A Study Of Rapid Intensification And Dissipation Of Typhoons Over Coastal Water Of China

The intensity change of typhoons over coastal water, especially the rapid change of it, is the key problem of typhoon forecast. But short of understanding of the intensity change mechanism and short of enough high resolution of data to reflect the finer physical processes in present numerical models that lead to unable to improve forecast accuracy over past ten years. This study tries to gain a better understanding of the mechanism of rapid intensification and decay of typhoons over coastal water.The statistical results suggest that the rapid intensification and decay of coastal typhoons are small probability events with 9.4% for rapid intensification and with 2.2% for rapid decay. They also have distinct temporal and spatial distribution characteristics. Rapid intensification typhoons occur in April-October, mainly in August and September. Rapid decay occurs in April and July-December, mainly in October and November. The rapid intensification and decay both occur more frequently over South China Sea. At the same time, rapid intensification also occurs over East China Sea and Yellow Sea.Composite data analysis and dynamic diagnosis are employed to reveal the circulation characteristics of these two kinds of phenomena. Composite data analysis with potential height field, temperature field, water vapor transportation, sea surface temperature(SST), vertical wind shear (VWS),density of core convection(DCC) and upper level jet and outflow of typhoons are completed.The results of height field analysis show that the rapid intensification of typhoon is related with the subtropical high situated to the north of it,on the other hand, the rapid decay is related with the westerly trough situated to the west of it. In the temperature field, there are always warm ridges encompassing the typhoons during their rapid intensification and cold tongues invading during rapid decay. In terms of water vapor transportation, the moisture supply to the typhoon of rapid intensification is abundant, the rapid decay is not exist moisture fluxes. In the upper level jet field,the outflow is strong during their rapid intensification and slow during rapid decay. The rapid intensification of typhoon is related with high DCC, the rapid decay is related with low DCC.As to the sea temperature field, the rapid intensification occurs over warm sea surface temperature region whereas the rapid decay of tropical cyclones occurs over cold sea surface temperature region. At the same time, the vertical wind shear also has influences on rapid intensity change. The high (low) VWS will make a contribution to rapid decay (intensification) of tropical cyclones.Six factors which impact the rapid intensity change from the diagnostic analysis above have been taken into account to explain the rapid intensity change. The results suggest that the water vapor transport and high level jet are the two high frequent factors for rapid intensification, and the strong cold air intrusion is the high frequent factors for rapid decay of tropical cyclones. Actually whatever the rapid intensification or decay were usually caused by multiple factors jointly. For example, the rapid decaying of tropical cyclone usually arise from the cold air intrusion, cold sea surface and strong VWS etc.Among the six factors,the SST,VWS and DCC are the basic factors reflecting the sea surface forcing,environmental control and TC inner thermodynamical process.The research results show that the high SST, low VWS and big DCC are favorable for TC rapid intensification,on the contrary,low SST,high VWS and low DCC would lead to decaying. Anyway,sea surface temperature is a basic factor.When TC in high SST,high VWS and low DCC can lead TC to decaying.When TC in middle SST,low VWS and high DCC can also help TC intensifying.Tropical cyclone with high DCC over middle SST ocean, usually it move from the high SST ocean and intensifying over middle SST ocean area.Numerical experiment is conducted to simulate the TC rapid intensity change.Typhoon Meranti(1010) is a rapid intensifying typhoon,when it move accross coastal water. The numerical simulation demonstrates that Meranti stay over a high SST ocean basin and move across into a middle SST sea area and intensified.Sensitive simulations show that SST,VWS and DCC affect the intensity change of Meranti remarkably and to fit in with the observed process of Meranti.Numerical simulations are also performed on the dissipation of typhoon Goni(0907).The result shows that the "pumping up" effects of typhoon Morakot(0908) lead to dissipation of typhoon Goni.Interaction of binary tropical cyclone usually induce one of the pair intensifying and the other one decaying.Finally a conceptual model of the TC rapid intensification and dissipation is formed,which can be provided for a references for operational forecast.