Application And Research Of Potential Vorticity In The Analysis For Rainstorm Caused By Landing Typhoon In Shandong Province

The mesoscale model MM5 was successfully used to simulate the rainstorm caused by the weaken 9907 typhoon circulation in Shandong Province during 11-12 Auguest 1999. Based on the outputs of the model, the mesoscale vertical structure of regular physical variables was analyzed. The potential vorticity(PV) on isobaric and isentropic surface was studied. Furthermore, the PV structure of another rainstorm caused by the 9711 typhoon was compared with it and the difference of the two rainstorm was discussed. The mechanism of rainstorm was revealed from the point of PV so that we can well understand the mechanism of rainstorm caused by typhoon or typhoon circulation in shandong province and improve our forcast ability for the rainstorm area and intensity.The results show that the "8.11" rainstorm had significant mesoscale character. Ascending motion , divergence and vorticity field intensified rapidly in the narrow area of rainstorm. The magnitude of these variables increased approximately ten times.It helped the rapid releasing of instability energy. The interaction of high-level and low-level mesoscale system leads to the rainstorm.The vertical cross section analysis of potential temperature tells us that rainstorm take place in the warm-moist and instability area .usually locate in the very stiff moist isentropes and it can lead to the development of vortex.The analysis of dry PV shows that the high PV dominate the area of typhoon , the higher is the center value of PV, the stronger is the typhoon.lt is also shown that the most important reason resulting in rainstorm is the coldair (high PV)which spreads to south from the upper troposphere or lower stratosphere and transports into the warm air. The joining situation of upper and lower PV area make vortex develop. PV transpotation reduces static stability, which increases the releasing of instability energy and makes rainstorm amplify.The study of PV on isentropic surface suggests that the active state of the midlatitude cold air and the transportation of positive PV advection is very important to the formation of the rainstorm. Positive PV advection is an important indictator to forcast typhoon' s movement.Moist PV serves as a bette.r indicator in typhoon rainstorm. When the low-tropospheric MPV1<0, MPV2>0 or negative MPV2 intensify, sudden rainstorm will be inclined to happen. The rainstorm usually happens in the dense section of the negative isoline of MPV1.Wind and pressure field on moist isentropic surface reveals that the interaction between cold air and warm air , it successfully explains the reason why "8. 11" rainstorm happened.