臺灣地區極端降雨颱風之環境特徵與渦旋尺度特徵合成分析研究

The Composite Study for Typhoons with Extreme Rainfalls on Taiwan

魏士偉(Shih-Wei Wei) ； 楊明仁(Ming-Jen Yang)

《大氣科學》 43卷3期 (2015/09) Pp. 233-263

摘要

本研究將1987至2010年共81個侵臺颱風透過各類型路徑的異常降雨指數（Abnormal PrecipitationIndex），分類成極端降雨颱風與普通颱風兩類。其中有12個颱風超過各類型路徑颱風的平均異常降雨指數一個標準差以上，我們定義這12個颱風為極端降雨颱風。進而針對2000年以後的颱風個案透過NCEP的FNL 1°x1°全球分析資料，將風場資訊做15天的低頻濾波（low-pass filter）以分離綜觀尺度環境場與颱風渦旋尺度環流場。將濾波結果透過合成分析，用以比較極端降雨颱風在綜觀尺度環境場以及颱風渦旋尺度環流場與普通颱風的差異。極端降雨颱風的綜觀尺度環境場顯示，在颱風環流上游區域存在較強的垂直風切以及較弱的相對渦度，並且有較高的相對濕度、較高的條件性不穩定以及在南海海域有著較強勁的西南風，而輻合輻散場在極端降雨颱風與普通颱風之間沒有明顯差異。颱風渦旋尺度的合成分析結果顯示，西行與北行路徑的極端降雨颱風個案都在登陸前12小時於第一象限以及登陸後12小時於第四象限有著較強的垂直風切，並且也有著較強的相對渦度。北行颱風在第三象限有較高的相對濕度與較強的輻合，西行颱風則沒有特定的象限分布。整體而言，各類型路徑的極端降雨颱風在環境特徵上與普通颱風有著許多系統性差異，透過這些差異分析，希望能對颱風降雨預報有所助益。

關鍵字

合成分析 ； 低頻濾波 ； 極端降雨

並列摘要

This study used the API (Abnormal Precipitation Index) to classify 81 typhoons which invaded Taiwan from 1987 to 2010 as extreme-rainfall typhoons and normal typhoons. In the 81 typhoons, there are 12 extremerainfall typhoons with total API above one standard deviation for each track type. For the extreme-rainfall typhoons, we applied the 15-day low-pass filter on the NCEP/FNL 1°x1° global analysis data to separate the synoptic-scale environment from the typhoon circulation. Then the composite analysis was conducted to examine the differences between the extreme-rainfall typhoons and normal typhoons. Through the synoptic-scale composite analysis, we found some general characteristics for extreme-rainfall typhoons. For the synoptic-scale environment, the extreme-rainfall typhoons had stronger vertical wind shear, higher potential instabilities and more moisture over the upstream regions. Over the South China Sea, southwesterly wind was stronger. But there was no significant difference on the divergence field between extreme-rainfall typhoons and normal typhoons. For the typhoon-scale circulation, we found that westward-and northward-moving abnormal typhoons had stronger relative vorticity near center and vertical wind shear in the 1^(st) quadrant 12 hours before landfall and in the 4^(th) quadrant 12 hours after landfall. For northward-moving abnormal typhoons, there was higher convergence in the 3^(rd) quadrant. In conclusion, these environmental characteristics difference between abnormal and normal typhoons may be a valuable forecast guidance when a typhoon is near Taiwan.

並列關鍵字

Composite analysis ； Low-pass filter