| Using meso-scale model MM5 and piecewise potential vorticity inversion technology, the evolution and formation mechanism of a meso-βconvective system (MCS) embedded in the Meiyu front on 9 June 2008 are investigated.The results of piecewise PV inversion show that the inverted balance flow can describe well the evolution of the formation and development of the MCS. The inverted non-balance flow can display the convergence and divergence the MCS. The piecewise PV inversion results indicate that the contribution from PV perturbations related to mid-level latent heat release is the most important toward the formation of vorticitiy on 850hPa, and the PV perturbations in the boundary layer firstly induce a closed circulation in the center of MCS. The upper-level PV perturbations have little contribution to the formation of the MCS. Further more, the PV perturbations unrelated to mid-level latent heat release have negative contribution.Some physical quantities such as vorticity, divergence in the center of the MCS indicate that the first growth happened in the boundary layer, the boundary layer plays a very important role on the earlier development of the meso-βconvective system. The evolution of potential equivalent temperature shows that the latent heat release is the mainly physical process at the later stage of the system. To further investigate the developing mechanism of the MCS, the Ekman pumping and Ekman non-balance flow are calculated from the model output. They indicate that the secondary circulation at the early stage of the MCS is mainly driven by the adjustment of Ekman non-balance flow in the boundary layer with little effects of latent heat release and minor topography dynamic lifting, though Ekman pumping contributes about 30% to the ascending motion. Moreover, this secondary circulation may arouse the outbreak of latent heat release and trigger the development of the MCS. |
