Formation and Development of a Mountain-induced Secondary Center inside Typhoon Morakot (2009)

Abstract

The structural evolution of Typhoon Morakot (2009) during its passage across Taiwan was investigated with the WRF model. When Morakot approached eastern Taiwan, the low-level center was gradually filled by the Central Mountain Range (CMR), while the outer wind had flowed around the northern tip of the CMR and met the southwesterly monsoon to result in a strong confluent flow over the southern Taiwan Strait. When the confluent flow was blocked by the southern CMR, a secondary center (SC) without a warm core formed over southwestern Taiwan. During the northward movement of the SC along the west slope of the CMR, the warm air produced within the wake flow over the northwestern CMR was continuously advected into the SC, contributing to the generation of a warm core inside the SC. Consequently, a well-defined SC with a warm core, closed circulation and almost symmetric structure was produced over central western Taiwan, and then it coupled with Morakot’s mid-level center after crossing the CMR to reestablish a new and vertically stacked typhoon. Therefore, the SC inside Morakot was initially generated by a dynamic interaction among the TC’s cyclonic wind, southwesterly wind and orographic effects of the CMR, while the thermodynamic process associated with the downslope adiabatic warming effect documented by previous studies supported its development to be a well-defined SC. In summary, the evolution of the SC in this study is not in contradiction with previous studies, but just a complement, especially in the initial formation stage.

摘要

本文利用WRF模式模拟和分析了台风“莫拉克”(2009)过台湾岛引发结构重组的演变过程:当“莫拉克”登陆台湾岛东部时, 其低层台风中心受地形阻挡和填塞导致最终消亡, 而台风外围低层气流绕过岛屿在台湾海峡南部上空与西南季风交汇并产生偏西气流并指向台湾南部, 受南部地形阻挡作用最终在台湾西南侧产生一个具有闭合环流结构(欠缺暖中心)的副中心. 之后, 受偏南气流引导, 该副中心北移至台湾中西部沿海. 与此同时, 偏东气流越过台湾北部山脉在地形背风侧因绝热增温和降压作用产生的低压暖空气持续向下游平移并注入副中心, 最终使得副中心具有暖低压中心和闭合环流结构, 并与越过中央山脉的“莫拉克”中上层台风中心在台湾中西部发生重组合并. 因此, “莫拉克”过台湾岛引起的副中心始发于台风环流、西南气流与中央山脉之间相互作用产生的闭合环流或涡旋, 属于一种动力机制;它区别于早前研究认为副中心是由过山气流在地形背风侧产生的低压中心发展演变而来的热力机制;但是“莫拉克”引起的副中心因产生于地形迎风侧, 后期还需台风偏东气流在中央山脉西北侧产生的暖空气注入才能发展并获得暖心结构, 最终成为完整定义的副中心. 因此, 本文所提“莫拉克”副中心的始发新机制是对台风过岛引发副中心生成机制的进一步补充研究.