MEI-YU FRONTOGENESIS

A hypothesis concerning the development of the western sector of a Mei-Yu front is proposed. An observational evidence showing the presence of a positive low-level potential vorticity (PV) anomaly along the western sector of a Mei-Yu front is first presented. It is then proposed that the frontogenetic process in this sector is maintained through the interactions between the PV anomaly and the cumulus convection induced by Ekman-layer pumping. The scale contraction produced by the convergence flow of the convection provides the basic frontogenetic forcing. In addition to the linear CISK process, the front also intensifies through a nonlinear feedback mechanism due to the fact that the effect of condensation on potential vorticity is proportional to the value of absolute vorticity. Nonlinear semigeostrophic model results indicate that due to this nonlinear feedback process, the fronts intensify and the cross-frontal horizontal scale contracts at a rate faster than the exponential growth. The nonlinear feedback process also causes the western sectors of Mei-Yu fronts to develop in such a way that a discontinuity would form in physical space at a finite time if there were no limit to the intensity of cumulus convection. The properties of Mei-Yu fronts forced by these processes are compared with those of the classical type of fronts observed in midlatitude cyclones.