A method to account for surface albedo heterogeneity in single‐column radiative transfer calculations under overcast conditions
A method to account for surface albedo heterogeneity in single‐column radiative transfer calculations under overcast conditions
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DOI：
10.1029/2008jd009815
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发表时间：
2008-10
影响因子：
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通讯作者：
R. Pirazzini;P. Räisänen
中科院分区：
文献类型：
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作者：
R. Pirazzini;P. Räisänen
[1] 提出了一种简单的参数化方法，用于导出阴天条件下高反射表面的宽带有效反照率。地表反照率的高空间变异性通过地表和云底之间光的多次反射影响邻近区域的下降太阳辐照度。有效反照率定义为均匀表面的反照率，该表面会产生与在存在非均质表面的情况下在观察点处观察到的相同的下降辐照度。所提出的方法使用云底高度和表面反照率图作为输入来参数化有效反照率。参数化基于对观测地点下降流辐照度有贡献的表面反射的空间分布，该分布用伽马分布来近似。针对四个理想化表面反照率图和各种云属性规范，根据从三维后向蒙特卡罗得出的有效反照率参考值和一维 DISORT 辐射传输计算来验证参数化。它给出的有效反照率值非常接近参考计算，其性能明显优于任何其他测试的方法，在应用于云光学深度的检索时也是如此。该方法可以应用于一维辐射传输模型，或用于解释极地沿海地区、边缘海冰区域或有森林和冰雪覆盖的斑块地形的宽带辐照度测量。
[1] A simple parameterization to derive the broadband effective albedo over highly reflecting surfaces under overcast conditions is presented. High spatial variability in the surface albedo affects the downwelling solar irradiance in neighboring regions via the multiple reflections of light between the surface and the cloud base. The effective albedo is defined as the albedo of a homogeneous surface that would result in the same downwelling irradiance as observed at the observation point in the presence of a heterogeneous surface. The proposed method parameterizes the effective albedo using the cloud base height and a surface albedo map as inputs. The parameterization is based on the spatial distribution of surface reflections contributing to the downwelling irradiance at the observation site, which is approximated with a gamma distribution. The parameterization was validated against reference values of effective albedo derived from three-dimensional backward Monte Carlo and one-dimensional DISORT radiative transfer calculations for four idealized surface albedo maps and various specifications of cloud properties. It gave values of effective albedo very close to the reference calculations, performing substantially better than any other approach tested, also when applied to the retrieval of cloud optical depth. The method can be implemented into one-dimensional radiative transfer models or used to interpret broadband irradiance measurements in Polar coastal regions, in the marginal sea ice zones, or in patchy terrain with forests and snow-covered fields.