| Based on the Tropical Cyclone (TC) Yearbooks data from 1979 to 2008, statistical analyses on temporal and spatial distribution were executed to research the Extratropical Transition (ET) of TC which made landfall on China and comparative compositing analyses and computation of kinetic energy budget were performed to study the environment fields of intensifying TCs during ET (IET) and weakening TCs during ET (WET), besides, the impacts of environment fields on the ET process of TC and Its possible mechanisms were studied by using the method of numerical simulation in accordance with the features of environment fields reavealed by compositing analyses. The main conclusions are as follows:(1) Statistics indicated that there are 30 TCs, which account for 15.23% of the total number of TCs landing in China during the 30 years, experienced ET. The translating time of the 30 ET cases are focus on May,June,July,August and September and there are 12 ET cases in August,6 in July,5 in each June and September, only 2 in May. The translating positions of these ET cases are mainly located in North China,Northeast China,HuaiHe Region,East China Sea and Yellow Sea. The temporal and spatial distribution of these ET cases landing in China is closely related with the average position of East Asian trough.(2) Comparative compositing analysis and computation of kinetic energy budget were performed to study six samples of IET and WET which transformed in Shandong, HuaiHe Region and offshore areas. The study shows the following results: IET shows the strong upper-level divergence, strengthened low-level convergence and significantly enhanced upward motion under the influence of strong upper-level trough and high-level jet, and it intensified after Extratropical Transition. WET exhibits the strong upper-level divergence, subdued low-level convergence and slightly enhanced upward motion under the influence of weak upper-level trough and high-level jet, and it weakened after Extratropical Transition; The increasing (decreasing) of the generation of kinetic energy by divergence wind in IET (WET) at low level is one of major reasons for IET's intensification (WET's weakening) after transforming and the generation of kinetic energy by divergence wind is closely related to the development of low-level baroclinic frontal zone; The growth of the generation of kinetic energy by rotational wind in IET at upper level is favorable for IET's maintenance, which is affected by the strong upper-level trough. The dissipation of the generation of kinetic energy by rotational wind in WET at upper level is unfavorable for WET's maintenance, which is affected by the weak upper-level trough.(3) We find that there is a nice corresponding relationship between the variation of TC's intensity after ET and initial strength of westerly upper-level trough by compositing analysis of ET cases which made landfall on China, So the TC Haima which made landfall on China in 2004 is taken as an example, the evolution of its transition process is reappeared by the WRF model. We modified the intensity of upper-level trough in the initial fields using the method of piecewise potential vorticity (PV) inversion and investigated the impacts of different troughs on Haima's re-intensification process during ET. The results indicate that the movement velocity of Haima accelerates (decelerates) when the trough enhanced (weakened), furthermore, the deeper (shallower) the upper trough is, the stronger (weaker) the Haima re-intensifies; The deeper trough shows wide and strong upper-level jet and enhanced divergence, which is favorable for Haima's redevelopment; Besides, the deeper upper trough is accompanied by stronger positive belt of PV, the upper-level positive PV propagates downward low-level to result in growth of Haima's positive PV and the intense development of low-level baroclinic frontal zone, finally, the low-level cyclone reinforced obviously. |
PDF Full Text Request