都市綠園道戶外熱舒適評估之研究 –– 以台中市為例

Abstract

隨著都市規模不斷擴張，都市熱島效應日益加劇，而都市中的綠地具有調節微氣候之功能，在大都市中，除森林公園外，也常見密植樹木的綠園道，是遊客和居民重要的活動空間，更被視為有效減緩都市熱島效應之手段，因此，確保都市中的綠地能提供良好的戶外舒適性相當重要。都市內透過喬木、植栽的綠化，在戶外環境中藉由植被提供遮蔭，阻隔輻射而達到降溫效果，可以有效的提升戶外之熱舒適性。本研究探討在濕熱氣候環境下，都市內綠園道之植栽對戶外熱環境舒適性的影響，利用葉面積指數反映植栽的結構和數量特徵，作為判斷阻擋之輻射量的依據，參考林妝鴻的研究得到屏蔽效應迴歸公式，計算出不同葉面積指數時的輻射量，加上台中氣象年TMY3中之溫度、相對濕度、風速、雲覆率四項資料，計算出全年不同葉面積指數下的生理等效溫度；以台灣戶外舒適範圍探討熱季的5∼10月間，各葉面積指數下之過熱時數以及過熱強度，並以單月PET>34°C之過熱時數為戶外過熱之評判標準，計算出不同過熱危險率所對應之最低葉面積指數設計值，而要將全年中最熱的七月份之過熱危險率控制在2.5％，所需提供之葉面積指數為2.64；過熱危險率控制在5％時，葉面積指數需達到2.19。最後，實際測量台灣綠園道規劃最完整的台中，四段最主要之綠園道上植栽的葉面積指數，對現有綠園道診斷並提供改善方向，選擇美術園道中一段遮蔭較不足的人行空間，利用微氣候模擬軟體ENVI-met模擬4種程度的遮蔭改善方式，希望藉由本研究之結果，找到促進都市戶外環境夏季熱舒適之樹木葉面積指數範圍，提供都市綠地規劃時，最佳的葉面積指數設計值，做為都市規劃的參考。

With the widespread of urban living area and due to the urban heat island effect, people in urban outdoors is experiencing more severe heat stress than in rural area during summer period. Street trees can enhance pedestrians’ thermal comfort mainly by providing sun shade via its leaf canopy to reduce human body’s solar radiant exposure. Urban greenway is an important greening strategies in fighting with urban heat island effect in Taiwanese major metropolitan area and has been promoting for years. It is a consecutive urban open space allocated alongside the street with tree greening to serve as recreation purpose. However, tree planting location and tree selection would significantly influence the space users’ thermal perception. As the density of a tree’s leaf would crucially affect the amount of radiation received underneath, this study aimed to identify the minimum required leaf density in terms of leaf area index (LAI) for tree planting selection to provide pedestrian sufficient thermal comfort in summer under hot and humid climate context of Taiwan. Correlation model of solar radiation reduction rate with LAI had been established and was used for calculating radiation received underneath tree’s canopy. In assessing hourly thermal condition, local typical meteorological year (TMY3) data containing hourly weather elements including total horizontal radiation, dry bulb temperature, relative humidity, wind velocity, and cloud cover, was used as outdoor climate conditions. These weather data together with the model estimated reduced solar radiation by trees were afterwards used for estimating the physiological equivalent temperature (PET). The outdoor thermal comfort PET range of Taiwan suggested by T.P. Lin was adopted as criteria in assessing overheating occurrence frequencies and overheating severities under various LAI values, thus to reversely identify the appropriate monthly LAI needed for ensuring thermal comfort. The results show that a minimum of LAI 2.19 is required to maintain outdoor thermal comfort in hottest July to ensure the overheating occurrence probability less than 5%. Other suggested values of minimum required LAI in June, August, and September are 1.52, 1.97, and 1.67, respectively. In the second part, this study measured leaf area index planting of greenway in Taichung. Trying to diagnose and provide improvement of the urban greenways. Then select Art Park Greenway, which was lack of shading, to improve thermal comfort by micro-climate modeling software ENVI-met. These results could be serve as a tree selection reference in planning or designing urban greenways in the viewpoint of outdoor thermal comfort.