| 摘要: | 在弱綜觀且較穩定的環境條件下，常可觀測到平行海岸線的對流雨帶在東部海岸生成。在前人的研究中，其分類大致可分為近岸型和離岸型兩種：在沿岸地區有陸風向外與盛行風輻合，為近岸對流線的形成原因(Yu and Jou 2005)。而盛行風受地形阻擋，激發地形回流、地形繞流或地形阻擋噴流並與盛行風輻合，為離岸對流線的主要成因(Yu and Hsieh 2009；Alpers et al 2010)。對流線的整體強度和離岸距離則與台灣的地形高度和坡度相關(蔡 2012)。但過去的研究多著重在地形效應造成動力上的輻合，有關熱力條件的對對流線的影響，過去僅Yu and Hsieh (2009)的研究中提及高海溫的黑潮其海氣交互作用在此區域的可能影響，然而因海面上觀測資料的缺乏，未能有效的進行分析。
本篇研究選取2012/02/01宜蘭外海的雙對流線個案，該個案依發展特性共可分四個時期。其中在第二期時對流線有羽狀回波帶結構向外海延伸，在第三期時，海面上同時有近岸對流線和離岸對流線同時存在，在過去對流線個案研究中較少見到類似的特徵。由於海面上的觀測資料不足，故使用WRF3.3.1模式模擬此個案。模式結果顯示：對流線發生時，環境盛行風在對流線北段為東北風，南段的風場在開始時為東北風，之後轉為偏東風，並與台灣地形產生作用生成地形繞流與地形阻擋噴流，並與更東側的入流風場輻合激發離岸對流線生成，入流風場間的水平風向轉換則形成羽狀回波帶。而在第三期時，東北季風開始增強，在台灣東北部沿著海岸形成一道地形分流與環境東北風入流風場輻合形成內對流線，此外在剖面分析中可以發現陸風在其中也有參與作用，使得沿岸的強風帶無法進入蘭陽平原，令內對流線在海岸生成。隨著東北季風進一步增強，對流線向東南外海推進，最後消散。
本研究共設計了三種敏感度實驗：地形高度降低、熱通量關閉實驗、改變海溫測試。在地形減半的測試下，前兩期的可見到對流線回波範圍明顯減弱，且離岸距離縮短，顯示地形效應對入流風場產生阻擋效應，然而後兩期的結果，對流線的位置及強度則變化不大。將地形降低至零時，在對流線發展的末期由東北季風輻合形成一道對流線，但對流線的位置與台灣地形無關。故地形敏感度實驗結果顯示台灣地形使東北季風分流，改變對流線發展方向。在關閉可感熱通量時，宜蘭區域的陸風消失，內對流線延遲生成，整體對流強度則因底層潛在不穩定度下降而減弱。而在海溫測試中，海溫降低時，海面上的不穩定度降低而使得對流線強度減弱，其發展位置也較為近岸，而改變海溫分佈也會改變對流線整體的發展形勢。熱力敏感度實驗在針對其它的對流線個案研究海溫的影響也有相同的結果，故總合熱力敏感度實驗結果，海面的熱力通量在對流線的生成與發展過程中扮演著有利對流線生成之角色。
Under weakly and relatively stable synoptic weather conditions, convective lines can be observed by radar observation at east coast of Taiwan, which orientation are usually parallel with the coast line. In previous studies, the convective lines can be divided into nearshore and offshore type. The nearshore type mainly cause by land breeze convergence with prevail wind (Yu and Jou 2005). The prevail wind blocked by steep terrain, generate terrain return flow, terrain split flow and barrier jet. Which converge with prevail wind induced offshore convective lines(Yu and Hsieh 2009；Alpers et al 2010). The intensity and offshore distance of convective lines are related with terrain height and slope. However, the previous studies are focus in dynamic mechanism by topographic effect, but rarely discuss the thermodynamic effect like air-sea interaction by Kuroshio’s high SST (Yu and Hsieh 2009).
This study selected a dual convective lines case which forming off the coast of Yilan in 2012/02/01. This case can be separate with 4 stages and has many features rarely seen in the past case studies like: feathery like structure extending seaward in stage 2, nearshore and offshore type of convective line exist in same time in stage 3. Since insufficient observations over ocean, we use WRF 3.3.1 model to simulate this case. Model results show that prevail winds of convective line were northeasterly in formative stage, and northeasterly change to easterly respectively at southern part of convective line. These prevail wind interacted with the Central Mountain Range (CMR) and induced terrain split flow and barrier jet. Which converged with prevail wind induced offshore convective line. The locally horizontal wind direction changes and converged with prevail winds induced the feathery like structure. In stage 3, the northeasterly monsoon strengthened and split by northeast part of Taiwan terrain. A strong wind belt along eastern coast converged with northeasterly inflow to induce the nearshore convective line. The cross-section analysis indicated the land breeze making the northeasterly monsoon inaccessible to the Yilan plain, help the convective line forming at coast. With the northeasterly monsoon further enhanced, the nearshore convective line moved to the southeast offshore, and finally dissipated.
This study design three of sensitivity experiments: terrain height reduced, heat flux test and SST change. In terrain height reduced experiment, the intensity and offshore distance of convective line reduced by setting the terrain height half in stage 1 and stage 2. This shows the topography blocking the inflow. But in the stage 3 and stage 4, the intensity and offshore distance of convective line did not change so much in half terrain experiment. By set the terrain height to 0, we can see the northeasterly monsoon converged with easterly flow in stage 3 and stage 4, but the convective line did not form along coast. This shows that topography affects the northeasterly monsoon and resulted in flow splitting and changes the orientation of convective line. If we turn off the sensible heat flux, the land breeze disappeared and the nearshore convective line did not form in stage 3, then the intensity of convective line weakened because of potential instability had been reduced. When reduce SST, the convective lines weakened because of the convective instability reduced. The SST distribution will also change the formation and development of convective line. Summery these results, the sensible heat flux of sea surface associated with high SST of Kuroshio play an important role for the formation and development of convective lines. |
