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。

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.

口試委員會審定書 i
誌謝 ii
中文摘要 iii
ABSTRACT iv
CONTENTS v
LIST OF TABLES vii
LIST OF FIGURES viii
Chapter 1 Introduction 1
Chapter 2 Data and Methodology 7
2.1 The Dataset 7
2.1.1 The Observational Dataset 7
2.1.2 Criteria for Selecting Cases 8
2.1.3 Criteria of Heavy and Non-Heavy Rainfall Events 8
2.2 The Thermodynamic Indices 8
2.3 The Dynamic and Topographic Indices 11
2.3.1 The Analysis Dataset 11
2.3.2 Domain Setting 12
2.3.3 The Definition of Onshore Wind 12
2.4 The Cloud Physics Indices 12
2.5 The Numerical Experiment Design 13
2.5.1 The Model Settings 13
2.5.2 The Idealized Experiment Design 13
Chapter 3 Results 16
3.1 Statistics of Heavy-rainfall Afternoon Thunderstorms 16
3.1.1 Regional Distribution in Northern Taiwan 16
3.1.2 Annual, Diurnal and Intensity Distribution 17
3.2 Decisive Factors of HATS 17
3.2.1 Analysis of Thermodynamic Conditions 17
3.2.2 Analysis of Dynamic and Topographic Conditions 19
.3.2.2.1 Surface Wind by Observation Data 19
.3.2.2.2 Analysis of Meso-Alpha Wind by FNL Data 21
.3.2.2.3 Analysis of Meso-Beta Wind with FNL Data 22
3.2.3 Analysis of Aerosol Conditions 23
3.3 Results of WRF Idealized Simulation 23
3.3.1 Results of WS run 24
.3.3.1.1 Rainfall distribution of WS run 24
.3.3.1.2 Rainfall statistics of WS run and the effect of advection 25
.3.3.1.3 The effect of Orographic lifting and leeside vortex 26
.3.3.1.4 Mechanisms double rainfall peaks 28
3.3.2 Results of WD run 29
.3.3.2.1 Rainfall distribution of WD run 30
.3.3.2.2 Rainfall statistics of WD run and the effect of advection 30
.3.3.2.3 The effect of leeside vortex 31
Chapter 4 Discussion and Conclusion 32
REFERENCE 38
TABLES 41
FIGURES 54

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