Analysis Of Organization Characteristics And Development Mechanism Of An Extreme-Rain-Producing Mesoscale Convective System Along The Meiyu Front

A heavy rainfall event that occurred over the Yangtze-Huai River Basin on 26-28 June 2015 was analyzed by using data from routine observation, automatic weather station, Doppler radar and WRF model. The initiation, organization mode, structure and mechanism of development associated with linear mesoscale convective systems(MCS) are studied. The major conclusions are as follows:(1)The heavy rainfall event resulted from the continuous convergence of weak cold air from the north and warm-moist air in the northwest side of the subtropical high. The low-level jet transported water vapor and energy, and the shear line and Meiyu front provided low-level convergence and rising conditions for the development of convection. The precipitation shows an east-west distribution, and hourly precipitation presents intermittent and multi peak characteristics, and the mesoscale characteristics are obvious.(2)The heavy rainfall contributed to the development of multiple MCSs. Many types of organization modes were found during the development of the linear MCS.There are two different scales of organization during the development of linear meso-scale convective system: One is the northeastward "echo-training" of new convective cells along individual rainbands, and the other is the eastward "band-training" of the rainbands along the linear MCS.(3)The low-level jet provides abundant water vapor and energy for the occurrence and development of convection is the important reason for the continuous generation and development of convection in the southwest of the rainfall critical area.(4)MCS is characterized by cold area in the high and low levels, while the middle level is characterized by warm area.The continuous interaction of the southward cold outflow in the srong convection and low-level warm and moist current is conducive to the initiation of convection, which is the development mechanism of the backbuilding.(5)The interaction between the cold pool and the vertical wind shear are the important reason for the development of the MCS. Horizontal vorticity is closely related to the formation of vertical circulation. The rise and sinking movement caused by horizontal vorticity plays an important role in the intiation of precipitation and deep convection.The curl of the horizontal vorticity is closely related to the vertical motion,which is a good indication for the precipitation forecast and the vertical velocity.