北台灣午後對流的強降雨事件發展機制

本研究主要目標為發生在夏季弱綜觀環境下北台灣午後雷暴強降雨（時雨量大於40毫米）機制。有利於午後對流強降雨的因素分為熱力因子、與地形相關的動力因子、以及雲物理因子。觀測資料顯示，除了垂直累積可降水量（column-integrated precipitable water），個別的熱力指標無法顯著區分出強降雨與非強降雨事件的差異；許多午後對流具有良好的熱力條件，往往無法造成強降雨，顯示動力作用也是造成可午後雷暴強降雨的重要配合條件，尤其是在台灣地形作用下。由於地形、地表作用的因素，向岸風與上坡風提供對流發展的有利條件，而海陸溫差造成的海風是一個向岸風的來源；但海風為很常見的現象，不會是偶爾發生的午後雷暴強降雨的決定性條件。此研究調查綜觀尺度風場及其在台灣地形作用下所的氣流變化（如尾流現象），是否有加強或減弱向岸風的作用，以理想化數值模擬實驗分析綜觀尺度的風向、風速在台灣地形作用下對於午後對流降雨強度的影響。結果顯示地形作用下的綜觀尺度風速及風向是決定午後對流的位置及雨量強度的重要的因素。相關機制包括對流胞的平流、尾流渦漩、渦漩脫離等。

關鍵字

午後對流；雷暴；強降雨；地形作用；理想化數值實驗

並列摘要

This study focuses on the mechanisms of heavy rainfall (intensity > 40 mm h-1) from afternoon thunderstorm ATS) that occurred over northern Taiwan in summer under weak synoptic forcing. Conditions favorable to the occurrence of heavy-rain ATS (HATS) include thermodynamic factors, topography-related dynamic factors, and cloud physical factors. Analyses of observational data indicate that, except for the column-integrated precipitable water, individual thermodynamic indices cannot differentiate HATS from ATS of weaker rainfall intensities. Many ATSs occurred under favorable thermodynamic conditions but often cannot produce heavy rain. This indicates the potential role of dynamic forcing, especially under the influence of Taiwan’s topography, which may provide orographic lifting for the onshore wind against the Snow Mountain Range or Central Mountain Range. Sea breeze is thermodynamically forced onshore wind (caused by land-sea temperature contrast), but it is too common to account for the rare occurrence of HATS. So, this study investigates the role of onshore wind associated with meso-alpha-scale winds under the influence of topography, including the lee-side wake flow. By running the WRF idealized simulations with controlled meso-alpha-scale wind, it was found that the direction and speed of the meso-alpha wind against the Central Mountain Range have critical influence on the location and intensity of HATS. The involved mechanisms include the advection of the convective cells, the wake flow effect, and vortex shedding effect.

並列關鍵字

afternoon convection ； thunderstorm ； heavy rain ； topography effect ； idealized numerical simulation

參考文獻

刘梦娟與杨引明，2017：基于风廓线网的散度和涡度计算 [Calculation of Horizontal Divergence and Vorticity Using Wind Profiler Network.]。气象科技进展，7(1)，27-32。doi:10.3969/j.issn.2095-1973.2017.01.004