颱風外圍雨帶的對流特徵及其與颮線的相似度

Abstract

颱風雨帶是一個複雜的降水系統，其可能的機制和不確定性甚多，也引發了相關研究人員的熱烈爭論。根據對流受內核渦旋環流影響的程度，颱風雨帶可以分為內圍和外圍雨帶。先前的研究表明，外圍雨帶發生在對流可用位能(CAPE)相對較大的環境中，並且經常表現出類似於颮線對流系統的結構和地面特徵。本研究使用氣象都卜勒雷達觀測、高時間解析度地面觀測、臺灣電力公司整合型落雷偵測系統(TLDS)和ERA5再分析資料來進一步探索外圍雨帶的對流特徵及其與颮線的相似度。本研究特別透過模糊邏輯方法開發出對流強度指數(Convective Intensity Index, CII)和颮線相似度指數(Squal-line Similarity Index SSI)，這兩項參數分別量化了外圍雨帶的對流強度以及外圍雨帶和颮線之間的相似程度，兩者的值介於0和1之間。2002-2019年總共從97個颱風中挑選出了824條外圍雨帶個案，其中有223個雨帶通過了地面測站，因此可檢視雨帶的地面特徵。統計分析表明，當對流可用位能小於500 J kg-1時，CII隨著環境對流可用位能的增加而增加。超過一半的外圍雨帶並沒有檢測到閃電，而當CII達到0.4時，外圍雨帶發生閃電的機率開始顯著增加，當CII達到0.9時，發生閃電的機率達到最大值90%。冷池強度與CII相關性較弱，然而越強的冷池通常出現在對流不穩定度較大的環境中，且中低層的相當位溫相對較低。經過分析只有約30%的外圍雨帶與颮線具有較高的相似度(即SSI > 0.5)。此外，外圍雨帶在垂直雨帶方向的移行速度與理論預測的冷池傳播速度普遍不一致且其相關係數較低，僅為0.2。這些結果不僅顯示出外圍雨帶對流特徵的多樣性與複雜度，而且多數外圍雨帶與颮線對流特徵存在著不小差異。

Tropical cyclone rainbands (TCRs) are a complex precipitation system with a lot of possible causes and uncertainties which trigger heated debate among researchers. TCRs can be divided into inner and outer rainbands based on the degree to which convection is influenced by the inner-core vortex circulation. Previous research suggests that outer TCRs (OTCRs) develop in an environment with relatively larger convective available potential energy (CAPE) and frequently exhibit structural and surface characteristics similar to ordinary convective systems such as squall lines. In this study, radar measurements, high temporal resolution surface observations, lightning data from the total lightning detection system (TLDS), and ERA5 reanalysis data are used to further explore the degree of similarity between OTCRs and squall lines from a comprehensive OTCR dataset. A convective intensity index (CII) and squall-line similarity index (SSI) are developed based on fuzzy logic. CII quantifies the convective intensity of OTCRs and SSI determines the degree of similarity between OTCRs and squall-lines, both of which range from 0 and 1. A total of 824 OTCR cases associated with 97 TCs as they approached Taiwan are identified during 2002-2019. In this OTCR dataset, 223 cases passed over the surface stations. The statistical analyses indicate that CII increases with the ambient CAPE when the CAPE values are less than 500 J kg-1. Over half of the OTCRs did not have any lightning detected. The probability of lightning occurrence in the OTCRs increases significantly when the CII reaches 0.4 and has a maximum of 75% when CII reaches 0.9. Intensities of cold pools and CII are weakly related; however, stronger cold pools occur in an environment with larger convective instability due to lower θ_e in the low-to-mid levels. Only about 30% of the analyzed OTCRs have a higher similarity with squall lines (i.e., SSI>0.5). In addition, the cross-band propagation speeds of OTCRs are generally not consistent with theoretically predicted propagation speeds of cold pools, with relatively low correlation coefficient at only 0.2. These results suggest not only that the convective characteristics associated with OTCRs are complicated and diverse but also that most of them are not closely related to squall-line convective processes.