西北太平洋热带气旋变性阶段强度变化的比较研究

Comparison of Intensity Changes of Western North Pacific Tropical Cyclones during Extratropical Transition

选取西北太平洋上两个生命史中发生变性的热带气旋Yagi和Francisco,前者变性后有一个24小时的再增强过程,而后者则继续减弱直至消亡。利用日本气象厅提供的热带气旋资料和美国环境预报中心(NCEP)提供的FNL全球分析资料,对比分析两个TC在变性阶段的形势场,发现两者在高低层的环境场均具有明显的差异:Yagi在变性阶段其高空槽较强且在低层有一个与中纬度原先存在的温带气旋合并的过程;而Francisco在变性阶段其高空槽较弱,且变性后自行消亡。另外探讨了导致Yagi变性增强的原因,结果表明:(1)Yagi变性阶段与高空槽前的急流相互作用时,高空急流入口区左侧和出口区右侧的次级环流将产生高空辐散低空辐合的趋势,有利于低层TC低压的发展。同时,当Yagi在穿越急流的过程当中,垂直风切变的增加将导致斜压不稳定增强,低层锋区强烈发展,锋区内的斜压能量可能向TC动能转化,从而使得Yagi发展增强;(2)高空槽所对应的高层湿位涡下传可使得低层正涡度增长,从而在低层诱生出气旋性环流,有利于Yagi变性后重新发展;(3)Yagi与中纬度原先存在的温带气旋发生合并,温带气旋所带来的较高纬度冷空气的入侵增强了低层的水平温度梯度,使得低层锋区强烈发展,从Yagi以一个锋面气旋的形式而再度发展,促使其变性后进一步增强。而这些特征都是Francisco所不具备的。

Two tropical cyclones over the western North Pacific that experienced an extratropical transition （ET）, Yagi and Francisco, are examined in this study. The former underwent a 24-h reintensification process after the ET, whereas the latter continued to weaken until dissipation. On the basis of tropical cyclone data from the Japan Meteorological Agency and Final Operational Global Analysis data from the National Centers for Environmental Prediction, a comparison of the upper- and lower-level environmental fields during the ET revealed clear differences. A strong upper-level trough existed during the ET for Yagi; moreover, Yagi merged with a preexisting extratropical cyclone at the lower level during the ET.In contrast, the upper-level trough related to Francisco was weak during the ET, and Francisco dissipated after the ET. In addition, the reintensification of Yagi could be attributed to the following causes： （1） Yagi interacted with the upper-level jet stream ahead of the trough during the ET stage and was influenced by the upper-level divergence and lower-level convergence tendency caused by the jet stream, which was favorable for reintensification after the ET. Furthermore, the vertical wind shear increased when Yagi was crossing the jet stream, which can lead to baroclinic instability and the development of a lower-level frontal zone. The baroclinic energy in the frontal zone may have been converted into kinetic energy through this process and contributed to the reintensification of Yagi. （2） The downward extension of the moist potential vorticity related to the upper-level trough could have increased the positive vorticity in the lower level and induced cyclonic circulation, which strengthened Yagi after the ET. （3） The merging of Yagi with a preexisting extratropical cyclone brought cold air from high latitudes, which can enhance the horizontal temperature gradient and cause intense development of the lower-level frontal zone. Thus, Yagi evolved into a frontal cyclone and reintensified after the ET. In contrast, these patterns mentioned above did not exist for Francisco.