Mesoscale convective systems along the Meiyu front in a numerical model

Two organized mesoscale convective systems (MCSs) developed sequentially al ong the Meiyu front over the Yangzi-Huai River basin and caused severe floo ding over eastern China during 12-13 June 1991. In this paper, the structur e and evolution of these MCSs are studied with a high-resolution (18 km) nu merical simulation using the Fifth Generation Penn-State/NCAR Mesocale Mode l (MMS). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite -derived cloud-top black body temperature (T-hh) maps. A mesoscale low-leve l jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respect ively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equiva lent potential temperature greater than 352 K into the MCSs, and strong low -level convergence can be identified on the left-front end of the mLLJ. Thi s strong convergence was associated with intense upward motion in the MCS w ith speed up to 80cm s(-1). Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase i n potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship betwee n the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. T his result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizonta l velocity gradient across the Meiyu front near the synoptic-scale low-leve l jet (LLJ) was quite large while the corresponding temperature gradient ac ross the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front.