A Study Of Mechanism On Rapid Weakened Typhoon Over The Offshore

A study about strength of typhoon is the focal point in typhoon study field at present, and is short of forecast ability on it, especially when typhoon strength happen sudden change. When typhoon intensity changes abruptly over coastal waters, it will lead to major disaster because of unprepared or cause a waste of property leaded by the excess defense. There is still a lack of study about sudden weakened typhoon currently .So, in order to advance the forecast ability about typhoon strength, it is necessary to emphasis the comprehend about the mechanism of sudden weakened typhoon over offshore.In this paper, through define the criteria of rapid intensification tropical cyclones (RITC) and rapid weakening tropical cyclones (RWTC), and then, utilizing the it to sift six RITC and six RWTC. Through comparative analysis the results of RITC and RWTC by using dynamic composite analysis technique, declare that: South Asia high trends to intensify during the phase when TCs rapid weakening; and it trends to weaken when the TCs happen rapid intensifying. RITC is related to the combined action of plenteous water vapor,strong convergence and propriety ascending motion; RWTC is affected by the combined action of weak ascending motion and weak convergence at the low-levels,strong ascending motion and strong divergence at the high-levels in the condition of water vapor shortage, in this case latent heat supply will decrease, and strong ascending motion and strong divergence will accelerate dissipation energy quickly from TC ,then TC happens rapid weaken ,this is the very importance reason for TC happens rapid weakening. Low-level cold air intrude into inner of typhoon is one of the important factors for TC happen weaken abruptly. Environmental wind shear by increasing the gradient strength is also in favor of offshore typhoon weakened suddenly.In order to have a further analysis on the main weaken factors, WRFV3 model is utilized to carry out the numerical experiment of sudden weaken offshore typhoon example ("Dujuan").After diagnosed the outcome of numerical experiment, we can find: first, during the phase when TC abruptly weaken, northwestern Pacific subtropical high is also trending to weaken, and South Asia high is trending to strengthen, and cyclonic vortex which near the eye-wall reduce ; the maximum vertical vorticity near the eye-wall is asymmetry ;all of these changes are favorable for the TC take off abruptly. In the phase of typhoon abruptly weakening, convergence at the low-levels trend to be weaken, eye-wall convergence intensifying at first, and then weak quickly; the divergence center which on both sides of the typhoon center is strengthen at the beginning, and then western of TC center start to weaken, but eastern of TC center is last intensifying; after weaken abruptly, the divergence center go on weakening and divided, but at the eastern of TC center the divergence center still strength. Second, water vapor decreased in the inner of typhoon is the main factor which can cause typhoon strength happen abruptly weaken, typhoon strength is osculating correlation with the receive capacity of water vapor. Third, there is a fine parallelism between warm-cored strength in typhoon and typhoon intensity, when warm-cored is intensifying, then typhoon intensifying, when warm-cored is weakening, then typhoon also weakening. Forth, non-adiabatic cooling affect is the main role of which lead typhoon internal heat dissipation, the role of temperature advection affect is more feebleness than non-adiabatic cooling affect. On-adiabatic cooling affect will to be weakening when TC is weakening, and the peak on-adiabatic cooling affect is at the level which is also the peak of warm-cored strength; all of these are favorable for typhoon sudden weakening. Non-adiabatic heating have positive contribute for the local action of temperature in the inner typhoon, it is a very important way for typhoon get heat energy. After diagnose the water vapor equation, can find that inner water vapor capacity is mainly influenced by water vapor vertical transportation and the condensation- evaporation. The water vapor condensation is more strength than the water vapor evaporation in the inner of typhoon, and when typhoon trend to weaken, condensation have the same trend.The results of sensitive experimentation about water vapor and typhoon intensity show: typhoon intensity have direct relation to the environmental field water content; when water content in environmental field is increasing ,then typhoon strength will intensifying ,and intensifying rate also increasing; contrarily, when water content in environmental field is decreasing ,then typhoon strength will weakening . These results can explain that water content in environmental field have hugeness influence for typhoon strength.Through diagnose the change characteristic of water vapor and characteristic of warm-cored by the data output from sensitive of under laying surface experiment, the results show: typhoon warm-cored in sensitive of under laying surface experiment is more stronger than it in the control experiment, the change of warm-cored cubic content and warm-cored intensity is the same with the control experiment. Cut down the derive capacity of water vapor into the inner typhoon circumfluence is the main way which offshore island or landform influenced typhoon over coastal waters of China.