The Diagnostic Analysis Of Instability About A Heavy Rain Process In Southwest China

The study concerns the diagnosis of an event of rainstorm between 2000 BST, May 29 to 0800, May 30, 2004 in Guizhou Province, SW China, with the interval of 6-hr maximum rainfall between 0200～0800 BST, May 30.The rainstorm begins in the neighborhood of negative MPV maximum in low-level SW winds, which reduces when the event starts, with the values <0 at mid-lower levels and >0 at high levels as well as |MPV1|>|MPV2|. When the rainfall begins to increase the MPV zero-value layer ascends and after the precipitation starts the negative MPV weakens and the zero-valued MPV layer descends. For the previous rainfall MPV1 is largely responsible and a negative MPV2 zone emerges in the subsequent precipitation relating to the southward displacement of the front. The prior symmetric instability is of convective nature, with convectively unstable energy (CUE) acting in the main. Energy starts to be amassed in convective unsteady and maximum CAPE zones before rainfall begins and CUE declines at rapidity posterior to rainfall increase and symmetric unstable energy (SUE) begins intensification.Data used are 6-h apart such that it is impossible to determine when and how the rainstorm changes from warm-area to cold front precipitation character as regards the mechanism. As a result, modelings at 1-hr intervals are employed to examine the change, with relevant physical variables and unsteady energy investigated.The intensive rainfall is simulated with MM5 (version 3.6) and the results are used to explore when and how the warm is converted into a front precipitation and examine the evolution of physical quantities, circulation and unsteady energy, with which to explain the mechanism.Analysis of physical variables and unsteady energy condition on a 1-h basis clearly reveals that the mechanism for the previous precipitation is thanks to CUE triggering, the development of positive and negative vorticity, intensification of low- convergence and high-level divergence, and the establishment of 0e around the mid-level frontal zone where slantwise vorticity is developing. Convective instability triggers the mid-level front symmetrical instability development that leads to a secondary circulation, with gravity wave produced. In the subsequent stage, in contrast, precipitation is due dominantly to the front lifting. The baroclinicity increases on account of topographic effect when the cold front is lifted while moving southward and the SCU maintenance above slantwise updraft serves as the main source of energy for the persistence of frontal rainfall.