An Observational Investigation And Numerical Study On Entering Back To Ocean Typhoon With Different Track After Landfalling

In this thesis, the thermal and dynamical structures and re-intensification mechanisms of three extratropical transition typhoon cases with different tracks, such as typhoon Winnie occurred in August 1997, typhoon Toraji and Mindulle occurred in 2001 and 2004 respectively, are investigated by using different available observational data and the results of simulation by PSU/UCAR mesoscale model-MM5. The results show that there are some common characters in synoptic situation, thermodynamics and dynamics structures between these three typhoons, but there also exist some differences among them. This paper aims to conclude the similarities and differences of extratropical transition typhoons in different tracks and further to give the conceptual model about this type of typhoon. This work is helpful to the forecast of typhoon tracks and can be used to reduce typhoon damages.Typhoon Winnie in 1997 is a typical extratropical transition intensification case. At beginning of extratropical transition, the intensification becomes weak because of the surface friction and other factors. But along with its transition up to the north and the interaction with mid-latitude westerly trough, the edge of Winnie and mid-latitude front become to combination, the cold air in the westside can be involved to the inside of Winnie and forms a very sharp contrast with the eastside warm air in the typhoon, this lead the thermodynamics structure to a imbalance distribution, the lower layer appears to a half cold and half warm situation (cold in west and warm east). Along with the continual injection of the cold air, a ring-like frontogenesis appears in the typhoon lower circulation and forms a inclined distribution which is relative to the cold-warm front, the west and the north has cold and warm front characters respectively, the typhoon become to a frontal cyclone. In the process that the cold air surrounds the typhoon center, which makes the baroclinic potential energies transform to kinetic energies. Meanwhile, the warm air is forced to rise and the vertical vorticity enforces, so the low-level circulation of typhoon grows. When cyclone couples with the upper level jet, the strong moist potential voticity(MPV) transmission from upper level strenthens the cold air, which results that equivalent potential temperature is more intensive and baroclinic enhances. From the conservation of MPV we know that in order to reduce the baroclinic, air mass must be stretched in vertical direction, and then relative vorticity develops, cyclonic circulation is induced on low-level frontal zone. In the process of extratropical transition, the thermal and dynamic structure evolve into asymmetric distribution, and negative temperature anomalies westward tilt with height in low-level of typhoon. The warm heart of upper-level falls from 200hPa to 400hPa and strengthens in the process. Meanwhile, the relative vorticity deviates from the center of the typhoon and locates at the west of the cyclone. The radius of maximum wind speed extends to three latitude. From the energy perspective, the strongest growth of regional average kinetic energy and eddy kinetic energy locates at 850-600hPa in the lower troposphere, which corresponds to the cold air. Results show that Winnie's re-intensification is associated with the meso-scale frontogenesis process in its circulation. A kind of frontogenesis zone in ring form, which wraps up the typhoon center, is found in the lower layer of typhoon remnant circulation. The intensification of the remnant of typhoon Winnie looks like the process of an extratropical cyclone developing in the surface frontal zone.Typhoon Toraji in 2001 goes through decaying, extratropical transition intensification and then decaying rapidly after landfalling. At the stage of extratropical transition intensification, besides the influence of southwesterly ahead of the upper trough, there is a low-level jet stream to the east of typhoon leading to a closed low-level low system above Shandong peninsula in front of the jet stream, where there is an intensive convergence zone. When residual eddies move northward and combine with the low pressure system, typhoon develops due to the combination of two homogeneous systems. Along with typhoon moving onto sea surface and approaching the low-level jet stream convergence zone, and the friction decreasing, the intensity of Toraji reaches peak when it moves to Shandong. Thereafter upper trough deepens southward, departs from westerly trough and becomes cut-off low. This system restrains typhoon moving northward and makes the cyclone get upper level energy hardly. Therefore, cold air fills in the low level of cyclone, which is occluded and vanishes. Due to the effect of southwesterly ahead of the upper trough and subtropical high, circulation on the east side of center is larger than that on the west side, while the maximum wind speed appears too high and locates at the middle-level of troposphere. Sensitivity experiment shows that latent heat release plays an important role in extratropical transition and re-intensification process of typhoon:If there is no latent heat release, typhoon will decay rapidly after landfalling due to the lack of heat exchange.Typhoon Mindulle experiences three landing process in 2004. The circulation both sides of the typhoon center strengthens with the development of the typhoon. the maximum of southerly circulation moves from middle troposphere to lower troposphere. The evolution of regional average kinetic energy and eddy kinetic energy show that, the strengthening process are mainly located in the troposphere underside 700hPa, and the growth is largest in the three typhoon cases below 900hPa because of low coefficient of friction.The research shows that, the extratropical transition and re-intensification of typhoon associates with Westerly trough when the typhoon enters mid-latitude zone. The positive vorticity advection and divergence of upper trough are all play a positive role in re-intensification. Meanwhile, the positive moisture potential vorticity anomaly downward transported from the upper troposphere induces cyclonic circulation in the lower front area and increased cold air which promotes the release of baroclinic potential energy into kinetic energy, and remnant vortex of typhoon develops. In ET process, whether the typhoon couples with the upper level jet, and whether the existence of low-level jet and the weakening of the friction surface, all affect the typhoon could strengthen, and enhance strength.