臺北午後對流在西北太平洋副熱帶高壓相關綜觀環境配置下之降雨分布

Abstract

熱帶島嶼上的日夜對流降雨分布對環境相當敏感。當臺北發生夏季午後對流時，西北太平洋副熱帶高壓（副高）可作為判斷綜觀環境的指標。過去的個案研究雖已指出副熱帶高壓環流對午後對流環境的影響，然而利用長期觀測資料的分析在臺灣仍相對缺乏。本研究利用28年期間資料（包含1796個弱綜觀日，其中有627個午後對流日），探討副高配置和臺北午後對流降雨分布之關係。首先，我們診斷了在不同綜觀環境配置下發生的四個代表性個案，突顯對流肇始位置及傳播方向的差異，推測中低層背景風與降雨分布有關。接著，對所有弱綜觀日的位勢高度擾動場 (eddy geopotential height) 進行階層式聚類分析 (hierarchical clustering)，辨識出六種典型的副高配置，其中五種型態常伴隨降雨發生。針對午後對流日的合成分析，則顯示不同副高分群在背景流、降雨熱區及對流發展特徵上的差異。 研究結果提出副高配置可能影響臺北降雨空間變異的機制：一、副高的位置調節臺灣附近的背景流，當脊線偏北，背景風場的西南風偏強；若脊線北抬，東風分量增加，風速減弱。二、對流的肇始位置受低層背景風影響，當背景風為西南風時，局地環流經過基隆河和淡水河谷的海風輻合，易促使對流發生於臺北盆地；當背景風具有東風分量時，盆地西側的局地環流則受北風主導，更易在西南側山麓一帶誘發對流。三、對流的傳播方向受中層風速影響，當700百帕層風速超過約每秒6公尺（12 節）時，對流傳播方向趨向和背景風一致；若風速偏弱，傳播方向則更多元，可能受對流外流及海風交互作用影響。 此外，針對降雨分布的階層式聚類分析顯示，儘管副高配置部分控制了強對流事件中降雨的位置，對流強度的發展則可能受其他因子影響，尚待後續研究進一步釐清。同時，午後對流的發生頻率及副高分群組成皆具有明顯的年際變異，其中對流發生頻率與聖嬰現象 (ENSO) 相位有關，但副高配置的組成與ENSO並無明確的關聯。整體而言，本研究的結果有助於拓展過去研究僅著重於對流是否發生之層面，提供在面對不同大尺度環境下預報午後對流降雨空間分布的指引。

Rainfall distribution of diurnal convection over tropical islands is sensitive to the environment. When Taipei experiences afternoon convection in summer, the Western Pacific Subtropical High (WPSH) can be the indicator of synoptic environment. While previous case studies have suggested that WPSH circulation modulates the convective environment, long-term observational analyses remain limited in Taiwan. This study investigates the relationship between WPSH patterns and rainfall distribution of afternoon convection in Taipei, using 1796 weak-synoptic days (including 627 convection events) over a 28-year period. We first diagnose four representative events under contrasting synoptic environments, highlighting differences in initiation locations and propagation directions that suggest a link between mid- and low-level background winds and rainfall distribution. To examine this relationship, we apply hierarchical clustering to eddy geopotential height fields on weak-synoptic days and identify six typical WPSH patterns, five of which are commonly occur rainfall events. Composite analyses of convection days reveal differences in background flows, rainfall hotspots, and convection evolution across clusters. These results suggest possible mechanisms linking WPSH patterns to rainfall variability in Taipei: (1) The position of the WPSH modulates regional background flow around Taiwan, with stronger southwesterly winds shifting to weaker winds with increasing easterly components as the ridge moves northward. (2) Convection initiation locations vary with low-level background winds, likely due to their modulation on local circulations. Under southwesterly flow, convergence between sea breezes channeled through the Keelung and Tamsui River valleys is frequently observed and favors initiation within the Taipei Basin. In contrast, when the background flow contains easterly components, the circulation over the western basin becomes dominated by northerlies directed toward the Dahan River Valley, favoring initiation near the southwestern foothills. (3) Propagation direction appears to be influenced by mid-level wind speed. When 700-Pa winds exceed ~6m/s (12 kt), propagation tends to align with the background flow. Under weaker winds, more diverse directions emerge, which may reflect interactions between convective outflows and sea breezes. Additional rainfall pattern clustering shows that WPSH patterns partly constrain where strong convection occurs, but its development likely depends on other factors modulating convective intensity, warranting further investigation. Afternoon convection frequency and WPSH cluster composition both exhibit substantial interannual variability. While convection frequency varies with ENSO phase, WPSH cluster composition shows no clear ENSO-related pattern. Overall, our results provide guidance for forecasting spatial distribution of afternoon convection under varying synoptic conditions, extending beyond previous studies that only predicted their occurrence.