樹木對熱環境變化的時空影響分析：以SkyHelios Pro與ENVI-met Lite模擬數據為基礎的比較研究

Spatio-Temporal Analysis of Thermal Changes Affected by Trees: A Comparative Based on Simulations with SkyHelios Pro and ENVI-met Lite

本文旨在以定量方式分析去植栽（de-vegetation）對熱環境的影響，研究案例為比哈爾博物館，其建造過程中砍伐了約200棵樹木。研究選定場地中的兩個區塊與四個測點，並使用三款免費模擬工具SkyHelios Pro、ENVI-met Lite與RayMan Pro進行熱環境模擬與結果比較。
其中，SkyHelios可用於特定時間與空間點的熱變化時空分析。模擬結果顯示，在中午12點、風速4 m/s，以及去除28%植栽的情境下，平均輻射溫度（TMRT）分別上升了1.2°C（SkyHelios）與4.5°C（ENVI-met），生理等效溫度（PET）則分別上升了0.5°C與2.7°C。
研究發現，位於樹木環繞但無遮蔭的區域，其熱感受高於無樹區域，且越靠近樹木，熱感受越明顯。SkyHelios與ENVI-met在樹木大小、樹木環繞空間、天空可視因子（SVF）與樹木距離等微氣候變化因素上呈現一致性；惟在樹蔭下溫度變化的空間分佈等質性表現，以及平均與網格級溫度變化的數值方面，仍存在差異。
此類質性差異主要源自兩模型性質不同——SkyHelios為診斷型，ENVI-met為預測型；而量化差異則可能來自ENVI-met免費版本所採用之較大網格尺寸的限制，以及兩模型中樹木輸入參數的差異。
綜合兩模型一致性結果，本文進一步提出戶外空間之被動式降溫設計建議，以供後續規劃與設計參考。

This paper attempts to quantitatively analyse the effects of de-vegetation on the thermal environment using the example of the Bihar Museum, recently built after felling around 200 trees at the site. Simulations were run and results compared at judiciously selected two parcels and four points at the site, using freely available models SkyHelios Pro, ENVI-met LITE, and RayMan Pro. SkyHelios allows for the spatial/temporal analysis of thermal changes at a point in time/space. The cumulative mean rise in mean radiant temperature (TMRT) was found to be 1.2°C/ 4.5°C, and in physiological equivalent temperature (PET) 0.5°C/ 2.7°C by SkyHelios/ ENVI-met results, both at 12 P.M. at wind speed 4 m/s, due to the tree-removal area of 28%. Unshaded areas surrounded by trees were found to be hotter than those in treeless conditions, more so closer to the trees. Sky Helios and ENVI-met simulations provided concurrent results in terms of the effect of size of trees/ space surrounded by trees, sky view factor, distance from trees, etc., on the microclimatic variations, but slightly different qualitatively in terms of the spatial distribution of temperature changes under a tree etc., as well as quantitatively in terms of mean and grid wise changes in temperature. The qualitative differences are manifested due to the different nature of the models (diagnostic SkyHelios versus prognostic ENVI-met), while the quantitative, probably due to the limitation of adopting a comparatively larger grid size, as provided with the free version of ENVI-met, as also due to the different tree input vectors in the two models. Recommendations for a passive cooling of outdoor spaces are framed based on the concurrent results of the two models.