Diagnostic Analysis of a Heavy Rain in South Central of Hebei Province

Based on observed data and NCEP reanalysis data,the circulation background of a heavy rain occurred in south cental of Hebei Province on May 9,2009 was analyzed.By calculating the dry and moist potential vorticity(PV and MPV) and the vertical helicity,the characteristics of PV,MPV and helicity of the rainstorm were researched.The results show that the dry potential vorticity at 500 hPa could better reflect both surface cold air moving direction and convergence intensity,the rain intensity became larger with PV2 increase and weakened with PV2 decrease.For this rainstorm case,the zone of strong convection was better related to the abnormal moving direction of moist potential vorticity at 850 hPa,and the rainstorm occurred in the isoline dense area in front of moist potential vorticity.At the overlapping area of abnormal MPV1 and MPV2,the strong convection was easily triggered.The appearance and enlarging of positive center of helicity at lower level showed in advance the breaking out of rainstorm.The close negative and positive centers of helicity on the same longitude was in favor of amplification of heavy rain in positive helicity region.