On The Mechanism Of The Heavy Rainfall Associated With The Mei-Yu Front And The Low Vortex By The Potential Vorticity

In summer season,heavy rainfall associated with the Mei-Yu front is one of the most important severe weather phenomena in the Reaches of Yangtse River of China. Its occurrence and development are the result of the interaction among the different scale weather systems within the Mei-Yu front system. In present, the prediction of the heavy rainfall is very difficult. The potential vorticty (PV) dynamics is an important theory to understand the evolvement of the synoptic scale weather system. In this work, the heavy rainfall event, occurred on 22-23 June 1999 within the Middle-lower Reaches of Yangtser River, is investigated. The effect of the latent heating from the heavy rainfall and of the interactions among the different scale perturbations within Mei-Yu front system, is mainly discussed through the piecewise potential vorticity inversion technique. The main results are as following:In this case, the development of the heavy rainfall associated with the Mei-Yu front is a nonlinear quasi-equilibrium process. In the potential vorticity dynamics, the development of the different scale weather systems and of the various physics processes is described by the change and transportation of the potential vorticity and its perturbation. Associated with the propagation of the upper trough and the upper level jet(ULJ) is the positive potential vorticity anomaly presented in the upper troposphere. The growth of the Mei-Yu front, the low vortex and the latent heating associated with the rainfall forms the band of the positive potential vorticity anomaly along the front in the lower troposphere. Through the potential vorticity inversion system, the nonlinear quasi-equilibrium evolution of the Mei-Yu front and the associated convection are investigated according to the distribution of the potential voritcity and its anomaly. Importantly, the impacts of the latent heat release and different weather systems are studied quantitatively.The latent heating is the most important factor during the formation and intensification of the heavy rainfall associated with the Mei-Yu front. The center of the latent heating from the deep convection related to the heavy rainfall within the middle-lower Reaches of Yangtse River, is located at the middle level in troposphere. The heating mainly increases the potential vorticity at the lower level within the frontal area, and intensifies the Mei-Yu front system. Further more, the efficiency of the generation to the potential vorticity from the latent heating is raised, and the convection increases more rapidly. This feedback process between the convection and the Mei-Yu front system is the dominating mechanics to form the heavy rainfall. Of course, the adiabatic effect can not be ignored. The distribution of potential vorticity is nonuniform along the front, and is partly the result of the horizontal advection on the isentropic surface. The local frontogenesis is related to the horizontal advection of the potential vorticity in adiabatic process in some sence. The nonuniform distribution of the rainfall near the front is formed partly by the adiabatic process.The heavy rainfall is affected evidently by the low vortex moving eastwardly. The dynamical enhancement to the Mei-Yu front presents the increasing of the vertical vorticity, the convergence in the low level and the updraft within the frontal area. The organized convection near the front developes as the front enhanced. In the mean time, the efficiency of the transiformation from the latent heating to the potential vorticity is promoted. The positive feedback between the front and the convection is enhanced.The trough and upper level jet are intensified as the cold air diffusing southerly. The direct impact to the front of the upper level jet, is small and can be neglected, and the indirect processe is the main effect. The upper level divergence in the entrance or the rear to the right of the upper level jet, is of advantage to the ascend current at the lower levels. The contribution to the frontal vorticity from the low level jet(LLJ) is weak, and the main contribution is to transport enough water vapor and warm air to the frontal area. In this event, the vertical circulations near the front are important which contact the different scale weather systems. There are two vertical circulations above the Mei-Yu front, the thermal direct circulation to the north and the thermal indirect circulation to the south. Both the synoptic analysis and the potential vorticity inversion result indicate that, the downward current in the north thermal direct circulation shows the effect of the cold air. Moving southerly, the cold air, raised the warm air at the low levels and induced divergence at upper levels, enhances the development of convection in the frontal area. The downward current of the south vertical circulation, deformed by the coriolis force, accelerates the south-west wind at the lower levels, forms and intensifies the low level jet.