西北太平洋颱風壯度變化之研究

本研究利用2007~2018年的ASCAT (The first Advanced Scatterometer)衛星觀測資料、重建西北太平洋地區之颱風近地表風場結構，並分析颱風壯度(strength，定義為100~300公里之平均角動量)。結果顯示，颱風之環境濕度越高，越有利颱風壯度增加；而環境垂直風切較高，則不利壯度增加。將颱風壯度變化前10%定義為快速擴張(rapid expansion , RE)，合成分析結果顯示，RE個案明顯位於季風距平環境中，而颱風之西南方則有強烈的低層西南風帶來水氣，增高中層溼度。此外，RE個案南方有強烈的高層輻散區；而衛星觀測亦顯示，RE颱風南方有較多的降雨。角動量流之分析結果顯示，在850百帕高度、RE個案於颱風外核區外，有相較於其他個案更多的角動量入流。另一方面，快速增強(RI)颱風則明顯位於東風距平環境；颱風東北邊之中層溼度較低，可抑制颱風外核區域之降雨，使降雨集中在颱風中心。分析結果同時顯示，颱風西南側中層溼度、南側高層輻散及西南側降雨和颱風壯度變化之相關性達0.4以上，顯示颱風西南部非軸對稱降雨對颱風結構變化具重大影響。中心西南部環境溼度較低的颱風個案，則需要較高的初始壯度才能到達RE。整體而言，颱風西南象限環境中層濕度，對於颱風是否出現RE的影響，大於其他區域環境濕度。

關鍵字

颱風結構變化；颱風壯度； ASCAT

並列摘要

This study uses ASCAT (The first Advanced Scatterometer) data from 2007 to 2018 to rebuild tropical cyclone (TC) symmetry surface wind structure. The TC strength is defined as average angular momentum between 100 to 300 km, calculated from our dataset. Results show that TC strength is less related to TC intensity but highly related to TC size. The environmental humidity favors strength development, but vertical wind shear is not. The first 10% of strength change per day is defined as rapid expansion (RE). The reanalysis data composite shows that RE cases are in a monsoon environment with a strong low-level southwest wind, high mid-level humidity, and high-level divergence in the southwest of TC. CMORPH data also shows that RE cases have more precipitation in the southwest than others. As a result, RE cases have significant angular momentum inflow outside TC outer-core. In contrast, rapid intensification (RI) cases are in the easterly environment, with low humidity in the northeast and less precipitation in the outer core. Mid-level humidity, high-level divergence, and precipitation in the southwest or south area all have over 0.4 correlation coefficients with TC strength. This result shows that asymmetry convection in the southwest can contribute to symmetry TC structure. Besides, cases with lower southwest humidity need higher strength to reach RE.