Numerical Study of a Mesoscale Vortex in the Planetary Boundary Layer of the Meiyu Front

It was found that the heavy rainfall event along the Meiyu front in the lower reaches of the Yangtze River on 23 June 2009 was connected with a mesoscale disturbance vortex, which originated from the planetary boundary layer (PBL) and developed upward later and was discovered by using the Shuman-Shapiro filtering method. The mesoscale disturbance vortex in the PBL (PMDV) in this process corresponded well to the short-time rainstorm in the Doppler radar echo. Analysis of the high-resolution simulation results from the Advanced Weather Research and Forecasting Model (ARW) showed that there were several surface disturbances along the southern warm section of the Meiyu front prior to the generation of the PMDV. The PMDV interacted with the mesoscale convective system (MCS) and intensiˉed the local convective precipitation. The north and southwest flows in the PBL converged at the time of the PMDV formation. Meanwhile, a southwesterly jet on the top of the PBL to the south side of the vortex reinforced the ascending motion and convergence. Hence, it is concluded that the PMDV was generated when the strong cold air flows north of the shear line encountered the southwest flow south of the shear line. The convergence line in the PBL, the intensification of the southwest wind, and the southward aggression of the north wind were critical for the development of the PMDV. The release of latent heat was found crucial for the formation of the PMDV as it facilitated the convergence at low levels.