Abstract: Based on the conventional surface observation data, the precipitation data from automatic weather stations and the NCEP reanalysis data, a rainstorm process on June 19-21, 2010 in the middle and lower reaches of the Yangtze River was diagnosed by Q vector and moist potential vorticity theory. The results indicate that the lower level shear moves slowly southeastwards which causes the rainstorm. The region that has big absolute values of the Q vector and Q vector frontogenesis function is corresponding to the strong rain band and their values change with rainfall, while the big value center of Q vector frontogenesis function and the rainfall center are sometimes not consistent. The change of Q vector divergence coincides with that of rain band center. The strong rainfall band is situated on the dense contour area with transition between positive and negative value of , and increasing gradient of  is corresponding to rainfall enhancement, and vice versa.

Key words: Regional rainstorm, Q vector, Moist potential vorticity