TAHOPE SOP4期間之準線性對流系統：都卜勒雷達分析下的對流特徵與演變

Abstract

本研究探討2022年6月29日於TAHOPE（Taiwan-Area Heavy Rain Observation and Prediction Experiment）第四次特別觀測期間(SOP4)所觀測到的一個準線性對流系統（QLCS）之演變與特徵。該系統在弱綜觀系統與垂直風切較弱的環境中發展，儘管如此，系統仍展現出短暫的弓形狀回波結構與地面風速增強的現象，這些特徵通常較常出現在強風切環境中。透過NCAR S-Pol都卜勒雙偏極化雷達資料、地面氣象站觀測以及SAMURAI雷達風場反演技術，本研究分析了該對流系統的結構演變、渦度生成與微物理過程。該對流系統逐漸發展為一條長度約90公里的QLCS，並形成短暫的弓形狀回波，其伴隨較強的降水與冷池現象。此弓形回波的生成可歸因於冷池所引發的水平浮力梯度，此梯度產生水平渦度，進而被低層上升氣流傾斜並拉伸為垂直渦度偶極結構。隨著系統分裂，對流系統向北移動並通過S-Pol雷達站上空，於新屋氣象站觀測到地面風速增強與風向轉變。由對流胞所伴隨的後到前（Rear to Front; RTF）氣流與下沉氣流，可能為導致此地面風速增強與風向變化的主因。根據地面氣壓差與對流系統前方由新屋風剖儀所提供的環境垂直風切資料計算顯示，此事件中的冷池強度大於環境垂直風切。依據Rotunno–Klemp–Weisman（RKW）理論，這種不平衡可以解釋系統隨後的減弱與消散現象。對流系統的移動主要受到新對流胞發展位置的控制，而新對流胞的生成除了受到海風輻合作用的影響外，亦與具有較高相當位溫（θₑ）(具有較大潛在不穩定度)的熱力條件有關。雙偏極化雷達分析（包括隨高度變化的頻率分布圖CFAD）顯示降水微物理特性隨對流強度及系統演變階段而變化，並觀測到RTF氣流通過低層時亦會影響其微物理過程。

This study investigates the evolution and characteristics of a quasi-linear convective system (QLCS) observed on 29 June 2022 during the TAHOPE (Taiwan-Area Heavy Rain Observation and Prediction Experiment) Special Observation Period 4. The system developed under weak synoptic forcing and low vertical wind shear. Despite this, it exhibited transient bow-shaped echo structures and enhanced surface winds, which are typically associated with stronger shear environments. Using dual-polarization Doppler radar data from the NCAR S-Pol radar, surface station observations, and wind retrieval techniques (SAMURAI), this study analyzed the convective system structural evolution, vorticity development, and microphysical processes. This convective system evolved into a 90-km long QLCS featuring a short-lived bow-shaped echo with enhanced precipitation and cold pool. The formation of the bow-shaped echo was attributed to cold-pool-induced horizontal buoyancy gradients, which generated horizontal vorticity that was subsequently tilted and stretched by low-level updrafts to form vertical vorticity couplets. As the system split, the convective system moved northward, passing over the S-Pol radar site and producing intensified surface winds and directional shifts at the Xinwu station. The rear-to-front (RTF) flow with downdrafts from the convection, likely contributed to the observed surface wind strengthened and direction changed. Based on pressure differences derived from surface stations and ambient vertical wind shear derived from the Xinwu wind profiler ahead of the convective system, it showed that the cold pool was stronger than the ambient vertical wind shear. According to the Rotunno–Klemp–Weisman (RKW) theory, this imbalance can explain the subsequent weakening and dissipation of the convection. The movement of the convection was governed by the location of new convective cell development, which was influenced not only by sea-breeze convergence but also by thermodynamically favorable regions with higher θₑ (greater instability). Dual-polarization radar analysis, including Contoured Frequency by Altitude Diagrams (CFADs), revealed changes in precipitation microphysics associated with size sorting, evaporation, and stratiform bright band features. These microphysics varied from convective intensity and the stages of convection evolution, and were also associated with observed rear-to-front (RTF) flow passing through low-level layers, influencing microphysical processes along its path.