The Characteristics And Mechanisms Of Two Different Typers Of Mesoscale Convective System Along A Mei-Yu Front

Based on intensive surface observation for 1-hour interval, geostationary satellite, NCEP reanalysis data and simulations of the Weather Research and Forecast model, the evolution characteristics and mechanisms of the mesoscale convective systems (MCSs) within a Meiyu front during a heavy rainfall event on 29-30 June 2009 in East China are examined through synoptic analysis, dynamic diagnose and numerical simulations.The TBB analyses show that this intensive precipitation event with two stages. In the first stage, a strip-shaped with fragments convective clouds (MCS1) migrated eastward produced heavy rainfall in Anhui Province, and there is no obvious cold center on the surface map. In the second stage, the southerly convective clouds developed and formed an oval convective system (MCS2) which produced torrential rain over the Yangtze River basin. There are a mesolow and a mesohigh which are located in the front of convective zone and behind the zone, respectively. The convective zone occurred at the transition zone between the mesolow and the mesohigh.Corresponding with the observed two-stage rainfall, the model simulations show that the evolutions of MCSs also experience two different stages:initial burst of convection (MCS1) and reinvigoration of convection (MCS2). MCS1 locates in the south of Meiyu front which there are the strong upward motion throughout troposphere. MCS2 locates in the front which develops in the lower-layer and there is relatively shallow height comparing with MCS1. The results indicate that downward extension of upper troposphere PV trigger the burst of MCS1.The PV budget analysis suggests that downward extension of upper troposphere PV is mainly ascribed to the horizontal and vertical advection. The convergence between the boundary outflow before the boundary layer cold pool resulting from the developing process of MCS 1 and lower-layer wind triggers MCS2.