| 摘要: | 本研究統計2011年至2021年01、10、11、12月（共計44個月）期間於宜蘭地區的平均風花圖，分析了蘭陽平原的南邊與西邊，以及白天與夜晚時段的風場特性，以瞭解宜蘭區域環流的變化。根據平均風花圖的結果，不論是白天或夜晚，北邊主要受東北季風影響。然而在白天受盛行風和海風影響下，南邊的西風比例仍然偏高。表示宜蘭的風場分布除了受區域環流影響外，還受到宜蘭特殊地形的影響。
為了深入瞭解宜蘭地區高時空解析度的三度空間環流，本研究使用2020年宜蘭劇烈降雨實驗(Yilan Experiment for Severe Rainfall, YESR2020)期間的觀測資料，根據日累積雨量分成為11月21日第一波強降雨個案、11月22日弱降雨個案以及11月23日第二波強降雨個案，探討地形效應下宜蘭局部環流的變化、降雨分布特徵以及豪大雨形成機制。在綜觀環境場中，11月22日弱降雨個案主要受弱東風影響，上游大氣環境相對穩定，水氣通量較少，因此白天時段宜蘭地區未出現降雨，並且有明顯的海陸風環流。相較之下，11月21和23日這兩波強降雨個案有不穩定且相當高的水氣通量，隨東北季風將水氣傳送到宜蘭地區，並在南邊山區受地形抬升而產生降雨。再者，台灣東部海上均有強對流隨東北季風往宜蘭地區移入，導致蘇澳沿海和南澳山區累積了大量降雨。由YESR2020的觀測資料發現，這兩波強降雨在宜蘭地區的地面風場有所不同，尤其是第二波強降雨個案中，宜蘭南邊山區前出現西風分量，且地面風場有風向不連續帶，而此現象在第一波強降雨個案中並未發現。此外，兩者的降雨強度也有所差別，第一波強降雨個案的降雨強度和降雨極值均較第二波大。
本研究使用WRF數值模式(Weather Research and Forecast Model)模擬11月21日第一波強降雨個案，探討這天宜蘭強降雨的生成與發展過程。透過模擬的結果，我們發現宜蘭當天降雨強度大的原因可歸納為以下幾點。首先，東北季風在南邊山區的迎風面受到地形抬升的影響，產生了降雨，同時中高層的南風分量抑制降水系統往下游移動，使得降水系統固定在此區域，進而造成南部山區發生持續性的降雨。其次，外海的雨帶隨東北季風移入宜蘭地區，增強了蘇澳沿海的降雨。最後，隨著綜觀環境風場發生變化，蘇澳外海出現了氣旋式環流的輻合雨帶，此環流與宜蘭的地形產生交互作用，延續了降水系統的生命期，再次造成南澳山區和沿海地區發生劇烈降水。由此可知，宜蘭的地形對此強降雨個案的形成和維持有著顯著的影響。
;This study conducted an analysis of wind roses in the Yilan region during the autumn and winter seasons from 2011 to 2021, covering a total of 44 months. The investigation focused on the variations in land and sea breezes in the southern and western areas of the Lan-Yang Plain (LYP) during both daytime and nighttime. The results of the wind rose analysis indicate that the northern part of the LYP is primarily influenced by the northeast monsoon, while the southern part exhibits a noticeable westerly wind component even during the daytime. This highlights that the wind field distribution in Yilan is influenced by the unique topography of the region.
To understand the three-dimensional circulation in the Yilan region, this study analyzed observational data from the 2020 Yilan Experiment for Severe Rainfall (YESR2020). Based on daily accumulated rainfall, the study categorized three cases: the first intense rainfall event on November 21, a weak rainfall event on November 22, and the second intense rainfall event on November 23. The research explored variations in local circulation under the influence of topography, characteristics of rainfall distribution, and the mechanisms leading to heavy rainfall. In the synoptic environmental context, the weak rainfall event is primarily influenced by a weak east wind. The upstream atmospheric environment is relatively stable with lower moisture flux, resulting in no daytime rainfall in the Yilan. Additionally, there is a noticeable sea-land breeze circulation. In contrast, the two intense rainfall events exhibit unstable and significantly high moisture flux. The northeast monsoon transports moisture to the Yilan, leading to rainfall in the southern mountainous region of the LYP. In addition, there is also a convective system off the eastern coast of Taiwan that moves into Yilan with the northeast monsoon, causing Yilan to receive heavy rainfall. According to the observation data of YESR2020, it is found that the surface wind fields of these two heavy rainfall are different in Yilan. Especially in the second heavy rainfall cases, there is a westerly component in front of the mountainous area in the south, and the surface wind field has a wind direction discontinuity zone. This phenomenon was not found in the first heavy rainfall cases. In addition, the rainfall intensity between the two is also different. The rainfall intensity and extreme rainfall values of the first heavy rainfall cases are greater than those of the second heavy rainfall cases.
This study uses the Weather Research and Forecast Model (WRF) to simulate the first heavy rainfall case on November 21 to understand the generation and development process of heavy rainfall in Yilan. Through the simulation results, we found that the reasons for the heavy rainfall intensity in Yilan that day can be summarized as the following points. First, the northeast monsoon produces rainfall due to orographic lifting on the windward side of the southern mountainous areas. At the same time, the southerly winds of the upper atmosphere stationary the precipitation system in this area, causing continuous rainfall in the southern mountainous areas. Secondly, the rainbands from the eastern sea of Taiwan move into the Yilan with the northeast monsoon, intensifying the rainfall along the Suaou coast. Finally, as the overall environmental wind field changed, a convergent rainband of cyclonic circulation appeared in the eastern sea of Taiwan. This circulation interacted with Yilan′s topography, again causing precipitation in the southern mountainous and coastal areas. It can be seen that the topography of Yilan has a significant impact on the formation and maintenance of this heavy rainfall case. |
