颱風生成與梅雨鋒之交互作用:凱米(2018)個案分析

Abstract

In this study, we followed the concept of Lee et al. (2006) to define that the tropical cyclones formed in the Mei-Yu frontal depression as the frontal-type tropical cyclone formation. The frontal-type cases during 1979 to 2018 were identified to uncover the essential signatures of the large-scale environments. The filtering method was applied to distinguish the signals from different temporal and spatial scales. The large-scale analyses suggested that the strong southwesterly flow, abundant low-level vorticity and reduction of baroclinicity fertilized the formation of tropical cyclone in the southwest end of the frontal depression. Furthermore, a representative frontal-type case, tropical cyclone Gaemi (2018), was analyzed to investigate the vortex-scale process and mechanism via the reanalysis and the numerical simulation. The southwesterly flow induced the active convection and strong horizontal wind shear in the frontal region. The perturbations were triggered via barotropic instability and the potential vorticity in the frontal depression nourished their intensification. Moreover, the mid-level trough was above one frontal vortex and helped the establishment of mid-level vortex. This perturbation finally developed into a tropical cyclon¬e ¬— Gaemi. In addition to the cyclogenesis mechanism, the frontolysis was an interesting phenomenon as well. The frontolysis was induced by the short-wave radiation prior to Gaemi’s development. The reduction of baroclinicity in this stage provided a suitable environment for Gaemi’s formation. After Gaemi enhanced, Gaemi’s strong potential vorticity distorted the frontal structure and Gaemi’s secondary circulation constrained the convection in frontal region. Thus, the Conditional Instability of the Second Kind (CISK) mechanism was restrained and led to the significant frontolysis. Last, a conceptual model was proposed to elucidate the interactions between the tropical cyclone formations and Mei-Yu fronts.