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伴隨強跨赤道流之熱帶氣旋形成的初步研究

A Preliminary Study on Tropical Cyclone Formation

摘要


本文分析西北太平洋地區,於形成時伴隨強跨赤道流之熱帶氣旋的氣候特徵,並利用MM5進行個案之模擬與分析,探討強跨赤道流在熱帶氣旋形成過程中的作用。結果顯示,伴隨強跨赤道流的熱帶氣旋形成個案佔全部之19.4%;此類個案大多集中在七~九月,其形成之位置平均較為偏南。分析ECMWF 850hPa流場顯示,大部份個案的強跨赤道流出現在擾動中心之西南方,且大部份個案之北方同時有東風存在。此外,約有四分之一的個案,在形成過程中,其低層曾具有兩個中尺度渦旋。 本研究模擬兩個南方有強跨赤道流、而北方有顯著東風的熱帶氣旋形成個案,其中葛樂禮(1996)850hPa初始風場有兩個中尺度渦旋,而歐珀(1997)則僅有一個。積分12小時後,兩個個案皆有兩個中尺度渦旋出現;此中尺度渦旋具顯著氣旋式環流,並有地面低壓中心配合。隨積分時間,兩個中尺度渦旋皆增強並向上發展,而且互繞、相互接近並合併。在渦旋合併時,就低層渦度場而言,較弱的系統逐漸變形並被捲入較強的系統中;然而,就近地面流場而言,則恰相反。此外,近地面流場顯示的合併時間,較低層渦度場顯示之合併時間為早。兩系統合併後,系統渦度顯著增強,並有水平收縮、垂直拉伸的現象。 最後,為驗證伴隨強跨赤道流之熱帶氣旋形成過程中,是否常具有雙渦旋合併過程,本研究亦分析NSCAT風場和其他個案的低層流場特徵。結果顯示,歐珀(1997)、羅絲(1997)及艾薇(1993)颱風,在發佈TCFA前,低層流場亦具有雙渦旋特徵,但ECMWF風場並不能完全解析此種中尺度環流特徵。因此,我們認為在此種熱帶氣旋形成個案中,有可能有中尺度渦旋的合併過程出現;此渦旋合併過程在系統渦度增強上,扮演重要的角色。

並列摘要


This paper analyzed the climatology of those tropical cyclones formed in the western North Pacific and while the strong cross-equatorial flows (SCEF) occurred. Case simulations using MM5 were performed and the results were analyzed to address the role of SCEF on tropical cyclone formation. Results show that 19.4﹪of all tropical cyclones formed while SCEF occurred. Most of these cases occurred in July ~ September and at lower latitudes. The ECMWF 850hPa streamlines showed that for most of such cases, the SCEF occurred at places to the southwest of the disturbance center. The easterly also prevailed to the north of the system center. In addition, a quarter of such cases appeared to have two meso-vortices during their formation stage. Two cases with SCEF to the south and significant easterly to the north were simulated. The initial 850hPa flows showed that one case (Gloria, 1896) had two meso-vortices while the other (Opal, 1997) had only one. However, both cases had two meso-vortices after 12-hour integration. These meso-vortices had pronounced circulation and low pressure center at the surface. Both systems intensified and developed upward at later time periods. They also rotated, moved closer with respect to each other and then merged. During the merging process, the weaker system deformed and was wrapped into the stronger system according the vorticity pattern. However, the process appeared to be different for the surface flow pattern. In addition, the merging was earlier for the surface flow pattern than for the vorticity pattern. After the merging process, the vorticity of the system increased significantly. The vortex also contracted horizontally and extended vertically. Finally, NSCAT flow field and the low-level circulation of some other cases were examined to address if the vortex-merging a common feature during the tropical cyclone formation stage. Results showed that low-level flow pattern appeared to have two vortices prior to the issue of TCFA for Opal (1997), Rosie (1997) and Irving (1993). However, the ECMWF grid point data could not resolve such mesosclae flow feature. Therefore, we concluded that the merging of meso-vortices might have occurred for some tropical cyclones and such process appeared to be crucial to the formation of these cases.

被引用紀錄


謝宜桓(2017)。低頻渦度對熱帶氣旋形成之研究〔博士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU201704485
張龍耀(2013)。伴隨信風增強之熱帶氣旋形成過程〔博士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2013.00290
林雍嵐(2004)。梅雨期間於南海弱斜壓環境下形成熱帶氣旋之分析〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2004.01411
莊惟然(2004)。颱風形成期間環境信風場之變化與影響〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2004.01407

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