Thermal Structure Characteristics of the Extratropical Transition of Tropical Cyclone Chaba(0417)

The thermal structure characteristics of tropical cyclone(TC)Chaba(0417)over the western North Pacific Ocean in 2004 in its extratropical transition(ET)period are analyzed with the temperature data from NOAA-15 AMSU-A, reanalysis data from NCEP and black body temperature(TBB)data from GMS-5. It is found that the original symmetric structure of warm core is destroyed and sinking with slope asymmetric distribution in the ET process, and there is a weak warm center in the upper and lower troposphere respectively in the core region. The remarkable front characteristic in Chaba circulation appears distinctly. Through the thermal structure analysis, it is found that the extratropical cyclone is developing while Chaba is decaying. The decay of the tropical cyclone and the development of the extratropical cyclone occur successively while the tropical cyclone moves to the higher latitude area and interacts with preexisting mid-latitude system. The obstruction of water vapor transport and the symmetric warm core destruction are caused by the intrusion of cold air from middle upper troposphere in west semi-circle of Chaba. At the same time, a dipole is caused and advantaged conditions for extratropical cyclone development is initiated by the weak warm air which ascends cyclonically in the east semi-circle of the tropical cyclone and the cold dry air which descends in the west semi-circle of the tropical cyclone. Consequently, a sort of conceptual model of ET process in the Western North Pacific Ocean is proposed. The development of the extratropical cyclone is the result of the interaction between the warm air ascending in the east semi-circle of the TC and the cold air from mid-latitude in the west semi-circle of the TC.The beginning of the extratropical transition period of tropical cyclone Chaba in the Western North Pacific is defined while the tropical cyclone circulation encounters the preexisting mid-latitude baroclinic front zone and interacts with it. The end of the extratropical transition period of tropcical cyclone Chaba is defined while the original closed circulation of tropical cyclone enters completely into the westerly circulation in 500 hPa and evolves into a baroclinic storm, it indicates that tropical cyclone Chaba is captured by westerly circulation and an asymmetric distribution of clouds in the extratropical cyclone is remarkable.