日月潭地區彩虹天氣條件之分析與應用
Authors: 許宸瑋 許宸瑋 , 周昆炫 周昆炫 
Published: April 2024

<p>彩虹是獨特的大氣光學現象，地球上許多地區因為不同的自然條件互相配合而容易形成彩虹，甚至成為當地觀光特色。在台灣南投的日月潭風景區因位處山區，夏季午後常有熱雷雨發生，雨後時常出現彩虹並被附近氣象觀測員及觀光客捕捉。因此若可利用氣象資料分析形成日月潭彩虹現象的氣象條件，甚或用於預測，不僅可促進當地大氣現象之學術研究價值，並對當地觀光提供更豐富特色。</p>
<p>本研究先將個案分為有彩虹、有熱雷雨無彩虹及無熱雷雨三類進行挑選，透過網路圖片先確認彩虹的現象發生，再使用日月潭測站、探空、雨量圖、雷達圖、日照時數等資料比對有彩虹及無彩虹個案中日月潭周遭降水情形及有無日照之大氣條件，並利用NCEP FNL全球模式分析資料合成CAPE值、K指數、850hPa高壓邊緣及風場進行三類個案分析，最後比對中央氣象署QPF預報資料分析其預報參考性。</p>
<p>從探空資料可看出大部分有彩虹及有熱雷雨無彩虹個案有較高的K指數，無熱雷雨個案則較低，有彩虹及有熱雷雨無彩虹差異較小，CAPE值也呈現相似結果。雷達分析結果顯示絕大部分有彩虹之個案，在日月潭東側有降水回波，同時在西側無回波或較弱回波，無彩虹個案則缺乏其中一項或均不滿足，此結果對於彩虹的形成扮演重要決定性因素。而比對日月潭測站日照時數及降水資料發現，有彩虹個案可同時觀測到微量日照及降水，無彩虹則兩者關係差異較大，同步性較低。從NCEP FNL模式分析資料分析K指數及CAPE值也近似於前述探空資料分析，有彩虹及有熱雷雨無彩虹個案有較高K指數，無熱雷雨則較低。高壓邊緣部分，無熱雷雨個案相較有熱雷雨及彩虹個案偏西；有熱雷雨及彩虹個案之近地面風向為南風至東南風，無熱雷雨則為偏東風。比對QPF預報資料則可看出有彩虹個案所預報之降雨類型較為零散，有熱雷雨無彩虹覆蓋性較廣，且雨量較大，無熱雷雨與實際觀測則容易有高估情形。</p>
<p>&nbsp;</p><p>Rainbows are unique atmospheric optical phenomena, and many regions on Earth are prone to forming rainbows due to various natural conditions, often becoming local tourist attractions. Sun Moon Lake Scenic Area in Nantou, Taiwan, is located in a mountainous region where thermal thunderstorms often occur in the summer afternoons. After the rain, rainbows frequently appear and are captured by nearby meteorologists and tourists. Therefore, if we can analyze the meteorological conditions that lead to the formation of rainbows at Sun Moon Lake using meteorological data or even predict them, it would not only enhance the academic research value of local atmospheric phenomena but also provide a richer experience for local tourism.</p>
<p>In this study, we first categorized cases into three groups: those with rainbows, those with thermal thunderstorms without rainbows, and those without thermal thunderstorms. We confirmed the occurrence of rainbows through online images and then compared the atmospheric conditions around Sun Moon Lake, including precipitation patterns and sunlight hours, using data from the Sun Moon Lake weather station, radiosonde data, rainfall maps, radar data, and sunshine duration records for both cases with and without rainbows. We also composited data from the NCEP FNL global model to analyze parameters such as CAPE (Convective Available Potential Energy), K-index, 850 hPa high-pressure edges, and wind fields for these three categories of cases. Finally, we compared the forecasting performance of the Central Weather Bureau’s QPF (Quantitative Precipitation Forecast) data to assess its reliability.</p>
<p>From the radiosonde data, it is evident that most cases with rainbows and cases with thermal thunderstorms but without rainbows have higher K-index values, while cases without thermal thunderstorms tend to have lower values. The differences between cases with rainbows and cases with thermal thunderstorms but without rainbows are relatively small, and a similar trend is observed for CAPE values. Radar analysis results show that the majority of cases with rainbows exhibit precipitation echoes on the eastern side of Sun Moon Lake, with little or weaker echoes on the western side, whereas other cases without rainbows lack one or both of these conditions. This result plays a crucial and determining role in the formation of rainbows. Comparing Sun Moon Lake weather station data on sunshine duration and precipitation reveals that cases with rainbows often exhibit both minimal sunlight and precipitation, while cases without rainbows show greater discrepancies and lower synchronicity between these factors. Analysis of K-index and CAPE values from the NCEP FNL model data closely aligns with the radiosonde data analysis. Cases with rainbows and cases with thermal thunderstorms but without rainbows have higher K-index and CAPE values, whereas cases without thermal thunderstorms tend to have lower values. Regarding the high-pressure edge, cases without thermal thunderstorms tend to be westward compared to cases with thermal thunderstorms and rainbows. Near-surface wind directions for cases with thermal thunderstorms and rainbows are predominantly from the south to southeast, while cases without thermal thunderstorms exhibit an eastward wind bias. Comparing the QPF data reveals that cases with rainbows forecast more scattered rainfall patterns, whereas cases with thermal thunderstorms but without rainbows have broader and more intense rainfall coverage. In contrast, cases without thermal thunderstorms tend to overestimate rainfall when compared to actual observations.</p>
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