A Diagnostic Analysis of the Simulated Structure of a Meiyu Front System in 1999

A numerical simulation of a torrential rain event occurring in the Jiang-Huai Valley of China from 2224 June 1999 is performed and analyzed by using the PSU/NCAR MM5 mesoscale non-hydrostatic model. The high-resolution model output data are utilized to diagnose the double front structure, and the distributions of potential temperature, equivalent potential temperature, and speci c humidity in the vicinity of the Meiyu Front System (MYFS) in the Jiang-Huai Valley. The results show that both the potential temperature gradient and the speci c humidity gradient have important impacts on the two strong equivalent potential temperature gradient zones associated with the double front structure of the MYFS, but the latter (mois ture gradient) is more important. In addition, the tendency equation of speci c humidity gradient is theoretically derived. It shows that variations of the speci c humidity gradient are related to the advection, convergence/divergence, horizontal and vertical vorticities (secondary circulation) e ects and the gradient of water vapor source/sink. As an example, the budget of the meridional component of the tendency equation is selected and diagnosed by using the above model simulation data of the torrential rain event. It is shown that the variation of the speci c humidity gradient averaged throughout the simulation is mainly controlled by the convergence/divergence e ect, the secondary circulation e ect associated with the horizontal vorticities, and the water vapor source/sink e ect. Since the water vapor source/sink is often formed from the phase change processes of water vapor in the air and thus directly associated with cloud and precipitation microphysics processes, the variation of the speci c humidity gradient is closely related with cloud and precipitation microphysics and the distribution, development and evolution of cloud and rainfall systems. The double front structure of the MYFS provides an advantageous environmental condition for the development and movement of the mesoscale torrential rain system nearby. In turn, the development of the torrential rain exerts a signi ant impact on the MYFS through changing the thermal and moisture distributions.