多重尺度交互作用對於複雜地形午後雷暴極端降雨之影響

Abstract

預測山區雷暴是一項極具挑戰性的工作，這是由於多種大氣過程之間的多尺度交互作用，包含太陽加熱、熱力驅動氣流、冷池作用、雲微物理過程與地形效應等。本論文旨在深化對午後雷暴於弱綜觀與強綜觀環境下，導致地形極端降雨之物理機制的理解。透過先進的雷達觀測與數值模擬，本研究針對兩個在台北盆地造成極端降雨的午後雷暴案例進行分析：一個為弱綜觀環境下的個案（2015年6月14日），另一個則為強綜觀環境下的個案（2022年5月31日）。 第二章 探討中層相對濕度如何影響弱綜觀午後雷暴的發展。以 2015 年 6 月 14 日個案為例，環境中層乾燥空氣增強了蒸發冷卻效應，使得冷池增強，進而增強低層輻合與垂直舉升。這些條件有利於形成較大的霰粒子與更強的高層潛熱釋放，導致對流系統的範圍與強度提升。對流上升氣流的內部區域受到周圍濕空氣包覆，使整體的逸入作用顯著降低。此外，台北盆地的“盆地限制效應（basin confinement effect）”進一步強化了低層輻合並促進對流發展。 第三章分析TAHOPE/PRECIP IOP 2（2022年5月31日）發生的強綜觀午後雷暴，使用資料包含雙偏極雷達觀測與多都卜勒風場反演。分析結果指出，ZDR 柱增寬、多重胞合併與後續上升氣流加強與強降水之間存在潛在關聯。寬廣的 ZDR 柱（超過 8 公里寬）可能可以作為判斷對流組織與降水劇烈程度的有用指標。 第四章 探討TAHOPE/PRECIP IOP 2 個案中，濕絕對不穩定層（MAUL）之形成機制，並透過雲解析模式模擬分析其與地形降雨的關聯。模擬結果顯示，地形與西南季風交互作用顯著增強雪山山脈一帶的低層水氣通量輻合，且支持中尺度氣層舉升，進而促成深厚的 MAUL 形成。相較之下，移除地形的模擬中僅產生淺層、氣塊的抬升。MAUL 體積與下一小時降雨強度之間呈現正相關，顯示 MAUL 相關的診斷指標可能具備預測極端降水潛勢的應用潛力。 綜合而言，本研究結果指出，地形對午後雷暴演變的影響高度依賴於其所處的綜觀背景與熱力環境場。本論文強調，若欲改善短延時強降雨的預報與風險評估，應採納跨尺度的分析架構。

Predicting thunderstorms over mountainous regions is challenging due to the multiscale interactions of various atmospheric processes, including solar heating, thermally driven airflow, cold pool, cloud microphysics and topographic effect. This dissertation aims to enhance our understanding of the physical mechanisms leading to orographic extreme rainfall in afternoon thunderstorms (ATSs) under both weak and strong synoptic environments. Using novel radar observations and numerical experiments, this study investigates a weakly-forced ATS case (14 June 2015) and a strongly-forced ATS case (31 May 2022), both of which produced extreme rainfall over the Taipei Basin. Chapter 2 explores how midlevel relative humidity may influence the evolution of weakly forced ATSs. In the 14 June 2015 case, drier midlevel conditions enhanced evaporative cooling, strengthened cold pool outflows, and promoted more pronounced low-level convergence and vertical lifting. These factors favor the formation of larger graupel particles and stronger latent heating aloft, resulting in broader and more intense convective systems. The inner portion of the updrafts was shielded by surrounding moist air, leading to a notable reduction in bulk entrainment rate. Terrain effects—particularly the “basin confinement” within the Taipei Basin—further contributed to sustained low-level convergence and enhanced convective development. Chapter 3 investigates the strongly forced ATSs during the TAHOPE/PRECIP IOP 2 using polarimetric radar observations and multi-Doppler wind retrievals. The analysis suggests a potential link between the widening of ZDR columns, multiple cell mergers, and the intensification of updrafts and precipitation. Broad ZDR columns (exceeding 8 km) may serve as useful diagnostics for identifying convective organization and storm severity in complex terrain. Chapter 4 focuses on the formation of moist absolutely unstable layers (MAULs) in relation to orographic rainfall, based on simulations of the same IOP 2 event. Terrain interactions with southwesterly monsoonal flow enhanced low-level moisture flux convergence along the Snow Mountain Range, supporting mesoscale layer lifting and the development of deep MAULs. In contrast, terrain-removed simulations exhibited shallower, parcel-based ascent with limited instability. A positive correlation was found between MAUL volume and next-hour rainfall, suggesting that MAUL-related diagnostics could be explored further as indicators of extreme precipitation potential. Together, these findings demonstrate that topographic effects on ATSs are highly sensitive to the background synoptic and thermodynamic environments. The work emphasizes the need for multiscale frameworks to improve forecasting and risk assessment of short-duration extreme rainfall in mountainous regions.