Diagnostic analysis of moist potential vorticity for a rainstorm in North China

In order to explore the relationship between rainstorm and moist potential vorticity field characteristics, we use NCEP reanalysis data with a resolution of 1°×1°to calculate the vertical component of moist potential vorticity (MPV1) and the horizontal component of moist potential vorticity (MPV2), and conduct a diagnostic analysis of moist potential vorticity for a rainstorm in North China. The results are as follows. The precipitation occurred in the neighborhood of the dense belt of potential pseudo-equivalent temperature isoline in the lower troposphere. The occurrence of MPV1 < 0 and MPV2 > 0 in the lower troposphere before the precipitation starts indicated the existence of convective instability and baroclinic instability. The rainstorm region was located near the dense belt of isoline in the positive and negative transition region of MPV1 and MPV2 in the lower troposphere, where it was favored to converge water vapor and develop vertical vorticity. During the main precipitation period, MPV1 played a leading role and the negative value of MPV1 near the precipitation center increased, while the positive value of MPV2 decreased at 850 hPa. Toward the end of precipitation period, MPV1> 0 and MPV2 almost became zero; thus the atmosphere was close to stable convection stratification. The spatial and temporal distribution characteristics of moist potential vorticity in the lower troposphere had a better correspondence with the occurrence and development of rainstorm.