不同熱帶氣旋背景降水類型下地形加強降水的運動場特徵與微物理機制

Abstract

本研究利用傳統及雙偏極化都卜勒雷達、密集雨量站網和雨滴譜儀等獨特觀測資料，深入探討「璨樹」颱風（Chanthu，2021）影響期間臺灣北部大屯山地區地形降水的運動場特徵和微物理機制。由於大屯山受到璨樹颱風環流的強勁低層偏北/西北風影響以及三種不同類型的颱風背景降水[外圍雨帶(OR)、弱層狀降水(WS)和淺對流胞(SC)]通過，導致該地區發生了強降水。 OR 和 WS 這兩種降水形態的水平範圍相較於大屯山的水平尺度更廣，降水分佈也相對均勻。OR 降水的特徵是從對流層低層到高層都有存在中等強度且深厚的上升氣流，這有利於冰晶的活躍生成 [即樹枝狀冰晶生長層；dendritic growth layer (DGL)]，隨後持續發生聚集與淞化過程，產生粒徑大且數量多的種雲粒子。WS 降水型態的特徵是垂直速度接近零，導致冰相和混合相生長過程不活躍，因此在融解層以下的水滴尺寸較小且數量濃度較低的種雲粒子。在 OR 和 WS 階段，大屯山的地形降水增強均表現為所有雙偏極化變量向地面（3.5 km MSL 以下）增加，突顯了碰撞合併過程的主導作用，清楚反映了颱風背景降水（種雲）對迎風坡抬升產生的雲（播雲）的收集作用(即種雲播雲機制)。研究發現，OR 和 WS作為種雲的微物理特性，結合環境風場條件，是決定地形降水加強程度及解釋兩階段最強降水區相對於山脊位置差異的關鍵因子。 相較之下，SC通過大屯山時的降水加強，不適合以典型的種雲播雲機制解釋。這些孤立的 SC發展迅速，在海岸地區即呈現對流加深（cell-deepening）的特徵，並在迎風坡出現顯著增強，其中最顯著的增強發生在較高的海拔高度（2 km MSL）。這些高層增強的降水隨後往下游進行水平平流到背風側，造成背風側降水增強。此結果顯示，對於更具對流性的背景降水而言，可能存在一種不同於典型種雲播雲機制的微物理過程來主導降水增強，這可能是由於對流運動和地形改變氣流之間更複雜的交互作用。 本研究除了提供直接觀測證據來支持在颱風環境中種雲播雲機制的重要性，也同時指出颱風背景降水本身的雲微物理特徵是影響地形加強降水過程的關鍵因素之一。

This study used unique observations from ground-based traditional and polarimetric Doppler radars, a dense rain gauge network, and disdrometers to document the kinematic and microphysical characteristics of orographic precipitation over Da-Tun Mountain in northern Taiwan associated with Typhoon Chanthu (2021). Heavy precipitation occurred over Da-Tun, as it was influenced by strong low-level northerlies/northwesterlies of Chanthu’s circulations flowing over Da-Tun, and the landfall of three distinct types of TC background precipitation: an outer rainband (OR), weak stratiform (WS) precipitation, and shallow convective cells (SC). The OR and WS precipitation features were characterized by a uniform precipitation distribution with a greater horizontal extent compared to the horizontal scale of Da-Tun. The OR precipitation was characterized by a deep layer of modest upward motion from the lower to upper troposphere, favoring the active production of ice crystals (evidenced by the dendritic growth layer; DGL signature) and subsequent aggregation and riming processes, resulting in large, abundant seeder particles. The WS precipitation was characterized by nearly zero vertical velocities, leading to an inactive ice- and mixed phase growth process and thus smaller drop sizes and lower concentrations below the melting level, serving as small, sparse seeder particles. The precipitation enhancement over Da-Tun was indicated by an increase in all polarimetric variables toward the ground below 3.5 km (MSL) for both the OR and WS stages, highlighting the dominance of collision and coalescence processes, a clear manifestation of the collection of upslope-lifting-generated clouds (i.e., feeder clouds) by typhoon background precipitation (i.e., seeder clouds). The inherent microphysical properties of the OR and WS precipitation serving as seeder clouds, combined with ambient wind conditions, were found to play a critical role in determining the degree of orographic enhancement of precipitation and explaining the different locations of observed heaviest precipitation with respect to the mountain ridge between the two stages. In contrast, orographic enhancement of precipitation during the passage of SCs over Da-Tun Mountain may involve different processes from the seeder‒feeder-like scenario. The isolated SCs underwent rapid evolution, exhibiting a cell-deepening signature over the coast and a significant enhancement over the windward slope that was most intense at an elevated level (2.0 km MSL). This elevated precipitation subsequently resulted in enhanced rainfall over the leeside, attributed to the horizontal spillover of enhanced precipitation from upstream. This finding suggests that for more convective background precipitation, a microphysical pathway different from the classical seeder‒feeder process may govern the enhancement, likely due to a more complicated interaction between convective motions and orographically modified flows. This study provides robust observational evidence supporting the importance of the seeder‒feeder process in a TC environment, while also conveying how the microphysical characteristics of the TC background precipitation fundamentally influence the enhancement process.