以測站長期觀測數據分析之臺灣各季節降水垂直分布氣候特徵
Authors: 洪致文 洪致文 , Chih-wen Hung 施明甫 
Published: February 2023

<p>不同於過往區域降水研究僅在水平空間上討論地區性的降水分布，本研究利用各海拔高度的降水觀測資料，聚焦於臺灣各地區在不同時節和不同海拔高度的降水資訊。藉由這些分析，可瞭解除了地區降水的平均分布外，亦可知其在時間和垂直空間上的分布特徵以及相關的形成機制，並為後續研究提供參考。</p>
<p>分析結果顯示，梅雨季時的臺灣西北部，滯留鋒附近的劇烈中尺度對流系統向西移入陸地，會在平地至高山形成強降水。夏季時雖然颱風也帶來廣泛性的強降水，然而另一降水來源的熱對流系統卻偏好發展於較低海拔高度，使得兩者的貢獻加總後，夏季降水量隨海拔高度上升而漸增，但高山地區降水卻又減少。西南部整體降水情況與西北部類似，降水主要集中在梅雨季和夏季，不同之處在於西南部位於西南季風的迎風面，因此該地區雨季整體降水量比其他地區更多。此外，夏季時除了颱風本身可為西南部各海拔高度帶來強降水，颱風環流和西南季風形成的暖濕西南氣流也會在迎風面山坡造成災害性強降水，然而高山地區卻因為可降水含量不足，因此雨水相對減少，使得夏季降水量極值主要出現在海拔2000 m以下區域。</p>
<p>東半部的降水多集中在夏季至秋季，形成與西半部明顯不同的降水季節特徵。梅雨季時儘管滯留鋒也會為東北部各海拔高度帶來明顯降水，然而由於位處鋒面對流系統移入路徑的背風側，因此旺盛對流系統不易發展而較少強降水。夏季時由於東北部位於西南季風的背風側，因此熱對流系統也較不易發展，然而該地區位於侵臺颱風的前線，颱風可為各海拔高度帶來強降水，補充夏季時的雨量來源。秋季由於環境風場漸轉為東北季風，季風本身或與颱風形成的共伴效應會在蘭陽平原的迎風面山坡帶來可觀降水，然而高山由於可降水含量下降，使得整體降水量高值集中在海拔2000 m以下地區。東南部與東北部的情況類似，卻因為整體地勢高度較為低矮，地形抬升作用造成降水的效果較不理想，以及並非如東北部般位於東北季風的直接迎風面，因此整體降水量比起東北部較低些。</p>
<p>&nbsp;</p><p>Unlike previous researches mainly examining regional precipitation in horizontal space, this study focuses on vertical spatial distribution characteristics of regional precipitation during the rainy season through analyzing precipitation observation data at various altitudes in Taiwan, which provides useful messages for future studies in terms of temporal and vertical distributions of regional precipitation as well as related mechanisms.</p>
<p>During the Meiyu season in the northwest, the stationary front accompanying mesoscale convective systems moves westward into the land, leading to heavy precipitation at all altitudes. In summer, typhoons bring extensive precipitation to lowland and mountainous areas, while the convection system, another main source of precipitation in this season, tends to develop at lower altitudes. Hence, the overall summer rainfall increases with height but gradually decreases in the mountains. The rainy condition in the southwest is similar to that in the northwest, which also mainly occurs in the Meiyu season and summer. Aside from this similarity, the overall rainfall in the southwest during the rainy season reaches another level as compared to other regions since this area is located on the windward side of the summer monsoon. In addition to typhoons with considerable precipitation at all altitudes in the southwest, the typhoon-monsoon-induced southwesterly flow also causes disastrous precipitation on the windward slopes; however, insufficient precipitable water vapor at high altitudes reduces precipitation in the mountains. All these contribute to a typical summer rainfall pattern with extreme values mainly seen at lower altitudes below 2000 meters.</p>
<p>Most of the water precipitates in the eastern coast in summer and fall, showing a huge dissimilarity from the western coast. During the Meiyu season, the northeast is on the leeward side of the stationary front, which does not benefit frontal convection systems to further develop and therefore less frequent heavy precipitation is the result. Although the northeast is also on the leeward side of the summer monsoon, this region receives plentiful rainfall from most of the invading typhoons due to its location and therefore the tropical cyclone becomes a main water source at all altitudes. In fall, since the environmental wind field gradually changes to the northeasterly, the northeasterly alone or the accompanied effect with the typhoon brings considerable precipitation on the windward slopes of the Yilan Plain, while most of the rainfall restricts at lower altitudes below 2000 meters. The precipitation pattern in the southeast somewhat resembles that in the northeast, yet the uplift effect caused by the topography does not effectively produce much precipitation due to the relatively low terrain in the southeast. In addition, the location of this region is not on the direct windward side of the northeasterly, which further creates a relatively unfavorable condition for rain to generate.</p>
<p>&nbsp;</p>