainstorms are associated commonly with the strong inertial instability in the upper troposphere,as well as respectively with symmetry and convective instabilities,and their rainfall is large;and the third and fourth categories generally produce stable precipitation with a small rainfall,and their upper tropospheres are usually inertially stable.Therefore,the effect of the inertial instability in the upper troposphere in enhancing the precipitation of Meiyu rainstorms is evident.