Effect of cloud inhomogeneities on the solar zenith angle dependence of nadir reflectance
Effect of cloud inhomogeneities on the solar zenith angle dependence of nadir reflectance
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云不均匀性对太阳天顶角最低点反射率依赖性的影响
DOI：
10.1029/96jd03719
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发表时间：
1997
影响因子：
--
通讯作者：
R. Davies
中科院分区：
文献类型：
--
作者：
N. Loeb;T. Várnai;R. Davies
卫星测量的最低点短波反射率与平面平行模式计算结果之间存在显著差异：对于中等到较大的太阳天顶角，观测到的最低点反射率随太阳天顶角的增加而增加，而平面平行值则减少。因此，利用一维（1-D）理论反演的云光学深度具有随太阳天顶角系统增加的偏差。利用蒙特卡罗模型模拟了具有不同云顶高度和体积消光系数的随机、各向同性、尺度不变云场的光子输运，结果表明，三维云场的最底层反射率随太阳天顶角的增加而增加，与观测结果一致。与一维情况的差异可以通过云侧照明以及结构化（即非平坦）云顶的存在来解释。云侧增加了被云拦截的入射太阳辐射量，允许更多的辐射在最低点方向向上散射。结构化的云顶改变了被照射的云顶表面的斜率，使得低太阳高度处的最低点反射率随着被照射表面的斜率而增加。对于简单的云几何形状，这两种效应对太阳天顶角增加的最低点反射率的贡献是相等的。虽然这种增加在垂直扩展的破碎云场中最为明显，但它也会影响云顶不均匀（凹凸不平）的阴云场的反射率。因此，观测到的太阳天顶角在云光学深度上的偏差在一般云景中也可能出现在广泛的阴云场中。云内小尺度液态水含量变化引起的内部不均匀性在低太阳高度下没有引起变化，在高太阳高度下仅引起最低点反射率的轻微降低。
A significant discrepancy has been noted between satellite measurements of shortwave reflectance at nadir and the results of plane-parallel model calculations: For moderate to large solar zenith angles, observed nadir reflectances increase with solar zenith angle, whereas plane-parallel values decrease. Consequently, cloud optical depths retrieved using one-dimensional (1-D) theory have a bias which increases systematically with solar zenith angle. Using Monte Carlo model simulations of photon transport through stochastic, isotropic, scale-invariant cloud fields with variable cloud top heights and volume extinction coefficients, we show that nadir reflectances from three-dimensional cloud fields increase with solar zenith angle, consistent with the observations. The difference from the 1-D case is shown to be explainable by cloudside illumination as well as by the presence of structured (i.e., non-flat) cloud tops. Cloud sides enhance the amount of incident solar radiation intercepted by cloud, allowing more radiation to be scattered upward in the nadir direction. Structured cloud tops change the slope of illuminated cloud top surfaces, such that nadir reflectance at low solar elevations increases with the slope of the illuminated surface. For simple cloud geometries the two effects make equivalent contributions to the increase in nadir reflectance with solar zenith angle. While this increase is most pronounced for vertically extensive broken cloud fields, it also affects reflectances from overcast cloud fields with inhomogeneous (bumpy) cloud tops. Thus the observed solar zenith angle bias in cloud optical depth for the general cloud scene likely also occurs for extensive overcast cloud fields. Internal inhomogeneities due to small-scale liquid water content variations within clouds are shown to cause no changes at low Sun and only slight decreases in nadir reflectance for high solar elevations.