1998 Second Plum Rainstorm And Nonlinear Sub-critical Symmetric Instability Diagnosis Analysis And Numerical Experiments The Initial Study

The nonlinear subcritical symmetric instability belongs to mesoscale dynamics. From a nonlinear equation with Boussinesq similarity on the F plane including friction dissociation and using a new kind of general energy as Lyapunov function ,we got the nonlinear subcritical symmetric instability criterion which a new kind of symmetric stability criterions . The nonlinear subcritical symmetric instability indicates that the larger amplitude disturb may cause the its instability to increase and then excite the mesoscale rainstorm.Meiyu rainstorm happens under the especial macro-scale background, and it is a result of the interaction of all kinds of synoptic system. Mesoscale system has a direct connection with the rainstorm, especially when it is on the front it will play an important role in exciting and increasing the rainfall. Because Meiyu rainstorm happens quickly, predicts in short time and often causes disaster. So study on the mechanism of mesoscale rainstorm system has a great value.Diagnostic analysis are carried out in the MeiYu storm rainfall process from 00UT of 20th July to 00UT of 01st August 1998 in this study. Results showed that the patterns in high benifit to this rainfall process. The elements of vorticity, divergence, specific humidity, moisture flux divergence and K index etc. were benefited to the form and development of storm rainfall over the rainfall region.Through diagnosing the connection in the nonlinear subcritical symmetric instability, linear symmetric stability, and rainfall, the article find : this rainfall process can be divided into three period, in which the nonlinear subcritical symmetric instability is the mechanism of the rainfall .The increase of rainfall has closely effection with the increase of the disturbance wind which often can be found before the increase of the disturbance wind which often can be found before the increase of the disturbance wind which often can be found before the increase of rainfall ,so the increase of the nonlinear subcritical symmetric instability can make the rainfall increase.And MM5V3 mesoscale model is used to similar from 00UT of 22th to 00UT of 23th July in 1998. The results show that (l)the nonlinear subcritical symmetric instability often happens at 200hPa and 500hPa ,which can make the atmosphere from linear symmetric stability to instability. Heavy precipitation often forms in the place between the source of high-disturbance wind field at 200hPa,especially of low-disturbance wind field at 850hPa.(2)The temporal and spatial scale about e-reflection index of nonlinear subcritical symmetric instability disturbance is 5-8hours and 200-300km respectively.