Effects of Cloud Heterogeneities on Shortwave Radiation: Comparison of Cloud-Top Variability and Internal Heterogeneity
Effects of Cloud Heterogeneities on Shortwave Radiation: Comparison of Cloud-Top Variability and Internal Heterogeneity
复制标题
云不均匀性对短波辐射的影响：云顶变率与内部不均匀性的比较
DOI：
10.1175/1520-0469(1999)056
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发表时间：
1999
影响因子：
3.1
通讯作者：
R. Davies
中科院分区：
文献类型：
--
作者：
T. Várnai;R. Davies
本文探讨了云的非均匀性对太阳反射的影响过程。这个问题很重要，因为目前的方法只能给出云异质性的总体辐射效应的数值结果，而不能确定各种机制对其负责的程度。本研究建立了一个定义这些机制的理论框架，并提供了一个计算其量级的程序。在推导框架时，作者引入了一维辐射传递近似，称为倾斜独立像素近似（TIPA）。TIPA利用斜光学厚度沿直射太阳光束的水平分布来描述在不考虑相邻云之间水平输运的情况下云异质性对辐射的影响。对水平传输的影响可以归结为两种基本机制：当辐射分别移动到较厚和较薄的云元素时，它会被捕获和逃逸。利用提出的框架，研究了云顶高度和云体积消光系数变化对短波辐射的影响。结果表明，如果云光学厚度的相同变化是由于几何云厚度的变化而不是由于体积消光系数的变化，则会引起更强的非均匀性效应。反照率的差异可超过0.05，顶日和斜日对天顶的相对反射率差异可大于25%和50%。本文还解释了以前观测到的一个现象：上升流辐射的捕获导致非均质云的天顶反射率随太阳高度的降低而增加。
This paper examines the processes through which cloud heterogeneities influence solar reflection. This question is important since present methods give numerical results only for the overall radiative effect of cloud heterogeneities but cannot determine the degree to which various mechanisms are responsible for it. This study establishes a theoretical framework that defines these mechanisms and also provides a procedure to calculate their magnitude. In deriving the framework, the authors introduce a one-dimensional radiative transfer approximation, called the tilted independent pixel approximation (TIPA). TIPA uses the horizontal distribution of slant optical thicknesses along the direct solar beam to describe the radiative influence of cloud heterogeneities when horizontal transport between neighbors is not considered. The effects for horizontal transport are then attributed to two basic mechanisms: trapping and escape of radiation, when it moves to thicker and thinner cloud elements, respectively. Using the proposed framework, the study examines the shortwave radiative effects of cloud-top height and cloud volume extinction coefficient variations. It is shown and explained that identical variations in cloud optical thickness can cause much stronger heterogeneity effects if they are due to variations in geometrical cloud thickness rather than in volume extinction coefficient. The differences in albedo can exceed 0.05, and the relative differences in reflectance toward the zenith can be greater than 25% for overhead sun and 50% for oblique sun. The paper also explains a previously observed phenomenon: it shows that the trapping of upwelling radiation causes the zenith reflectance of heterogeneous clouds to increase with decreasing solar elevation.