台灣地區極端降雨颱風之環境特徵合成分析

The Composite Study for Typhoons with Extreme Rainfalls on Taiwan

魏士偉(Shih-wei Wei)

指導教授 ： 楊明仁

國立中央大學/地球科學學院/大氣物理研究所/碩士(2011年)

摘要

台灣地區位處西北太平洋副熱帶海域，主要的水資源來自於每年夏季的颱風降水，但是颱風所帶來的過多降水亦會帶來許多災害像是淹水、土石流等等，造成台灣民眾生命財產的嚴重損失。因此為了減少颱風帶來的損失，對於極端降雨颱風降水要有更深入的認知與更好的預報，及早防範並減低可能的災害。
本研究將1987~2010年24間九類颱風路徑共81個侵台颱風(指中心位置經過119°E ~125°E、21°N ~26°N)透過各路徑的異常降雨指數(Abnormal Precipitation Index)分類成極端降雨颱風與普通颱風。在這81個侵台颱風中有12個極端降雨颱風超過各路徑颱風的平均降雨一個標準差以上，我們針對這12個極端降雨颱風中2000年以後的個案透過NCEP的FNL 1°× 1°全球分析資料，將風場資訊做15天的低頻濾波(low-pass filter)以分離出環境場與颱風環流。我們以登陸時間為中心點合成分析登陸前後兩天共五天的時間內，各路徑的降雨極端降雨颱風在綜觀尺度環境場上的結構與普通颱風有何異同。更進一步提取短於15天週期的風場資訊，取颱風登陸前後12小時共三個時間點，以850hPa最大相對渦度為中心，合成分析範圍16° × 16°的區域以探討於颱風時間尺度下，極端降雨颱風與普通颱風之颱風渦旋尺度的差異。
在本論文中，各路徑極端降雨颱風在綜觀尺度環境場(15天週期以上)上各自都有一些明顯的特徵，就環流上游區域而言，極端降雨颱風個案之低層大氣的位勢不穩定度都較普通颱風來得高，並且極端降雨颱風個案在南海海域的低層850hPa平均風向比普通颱風的平均風更偏西南風。海表面溫度在各種類型路徑颱風中，並不是每個極端降雨颱風個案都有著較高的海表面溫度，代表海表面溫度對於颱風帶來極端降雨並非充分條件；但是在環流上游區域有著較高的海表面溫度，將利於颱風自海面帶來更多的水氣。低層850hPa的環境場相對濕度將有利於颱風外圍雨帶的發展，例如路徑類型一的極端降雨颱風；而路徑類型二、六、九環流上游區域也都至少有一區域存在著較高的相對濕度。而在低層850hPa相對渦度與中層500hPa相對濕度方面，幾乎六類的極端降雨颱風都有著較普通颱風更小的相對渦度與更低的相對濕度，但是這特徵與颱風帶來的極端降雨之間的關係尚不明朗，需要更進一步的研究。環境場的垂直風切與輻散場在六種類型路徑颱風個案中都無明顯的特徵。
在颱風時間尺度(15天週期以下)下能夠同樣地看見各類型路徑的極端降雨颱風都有著比普通颱風個案更不穩定的空氣，並且透過中層500hPa的相對濕度能夠得知水氣的主要來源，像路徑類型六的極端降雨颱風於中心西南側有著較高的相對濕度，而路徑類型二的極端降雨颱風水氣來源主要來自東北區域，因此除了南海海域的充沛水氣值得關注之外，台灣東方海面的水氣來源也是不可忽略的。並且在侵台期間於颱風時間尺度下，路徑類型四的極端降雨颱風並無明顯的水氣供應，可以更加確認颱風與台灣地形的交互作用亦是造成極端降雨的重要原因。
在本論文中主要透過系統性的合成分析，整理了六種路徑類型的極端降雨颱風在綜觀尺度環境場上的一些特徵，並且分析了颱風渦旋尺度下各類路徑極端降雨颱風與普通颱風的差異，但是並未深入探討這些特徵與差異與颱風帶來的極端降雨之間的物理機制與因果關係，未來希望能夠透過更高時間與空間解析度的中尺度模式模擬進行深入研究。

關鍵字

低頻濾波 ； 異常降雨指數

並列摘要

Taiwan is located over the subtropical western north Pacific Ocean, where there are about 20 typhoons every year. The rainfall brought by typhoons is very important to Taiwan, but it could lead to disasters if the rainfall is too much. In order to reduce the damages by typhoons, we need to study more about extreme-rainfall precipitation typhoon.
In this study, we used the API(Abnormal Precipitation Index) to classify 81 typhoons (with the centers of typhoons passing the area of 119°E ~125°E and 21°N ~26°N) which invaded Taiwan from 1987 to 2010 as extreme-rainfall typhoons and normal typhoons. In the 81 typhoons, there are 12 extreme-rainfall typhoons with total API from every station above one standard deviation for each track. The typhoon-track classification defined by the Central Weather Bureau is used in this study. For every extreme-rainfall typhoon, we apply the 15-day low-pass filter on the NCEP/FNL 1°× 1° global analysis data to separate the typhoon circulation from the environment. We then use the 5-day composite analysis to examine the synoptic environmental differences between the extreme-rainfall and normal typhoons.
For the synoptic scale environment, the extreme-rainfall typhoons from each type of typhoon tracks have some apparent characteristics. 1) For upstream region of the extreme-rainfall cases, there are higher potential instabilities and the average winds at 850hPa over South China Sea are much closer to the southwesterly than the normal cases. 2) Although extreme-rainfall cases do not always have higher SSTs than normal cases over the South China Sea and eastern area of Philippine, SST is possibly not the necessary condition to extreme rainfall, but higher SST is favorable for typhoon to bring more water vapor from the ocean. 3) Higher low-level relative humidity is conducive to outer rainband’s development and expansion, such as typhoons in Type 1 track. For typhoons with tracks in Types 2, 6, 9, the extreme-rainfall cases have at least one region average higher than normal cases. 4) Almost all extreme-rainfall cases have lower relative humidity at 500hPa and lower relative vorticity at 850hPa over the upstream region. 5) Deep vertical shear and divergence do not have the apparent differences between the extreme-rainfall and normal typhoons.
For the typhoon scale, we can find the potential instability and low-level relative humidity that is higher in the extreme-rainfall cases are same as the synoptic-scale characteristics. And the relative humidity at 500hPa can be used to identify the water vapor’s transmission, like in the track type-6 there are higher relative humidity at the 3rd quadrant. For the type-2, the water vapor mainly transmit from the east-northern area, so the water vapor from the eastern ocean of Taiwan is also important to extreme rainfall that brought by typhoon. There are no apparent water vapor supply for the extreme-rainfall typhoon of type-4, so we can infer that the interaction between the topography and typhoon circulation is important to extreme rainfall case.
Because this study has focused on the synoptic-scale composite analysis, we didn’t discuss the dynamic mechanism. In the future, we will conduct more research on the interaction between extreme-rainfall typhoon and environment by the high resolution numerical model.

並列關鍵字

Low-pass filter ； Abnormal Precipitation Index

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