Monsoon trough and MCSs interactions during the persistent torrential rainfall event of 15-18 July 2011 along the South China coast

By using the gridded analyses from the NCEP FNL and the methods including the piecewise potential vorticity inversion (PPVI), interactions between the Monsoon Trough (MT) and Mesoscale Convective Systems (MCSs) during the persistent and torrential rainfall event occurred along the Southern China coast on 15-18 July 2011 were examined with the focus on mechanisms responsible for the upward development of MT's cyclonic vorticity and their influences on the MT persistence and MCSs activity. The results show that the MCSs development within the MT was not baroclinic in nature, most of them intensively developed in MT's eastern and western sections with high concentration of vorticity. It was the southwesterly Low Level Jet (LLJ) over south of MT to provide an environment of convective"sustainability"to support the reoccurrence of convection and development of MCSs. The vorticity budget analysis shows that vorticity generation within the MT was contributed mainly by the convergent effect associated with the MCSs low-level confluence. The PPVI results confirmed that the intensification of cyclonic circulation over MT was largely attributed to the mid-to low-level PV perturbations associated with MCSs condensation heating, and as the vertical height of MCSs peak heating elevated this kind of intensification developed upward, indicative of a result from large-scale dynamical response to the development of MCSs. Especially over eastern and western sections of MT, where MCSs development presented intense and persistent, latent heating was highly efficient, the upward development of MT's cyclonic circulation was the most significant. As a result, it led to northward ageostrophic flows genesis southwest of MCSs, and through the Coriolis torque, the westerly wind speed over LLJ increased, which providing a favorable condition for the development of LLJ and reoccurrence of convection. All these evidences suggest that interactions of the large-scale MT circulation and MCSs exert pivotal influence on the development of MCSs and occurrence of the persistent torrential rain.