| 摘要: | 臺灣秋冬季期間，蘭陽平原南側常有強降雨發生，當對地居民生活與農業活動等方面都相當的影響。早在1950年，樺澤實(1950)提及宜蘭地區的特殊降雨特徵，並就宜蘭地形提出可能的降雨機制之概念模型；歷經過去七十多年，科學家對於此研究議題利用模式模擬或資料分析奠定了一定的科學理解，但仍缺乏完整的大氣觀測實驗提供詳細的三維觀測作為驗證。因此自2020年開始，一系列的宜蘭劇烈降雨實驗(YESR)提供了無人機、探空、雷達和剖風儀等密集垂直觀測資料，致力於探索與暸解宜蘭的降雨機制。本研究主要利用宜蘭劇烈降雨實驗期間無人機垂直密集觀測資料(共82趟)，並整合分析中央氣象署氣象觀測站過去2018至2024年秋冬季的長期資料(共721天)，探討宜蘭地區於冬季季風背景下的降雨與風場特徵，並利用垂直觀測資料構建在無綜觀系統影響下蘭陽平原南側的降雨機制。
以桃源谷和鶯子嶺測站的平均作為宜蘭地區的背景場，並以背景風向劃分觀測時間段，選用背景風向在北至東風(冬季季風)的觀測時間段作為研究對象。統計結果顯示，地形迴流的影響範圍為大福測站以南及蘇澳測站以北，其厚度最高可達800m或以上，1100m以下；在背景風向為東北偏東時為地形迴流最容易生成的環境條件，蘭陽平原中和南部的西風佔比為42%至63%。利用宜蘭劇烈降雨實驗中無人機密集垂直觀測資料，進一步對蘭陽平原南側的降雨垂直結構進行動力、熱力及溼福祿數分析，發現在不同背景風向下蘭陽平原南側由不同的降雨機制所主導︰在背景風向為北風時，風速為9至11 m s-1，降雨主要受到地形抬升影響，並同時高背景風速抑制對流往上遊地區移動，因此降雨區域主要集中在中央山脈(降雨發生率大於50%)。而在背景風向在東風時，風速為6至9 m s-1，背景風因受雪山山脈的阻擋而導致近地面產生的地形迴流，在動力上於宜蘭沿岸外海與背景風輻合，熱力上於內陸與背景風產生約1200m厚的絕對不穩定環境，為對流提供有利的降雨環境。當背景風為東北風時，風速為6至9 m s-1，受到地形抬升與地形迴流的交互作用下，宜蘭地區出現頻率最高持續性強降雨(降雨發生率為30%至60%)，對流首先會因地形抬升而在中央山脈生成，隨後對流所產生的陣鋒鋒面與背景風輻合並生成新對流，由此對流逐漸往迎風面上游區域移動；同時，地形迴流會有助於外海的對流的持續發展，而外海對流在隨背景風平流至中央山脈後會因地形抬升而增強，令蘭陽平原中與南部出現持續且劇烈的降雨；因此，在東北背景風的條件下，地形抬升與地形迴流降雨維持機制的複合作用令宜蘭地區皆有最多且強的降雨發生。
總結而言，宜蘭地區降雨機制複雜，本研究發現地形分別以地形抬升和地形迴流的方式幫助對流發展，背景風向決定蘭陽平原南側的降雨機制，而背景風速決定了地形降雨的類型。北風背景風下，蘭陽平原南側為地形抬升主導，降雨主要集中在中央山脈區域；東北風背景風下，在地形抬升與地形迴流維持機制的複合作用下宜蘭地區容易發生持續且強的降雨，主要降雨區域為蘭陽平原中與南部；東風背景風下，因外海對流範圍覆蓋整個宜蘭地區，因此宜蘭地區降雨發生率均一，而地形迴流分別以動力和熱力為對流提供有利條件，令對流有效發展。
;During autumn and winter, the southern Lanyang Plain in Taiwan frequently experiences heavy rainfall, significantly impacting local residents and agricultural activities. As early as 1950, Kabasawa(1950) identified unique rainfall characteristics in Yilan and proposed a conceptual model of possible rainfall mechanisms related to Yilan′s topography. Despite over seventy years of research utilizing model simulations and data analysis, a comprehensive understanding still lacks detailed three-dimensional observational data. Since 2020, the Yilan Experiments of Severe Rainfall (YESR) have provided intensive vertical observations using UAVs, soundings, radars, and wind profilers, aiming to explore and understand the rainfall mechanisms in Yilan. This study primarily uses vertical intensive observations from UAVs (82 flights) during YESR and integrates long-term data from the Central Weather Bureau′s meteorological stations from autumn and winter seasons of 2018 to 2024 (total 721 days). It investigates rainfall and wind field characteristics in Yilan under winter monsoon condition, constructing a rainfall mechanism model for the southern Lanyang Plain in the absence of synoptic systems.
Using the average data from Taoyuangu and Yingziling stations as the background field for Yilan, the study segments observation periods based on background wind directions, focusing on periods with wind directions from 350 to 110 degrees (winter monsoon). Statistical results indicate the influence range of topographic return flow spans south of Dafu station and north of Su’ao station, with a maximum thickness of 800m or more, up to 1100m. The most favorable conditions for topographic return flow formation occur with background wind directions between 50 and 70 degrees, with westerly winds dominating 42% to 63% of the time in central and southern Lanyang Plain.
Further analysis using intensive vertical observations from the YESR reveals distinct rainfall mechanisms in the southern Lanyang Plain under different background wind directions. With background wind directions from 350 to 30 degrees (north wind) and wind speeds of 9 to 11 m s-1, rainfall is primarily influenced by orographic lifting, with high background wind speeds suppressing upstream convective movement, concentrating rainfall over the Central Mountain Range (rainfall occurrence rate > 50%). When the background wind is from 70 to 110 degrees (east wind) with speeds of 6 to 9 m s-1, background wind is blocked by the Xu Mountain Range, creating near-surface terrain-blocking flows. It converges with background wind offshore and create the absolute instability layer inland (with a thickness of about 1200m), provides favorable conditions for convective rainfall. During background wind directions of 30 to 70 degrees (northeast wind) with speeds of 6 to 9 m s-1, the interaction of orographic lifting and terrain-blocking flows leads to the highest frequency of intense, sustained rainfall (rainfall occurrence rate of 30% to 60%) in Yilan. Initial convective activity generated by orographic lifting over the Central Mountain Range is followed by new convection formed by the interaction of gust fronts and background winds, progressively moving upstream. Concurrently, terrain-blocking flows aids in the continuous development of offshore convection, which is enhanced by orographic lifting when advected over the Central Mountain Range, resulting in sustained heavy rainfall in central and southern Lanyang Plain. Therefore, under northeast wind conditions, the combined effects of orographic lifting and return flow mechanisms lead to the most intense and frequent rainfall in Yilan.
In summary, rainfall mechanisms in Yilan are complex. This study finds that topography aids convective development through orographic lifting and terrain-blocking flows, with background wind direction determining the rainfall mechanism in the southern Lanyang Plain, and background wind speed influencing the type of orographic rainfall. Under north wind conditions, orographic lifting dominates, concentrating rainfall over the Central Mountain Range. Under northeast wind conditions, the interaction between orographic lifting and terrain-blocking flows cause persistent and intense rainfall in central and southern Lanyang Plain. Under east wind conditions, offshore convection uniformly affects the entire Yilan, with terrain-blocking flows dynamically and thermally supporting convective development. |
