| Summary: | 博士 === 臺灣大學 === 大氣科學研究所 === 98 === The extratropical transition of Typhoon Winnie, the self-amplification process of the cyclone, and the tropopause fold accompanying the extratropical transition are diagnosed using potential vorticity inversion. Typhoon Winnie weakened rapidly, re-curved, and transformed into an extratropical cyclone after making landfall over mainland China on 18 August 1997. The cyclone transformed from a vertically deep system into a shallow one limited in the lower-to-middle troposphere. The initially symmetric structure associated with a tropical cyclone also changed into an asymmetric one with cold and warm fronts in the lower troposphere. Analyses suggest that the mechanism responsible for the development of this cold front was very likely evaporative cooling, which strengthened the cold tongue at the western section of the cyclone. Such a frontogenetic process is very different from that for an ordinary mid-latitude cold front associated with airmasses.
The cyclone coupled with the upper-level jet streak shortly after making landfall, and the jet streak seemed to play some role in maintaining the circulation strength of the cyclone. The re-intensification started with a burst of convection over the warm-front area. The latent heat was not only very crucial for cyclogenesis, but also affected the movement of the cyclone. The PV anomaly associated with the cyclone induced warm and cold advections at mid-levels when the cyclone migrated into a mid-level baroclinic zone. The dipole structure of the temperature advections, as well as a diabatic contribution through latent heating caused an amplification of the initially weak upper-level wave. With proper phasing with the upper-level system, the cyclone intensified first by vertical coupling and then by superposition. Many previous studies of ET suggested that the re-intensification process of the cyclone was similar to the Petterssen type B scenario, and the role of the cyclone was secondary. In contrast, the re-intensification of Winnie was a manifestation of self-amplification process and the role of the cyclone is primary.
Strong tropopause fold events are usually associated with upper-level frontogenesis during cold seasons. In this study, a rather rare strong mid-latitude tropopause fold event in summer was found to be associated with the extratropical transition of Typhoon Winnie. The vertical motions associated with the cyclone and the mid-latitude trough tilted the tropopause, the horizontal confluence caused by the cyclone further steepened the slope of tropopause, and finally the vertical wind shear induced by the cyclone produced a fold.
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