颱風外圍雨帶降水特性與其環境條件之統計分析

Abstract

颱風雨帶在颱風中是除了眼牆以外風雨最強烈的區域之一。當雨帶位於距離颱風中心超過3倍最大風速半徑之外，通常被稱為外圍雨帶。儘管對於外圍雨帶之生成過程已有初步了解，但其降水強度會受哪些環境因子影響仍不清楚。本研究承襲 Yu et al.（2023）所建立之颱風外圍雨帶資料庫，針對2002至2019年間生成於廣闊海域之1029條雨帶進行統計分析，探討其初生期降水強度特徵與生成前環境條件之關聯性。資料來源包括中央氣象署四部S波段都卜勒氣象雷達、ERA5再分析資料，以及JTWC最佳颱風路徑資訊。統計結果顯示，雨帶降水強度與中高層環境相對濕度、低層輻合、上升運動呈現高度正相關，而相當位溫的垂直差異（θₑ deficit）與對流可用位能（CAPE）雖未達統計顯著水準，亦呈正相關趨勢。此外，位於颱風垂直風切下風側之雨帶亦較易發展出強降水。由於先前研究指出許多颱風外圍雨帶具有颮線（squall line）之氣流結構與降水特徵，本研究依雨帶垂直方向移速與降水梯度進行分類，分成2種類颮線雨帶，包括移速快且降雨較對流性的雨帶（SL1）與移速快但對流性較弱的雨帶（SL2）；及2種非颮線雨帶，包括移速慢的對流性降水雨帶（NSL1）與移速慢且對流性較弱的雨帶（NSL2）。分析顯示SL1雨帶之低層垂直雨帶方向風切可能處於RKW理論（Rotunno-Klemp-Weisman theory）中冷池和風切平衡的最適狀態（optimal state），有助於強對流之發展與維持；而SL2雨帶降水強度較弱的原因可能是由於低層風切較冷池強，導致整體環境處於次理想狀態（suboptimal state），進而抑制上升運動的發展。不過，本研究缺乏冷池觀測資訊，這些推論的可靠性仍有待未來進一步探討與釐清。此外，非颮線雨帶移速較慢可能與其低層平行雨帶方向風切較大有關，而CAPE和θₑ deficit也可能對其降水對流度有所影響。

Tropical cyclone rainbands (TCRs) are prominent features of tropical cyclones (TCs) that, aside from the eyewall, often produce heavy precipitation and hazardous weather. Those located beyond three times the radius of maximum wind (RMW) are typically classified as outer TCRs. While their formation processes have been previously explored, the environmental factors governing their precipitation intensity remain unclear. This study builds on the TCR database established by Yu et al. (2023), analyzing 1029 outer TCRs over open ocean near Taiwan between 2002 and 2019. Using S-band Doppler radar from the Central Weather Administration (CWA), ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), and best track data from the Joint Typhoon Warning Center (JTWC), we examine the relationship between preformation environmental conditions and formative stage precipitation intensity. Results show strong positive correlations between outer TCR precipitation intensity and mid-to-upper-level relative humidity, low-level convergence, and upward motion. CAPE and vertical θₑ deficit also exhibit positive trends, though not statistically significant. Outer TCRs on the downshear side of the environmental wind shear vector are more likely to produce heavy precipitation. To further explore differences among outer TCRs with and without squall-line-like features, we classify them into four types based on propagation speed and precipitation gradient: two squall-line-like groups (SL1: fast-moving with strong convection; SL2: fast-moving but weak convection) and two non-squall-line-like groups (NSL1: slow-moving with strong convection; NSL2: slow-moving and weak). Analysis suggests that SL1 may exist in an optimal balance between cold pool strength and vertical shear, as described by RKW theory, favoring sustained deep convection. In contrast, SL2 cases may be in a suboptimal state with excessive shear, suppressing upward motion. NSL rainbands, though lacking dynamic squall-line structure, may still be influenced by strong CAPE and θₑ deficit, while their slower propagation could be linked to stronger band-parallel shear.