西北太平洋颱風暴風半徑之分析

Abstract

颱風暴風半徑的大小，常為決定颱風警戒範圍的重要參考資訊；如何客觀估計颱風暴風影響範圍，並了解其變化的物理過程，是一個重要的研究課題。本研究利用QuikSCAT海面風場資料，估計2000 ~ 2005年間、145個西北太平洋颱風之15 m s-1暴風半徑（R15），分析其氣候特性和伴隨的綜觀環境場變化特徵；此外，並選取綜觀環境類似合成場之個案Aere（2004）（較大颱風）與Roke（2005）（較小颱風），利用WRF進行數值模擬，探討其角動量通量之變化。 分析颱風發展期間R15的變化結果顯示，強度達34 kt（TS）時，其R15較大（> 1.8° Lat.）的颱風，在強度增加至颱風強度（64 kt）時，其R15仍維持較大（> 2.6° Lat.）的比例達72%，較小（TS：< 1.1° Lat.、TY：< 1.8° Lat.）颱風亦達67%；即有大者恆大、小者恆小的趨勢。為探討此兩類型颱風在形成和發展期間，結構改變與綜觀環境間的關係，本研究針對強度達到34 kt前三天與強度從34 kt發展至64 kt這兩段時期，利用NCEP reanalysis2網格資料進行合成分析。結果顯示，在強度達34 kt前72小時內，較大颱風在系統南、北兩側，在850 hPa分別有大於10 ms-1之西南風與東風，且南側之西南風隨時間增強較顯著。較小颱風附近之綜觀風場則較弱，隨時間亦無明顯變化。在增強階段（34 kt發展至64 kt），500 hPa高度場顯示，較大颱風由位於副熱帶高壓之西南側，移至高壓之西側；較小颱風則持續位在副熱帶高壓南緣。 使用WRF模擬綜觀環境類似合成之個案結果顯示，模式可合理掌握颱風結構變化。分析模式中近地面10 m風剖面顯示，在形成和發展初期，較大颱風Aere（2004）之風速增強不僅於中心附近，風速大於10 m s-1之延伸範圍達半徑達8° Lat.以上，呈現具有較大暴風半徑颱風之特性。較小颱風Roke（2005）風速增加之形式則與Aere不同，僅侷限在距中心1.5° Lat.半徑內，風速大於10 m s-1之延伸範圍在半徑3° Lat.以內。角動量通量分析結果顯示，Aere於低層有大量的角動量輸入，且平均相對角動量與平均科氏力矩項均有顯著貢獻；在7° Lat.範圍外，又以後者較大；Roke則無顯著角動量通量之輸入。綜合分析結果顯示，在熱帶氣旋形成及發展至TS強度期間，系統南側若有顯著西南風，將有利於其發展為較大的熱帶氣旋。

This study uses QuikSCAT satellite to estimate the radial extents of 15 m s-1 wind (R15) of the tropical cyclones (TCs) occurring over the western North Pacific from 2000 to 2005 (145 TCs). The results of analyzing of the development during the R15 changing from tropical storm (TS) to typhoon (TY) show that if the R15 of a TC is relatively large (> 1.8°) (, or small (< 1.1°)) when it develops to TS, there is a 70% (67%) of possibility for it to remain as a large TC (> 2.6°) (, or small (< 1.8°)) when it intensified to TY. In other words, a TC tends to retain its size during the development of the intensity from TS to TY. The composite analyses show that there are strong southwesterly and easterly (> 10 m s-1) at 850 hPa to both the north and the south of the large cases within 72 h before their intensity to 34 kt, and especially the southwesterly increase about 5 m s-1 during the period. However, the synoptic flows of small cases are relatively weak, and remain almost the same during the whole process. Furthermore, when large cases move from the south to the west of subtropical high (STR), the small cases move with STR by lingering around its southern edge. Aere (2004) and Roke (2005) are selected cases respectively represent large and small cases considering their associated synoptic flows are similar to composite wind field. The 10 m radius section shows that the wind speed of Aere increases not only at the inner but also the outer core, and the area of wind speed above 10 m s-1 extends to 8° radius; nevertheless, the 10 m wind speed of Roke increases at inner core only. The angular momentum flux analysis also shows the angular momentum flux of Aere extends over 10°, which is much larger than that of Roke. Therefore, if there is a southwesterly surge located to south of a large TC during the formation stage, the TC tends to be with large size.