利用WRF模式探討台灣東部海上對流線之個案研究

Cases syudy of convective lines over the eastern sea surface of Taiwan by WRF model

蔡宗樺(Zong-hua Tasi)

指導教授 ： 林沛練

國立中央大學/地球科學學院/大氣物理研究所/碩士(2012年)

摘要

台灣冬季弱綜觀環境下東部海面上常觀測到線狀對流的發生。這些對流線多呈東北-西南或南-北走向，且平行於海岸線，通常發展於近海地區（<40 km），但有些對流線能發展於外海地區（>40 km）。由先前的研究中提出，沿海地區的離岸流與大環境的向岸流所導致的低層輻合為近海對流線主要的生成機制。然而對於發展於外海的對流線，不同的個案之生成物理機制似乎有不同的說法。因此地形效應所造成地形回流或地形噴流是否可造成外海對流線生成，以及是否有其他原因導致對流線生成，為本研究主要探討內容。由於海洋地區缺乏觀測資料，增加其研究難度，因此本文藉由高解析數值模式來探討其對流線之動力生成機制。
　本研究利用美國國家大氣研究中心（NCAR）的WRF（Weather Research and Forecasting）數值模式進行模擬，水平網格解析度分別為40.5，13.5，4.5，1.5 km，並配合不同觀測資料進行驗證。選定三個個案2004/01/03，2006/12/11，2008/10/29，三個個個案發生時上游風場有所不同，且對流生成位置皆不相同，本研究藉由此三個個案，詳細探討台灣東部近海和外海對流線的生成機制與對流線特徵。模擬結果顯示，三種個案其主要生成機制皆有所不同，但都與台灣的地形效應有密切相關。個案一形成機制為，上游氣流受到山脈阻擋，使得對流線左側有地形噴流造成較強的北風，而對流線東側為東北風，此兩個不同方向之氣流，在外海約50 km處產生輻合，激發出對流線。個案二的生成原因為盛行風受到山脈阻擋，導致氣流無法過山，因此在山前形成地形回流與地形繞流（西南風）。而兩種機制產生之離岸流，再跟大尺度風場在外海低層輻合，生成對流線。個案三的生成機制為夜間局部環流和大尺度風場兩反向氣流在近海附近輻合，外加上850 hPa這層於成功沿岸附近有區域性的風切輻合，增強中層對流線發展，此個案生成機制與台灣東部局部環流有較大相關性。
本研究另外設計台灣地形高度敏感度實驗，將台灣地形高度降低50%（TER50 RUN）後，模擬結果顯示，東部山脈阻擋效應較不明顯，部分氣流容易直接過山，不論是地形作用或局部環流所引發之離岸流皆比（CTRL RUN）較微弱，間接導致對流線生成位置靠近岸邊，生命週期也較短。而當地形高度降至0%（TER00 RUN）時，對流線左側離岸氣流消失，無離岸氣流可維持對流生成，對流線隨之消散掉，可見台灣地形是影響對流線生成與發展的關鍵因素之一。

關鍵字

福祿數 ； 地形噴流 ； 羅士比半徑

並列摘要

Under weakly synoptic weather conditions, the occurring of convective lines are often observed over the eastern sea surface of Taiwan in the winter. These convective lines are oriented northeast – southwest or south – north direction and approximately parallel to the coastline. Additionally, these lines usually develop near the coast (<40 km) but they could develop off the coast (>40 km) occasionally. The main purpose of the study is to discuss the main formation mechanism of convective lines which developed far from the coast. Whether the orographic effects resulted in terrain returned airflow or barrier jet will be investigated in this study. Since, the lacking of observation data over the ocean area increases the difficulty of observational study, so this study aims to investigate these questions by high-resolution numerical model.
This study conducts simulation by WRFV 3.2.1 (Weather Research and Forecasting) numerical model and validates with different observation data. Four nested domains were used with horizontal resolutions of 40.5, 13.5, 4.5 and 1.5 km, respectively. Three different types of case are chosen, the first one is the case Jan. 3 in 2004, the second is the case Dec. 11 in 2006 and the third is the case Oct. 29 in 2008. The formation mechanism of the first case (200401) is that the airflow in the upstream was blocked by mountains so that barrier jet formed on the west side of convective lines, and resulted in stronger northly winds. Since, there are north-easterly winds prevailed in the east side of convective lines; these two different directions of flows collide and converge about 50 km off the coast, to stimulate initiation of the convective lines. The formation mechanism of the second case (200612) is blocking of prevailing winds by mountains so that prevailing winds couldn’t across mountains. Therefore, there are terrain returned airflows and terrain around flows (south-westerly winds) forming in front of mountains. These two different terrain induced flows could formation of converge with large scale prevailing wind in the low level off the coast, and lead to the convective lines. The formation mechanism of the third case (200810) is the local circulation during the night time and the large scale prevailing wind which are two reverse flows converge near the coastal area. Additionally, regional wind shear convergence enhance the development of mid-level convective line near the coast of Cheng Kung in the 850 hPa.
This study also designs the sensitivity test besides the control simulation with real terrain of Taiwan (referred to as CTRL), we conducted simulations with reduced terrain of 50% (TER50) and 0% (TER00) of the real terrain. The sensitive test shows that when the terrain height is reduced, the blocking of terrain is not obvious, and part of airflows will cross mountains. Therefore, the reduced orographic effects caused the offshore flow weaker than CTRL; the forming position of convective lines near the coast , and the life cycle is shorter. This sensitive test could confirm one of the key factors to the control formation and development of convective lines is terrain height.

並列關鍵字

Froude number ； Barrier jet ； Rossby radius ； Burger number

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