Abstract: In this paper,the FY (Feng Yun) geostationary satellite,Doppler radar products and 4DVAR (Four-Dimensional Variational Data Assimilation),regional automatic station,conventional meteorological and the NCEP reanalyzed data were used to analyze the structure,evolution and formation mechanisms of the MCS (Mesoscale Convective System) on the north side of the Subtropical High and in upper reaches of the Huaihe River on August 16,2011.The MCS formed in the shear line near the edge of the subtropical high.The cold air diffusion to trigger the release of a higher convective instability energy is the main mechanism for the system development.The lifecycle of the convective system consists of four periods:primary stage,merger and development stage of M_βCS,maturity and decay stages of round M_αCS.The heavy rainfall mainly occurs during the former three stages.On the radar image,the MCS is mainly composed of single and multi-cell storms at the primary stage and shows a β or α mesoscale convective system at the rest stages.The system often appears a linear convective one at the M_αCS maturity stage.Convection merger results in an explosive development of the MCS.The positive and negative vorticity layers of the convection system show alternate distribution characteristics.The high-level divergence and vertical upward movement are relatively weak.The width of the low-level convergence is associated with the scale of the system.The intersection of cold and warm air flow and topographic convergence lines play an important role in triggering the convection,and the area current confluence is advantageous to form and develop the round MCS.The mesoscale mountainous topography in the west of He'nan province has an influence on the range and intensity of the heavy rainfall.

Key words: Subtropical high, Round M_αCS, M_βCS