微波衛星觀測西北太平洋雙眼牆颱風特性之探討

Microwave Satellite Observation of Tropical Cyclone with Concentric Eyewall in Western North Pacific Basin

楊憶婷(Yi-Ting Yang)

指導教授 ： 郭鴻基

國立臺灣大學/理學院/大氣科學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.02480

摘要

本研究發展一客觀方法判斷雙眼牆結構。此法直接從SSM/I及TMI 85GHz衛星雲圖得到颱風的雲頂黑體輻射溫度，並設定標準判斷雙眼牆結構。透過這個方法我們一共分析了1997至2011年間西北太平洋26774張微波衛星雲圖，共判斷出234個雙眼牆微波衛星雲圖、77個雙眼牆颱風以及95個雙眼牆颱風個案，其中包含了16個多次形成雙眼牆的個案。研究中包含：(1)西北太平洋15年間雙眼牆颱風的氣候統計、(2)雙眼牆形成後的結構變化，及(3)雙眼牆形成前的不對稱對流分析。
(1) 在氣候統計的研究中，我們將Niño Index達到+0.5oC和-0.5oC連續5個月時，定義為暖期及冷期，比較了暖期和冷期形成的雙眼牆颱風。在暖期時，因為太平洋東側的海水相對溫度比平常時期高，雙眼牆颱風形成位置偏東，而且有較高比例的雙眼牆颱風生成，並且雙眼牆形成前後24小時期間可能因為沃克環流相對減弱，850-200 hPa的垂直風切較弱，可能的原因是由於暖期的東側太平洋海水溫度較高，導致暖期的雙眼牆颱風強度較強，較能維持其強度。
(2) 雙眼牆的結構變化研究中發現，雙眼牆形成後不單只有一般所熟知的外眼牆內縮並取代內眼牆的眼牆置換過程(Eyewall replacement cycle, ERC)，有23%的雙眼牆颱風可以維持雙眼牆結構很長的時間(Concentric eyewall maintained, CEM)，另外約24%個案是由外眼牆部分消散，而內眼牆仍然存在(No replacement cycle, NRC)。分析這3種結構特徵及環境因子隨時間變化，顯示CEM個案強度較強、兩個眼牆間的弱對流區(moat)及外眼牆較寬，根據正壓理論較為穩定，此外這一類個案形成於較好的環境，有利於其維持結構。NRC個案則是平均形成位置較高緯度並且向北移動的速度較快，容易遭遇到強垂直風切等不利的環境，導致外眼牆部分開始消散。ERC個案則因為環境因子沒有明顯的特徵變化，可能是由內在動力控制。本研究也發展了T-Vmax圖，將以颱風為中心的400 km×400 km範圍內對流強度(Convection activity, CA)與強度隨時間變化同時包含於圖中，希望可以提供颱風預報參考。
(3) 在雙眼牆形成前的不對稱對流分析中，我們發現對流分布都坐落在下風切處的左側，沒有形成雙眼牆的個案在垂直風切下風切處左側的不對稱對流，比雙眼牆的個案略強，可能的原因是沒有形成雙眼牆的個案垂直風切也較強的緣故。從雙眼牆生成季節來看不對稱對流的分布，我們發現4-6月及7-9月間，不對稱對流分布於東南側至南側，10-12月間則分布於北側。將雙眼牆個案分為南側不對稱對流主宰及北側不對稱對流主宰，兩者的平均垂直風切方向分別為東北風及西風，南側主宰的個案颱風強度略強，但是在形成雙眼牆的地理位置上來看，形成的緯度沒有明顯的差異，形成的經度則是北側主宰偏東，南側主宰平均偏西，也處在較有利的環境下，可能也因此強度較強。

關鍵字

雙眼牆 ； 微波衛星觀測 ； 眼牆置換 ； T-Vmax圖 ； 不對稱對流

並列摘要

An objective method is developed to identify concentric eyewalls (CEs) for typhoons using passive microwave satellite imagery from 1997 to 2011 in the western North Pacific basin. There were 26,774 SSM/I and TMI satellite images examined. Out of these, 95 CE cases with 234 CE images were identified, including 16 cases of multiple CE formation. (1) The 15-years climatology, (2) CE structural changes and (3) Asymmetric convection before CE formation studies are included in this dissertation.
(1) We compared the typhoons with CE structure in the warm and cold episodes. (Warm and cold episodes based on a threshold of +0.5oC and - 0.5oC for the Oceanic Niño Index, respectively). The SST in the eastern Pacific was warmer in warm episode resulted in that the CE structure tend to occur farther east in the basin. Moreover, the weaker vertical shear, which because of the weaker Walker circulation led the CE typhoons were with higher intensity and maintained the intensity in the warm episode.
(2) Three CE structure changes are identified: CE with an eyewall replacement cycle (ERC; 37 cases), CE with no replacement cycle (NRC; 17 cases), and CE is maintained for an extended period (CEM; 16 cases). The inner eyewall (outer eyewall) of the ERC (NRC) type dissipates within 20 h after CE formation. The CEM type has its CE structure maintained for more than 20 h (mean duration time is 31 h). The NRC (CEM) cases typically have fast (slow) northward translational speeds and encounter large (small) vertical shear and low (high) sea surface temperatures. The CEM cases have a relatively high intensity, and the moat size (61 km) and outer eyewall width (70 km) are approximately 50% larger than the other two types. Both the internal dynamics and environmental conditions are important in the CEM cases, while the NRC cases are heavily influenced by the environment. The ERC cases may be dominated by the internal dynamics due to more uniform environmental conditions. We also develop the T-Vmax diagram (where T is the brightness temperature and Vmax is the best track estimated intensity) demonstrates structural and intensity changes of CE typhoons for a time sequence of the intensity and convective activity (CA) relationship.
(3) We compared the asymmetric convection which is between 150 km and 400 km from TC center 24 h before CE formation/the maximum intensity in CE typhoons/ no-CE typhoons. The asymmetric convection was located in south to southwest region and downshear to the left region both in CE and no-CE typhoons. Our results also showed the asymmetric convection was located in south to southwest region between April and June, and between July and September. However, the asymmetric convection located in north region between October and December. We also compared the cases which the asymmetric convection located in the north and south. The intensity of south asymmetric convection dominated cases (SAC) is stronger than that of north asymmetric convection dominated cases (NAC). The SAC and NAC cases were with northeasterly and westerly windshear, respectively. The SAC cases occurred farther west in the basin and encourage favorable environment.

並列關鍵字

Concentric Eyewall ； Microwave Satellite Observation ； Eyewall Replacement Cycle ； T-Vmax diagram ； Asymmetric Convection

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