西北太平洋極低緯度熱帶氣旋快速增強過程之探討

A Study of Rapid Intensification of Tropical Cyclones over the Low-Latitude western North Pacific

陳建安(Chien-An Chen)

指導教授 ： 李清勝

國立臺灣大學/理學院/大氣科學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.01946

摘要

極低緯西北太平洋地區(low-latitude western North Pacific，LLWNP) 絕大部分地方之年平均maximum potential intensity (MPI)均在70 m/s以上，即此區域之熱力條件極有利熱帶氣旋(Tropical cyclone，TC)之發展。 本研究即針對此區域TC之快速增強(Rapid intensification，RI)過程進行診斷分析與數值模擬，探討此種個案之綜觀環境和其中之重要機制。
分析結果顯示，LLWNP RI較常發生於信風較強之5、10及11月，強信風對LLWNP RI似扮演重要角色，且使此種TC於RI期間之移速比平均值快21%。 有38% 之LLWNP RI發生於TC強度達熱帶風暴前，76%個案之RI持續時間為24~54小時；經歷LLWNP RI後的TC，高達36%之最大強度達到category 5等級。針對有RI和無RI (non-RI)個案之合成分析結果顯示，低層與高層之動量場在LLWNP RI過程中扮演重要角色。 LLWNP RI發生前12小時(pre-RI組)，在低層，TC北側有大範圍強東風、南側則有強西風(與位於南半球低緯地區之氣旋式環流密切關連)，此流場配置提供較強之背景風切渦度。在高層，南半球之西風帶常較強且較偏北，TC東南側約3500公里處有一強而穩定之高層反氣旋，而TC東北側之高層反氣旋則較弱；此配置使系統有較強之往南外流分量(或外流通道)，而利於LLWNP RI發生。 個案分析結果則顯示，高層流場可分為三個類型：Type A(西南向外流通道)、Type B(西向外流通道)及Type O(剩餘類型)；pre-RI組屬Type A、B及O之比例分別為48%、33%及19%，但non-RI組則分別為11%、10%及79%，其中又以北向外流通道配置占多數。結果亦顯示，若TC西到西北側約2000~3000公里處之低層有氣旋式環流，則該處高層之反氣旋將被加強或維持，且將加強TC往西南之外流並利於RI之發生(屬Type A)。 針對典型LLWNP RI個案-芭瑪颱風(2009)之WRF模擬結果顯示，模式可合理模擬低層和高層(Type A)之大致環流特徵與RI過程；在芭瑪發生RI前，低層強風區有以氣旋式方向繞著中心移動，並逐漸對稱化的現象。渦度收支分析結果顯示，在RI時，輻散項是主導渦度增加最重要之項。

關鍵字

熱帶氣旋 ； 颱風 ； 快速增強 ； 極低緯度西北太平洋 ； 信風 ； 外流通道

並列摘要

The low-latitude western North Pacific (LLWNP) is a region with high maximum potential intensity (MPI) for tropical cyclone (TC) because of its thermodynamic environments. MPI is higher than 70m/s over most part of the LLWNP. Since the intensity of an incipient TC, which often forms at lower latitudes, is generally low, the probability of a TC to experience rapid intensification (RI) thus is not low at LLWNP. Observations show that most RI cases at LLWNP occur in May, October and November. The average moving speed of RI cases is 21% faster than that of the total cases at LLWNP. These two characteristics are likely to be associated with the stronger trade wind, which is an important factor to the LLWNP RI. About 38% of all RI cases start at TD stage and 76% of RI processes last for 24 to 54 hours. About 36% of all RI TCs reach category 5 intensity during their life periods.
Prior to the RI process, there are strong low-level trade winds (westerlies) to the north (south) of TC and a cyclone exists in the low-latitude of South Pacific. The aforementioned atmospheric patterns create a zonally-extended region of strong shear vorticity over the LLWNP. In upper-level, favorable outflow channel can be provided by (a) stronger and more equatorward upper-level westerlies over the south Pacific, (b) stronger upper-level anti-cyclone locate at 3500km to the southeast of TC, and (c) weaker upper-level anti-cyclone locate at 1200km to the northeast of TC. In this study, the upper-level patterns are classified into three types － type A (southwestward outflow channel), type B(westward outflow channel) and type O (residual patterns). For the LLWNP RI cases, 48% are type A, 33% are type B and 19% are type O. For the LLWNP non-RI cases, however, 79% are type O (mainly contributed by northward outflow channel) and only 21% are type A or type B. The aforementioned results imply that the upper-level condition is very important to the LLWNP RI. Results also show that, the LLWNP RI seems to be easier to occur if another TC exists at about 2000~3000km to the west of TC. This result may be due to the enhancement of southwestward outflow by the upper-level anti-cyclone near the west TC. In the WRF simulation of Parma(2009), strong wind region move cyclonically around TC center during LLWNP RI and the divergence term dominates the low-level vorticity increase of Parma.

並列關鍵字

tropical cyclone ； typhoon ； rapid intensification ； low latitude western North Pacific ； trade wind ； outflow channel

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