Grant Richard H, Heisler Gordon M
Department of Agronomy, Purdue University, West Lafayette, IN, USA.
Photochem Photobiol. 2006 Mar-Apr;82(2):487-94. doi: 10.1562/2005-07-07-RA-604.
The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated with a three-dimensional canopy radiation transport model. The spatial distribution of UVB irradiance at the base of a regular array of spherical tree crowns was modeled under the full range of sky conditions. The spatial mean relative irradiance (I(r)) and erythemal irradiance of the entire below-canopy domain and the spatial mean relative irradiance and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles from 15 degrees to 60 degrees. The erythemal UV irradiance under skies with 50% or less cloud cover was not remarkably different from that under clear skies. In the shade, the actual irradiance was greater under partly cloudy than under clear skies. The mean ultraviolet protection factor for tree canopies under skies with 50% or less cloud cover was nearly equivalent to that for clear sky days. Regression equations of spatially averaged I(r) as a function of cloud cover fraction, solar zenith angle and canopy cover were used to predict the variation in erythemal irradiance in different land uses across Baltimore, MD.
利用三维树冠辐射传输模型评估了云量对树冠下行人所接收到的太阳紫外线辐射量的影响。在全天空条件下，对规则排列的球形树冠底部的中波紫外线（UVB）辐照度的空间分布进行了建模。针对15度至60度的太阳天顶角，确定了整个树冠下区域的空间平均相对辐照度（I(r)）和红斑辐照度，以及该区域阴影部分的空间平均相对辐照度和红斑辐照度。云量为50%或更低的天空下的红斑紫外线辐照度与晴朗天空下的红斑紫外线辐照度没有显著差异。在树荫下，部分多云天气下的实际辐照度比晴朗天空下的实际辐照度更大。云量为50%或更低的天空下树冠的平均紫外线防护系数几乎等同于晴天的平均紫外线防护系数。利用作为云量分数、太阳天顶角和树冠覆盖率函数的空间平均I(r)的回归方程，预测了马里兰州巴尔的摩不同土地利用类型中红斑辐照度的变化。
