Albedo bias and the horizontal variability of clouds in subtropical marine boundary layers: Observations from ships and satellites
Albedo bias and the horizontal variability of clouds in subtropical marine boundary layers: Observations from ships and satellites
复制标题
副热带海洋边界层云的反照率偏差和水平变化：船舶和卫星的观测
DOI：
10.1029/1998jd200125
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发表时间：
1999
影响因子：
--
通讯作者：
S. Klein
中科院分区：
文献类型：
--
作者：
R. Pincus;S. McFarlane;S. Klein
在大尺度模式中，云的光学性质在比典型网格单元小的空间尺度上变化很大，这会导致模式对云的散射系数的显著高估。如果云的平均光学厚度和变化量是可用的，这种平面平行均匀（PPH）的偏置存在可能会减少，但很少有人知道有多少变化存在于自然界中，什么因素是敏感的。作者结合联合收割机1331年的观测志愿者地面观测卫星图像，以评估云的分数，云的光学特性，和云类型之间的关系，在海洋边界层云的加州海岸在夏季。当使用0.09和0.10之间的反射率阈值时，从两个数据集的云分数的估计是最一致的。在1至32公里的空间尺度上，卫星导出的云分数随着传感器分辨率的增加而缓慢增加。云分数在场景中占主导地位的积云是更敏感的反射率阈值用于云检测的场景比含有层状云。这里发现的PPH偏倚的平均幅度为0.025，比最近其他研究中发现的要小得多。当拟合到所观察到的光学厚度分布时，对数正态分布和伽马分布都大大降低了PPH偏差。对数光学厚度的平均值和离散度与云的类型有关：光学厚度随云的类型从积云到层状云的变化而增加，而相对变率减小。作者提出了大尺度模式中未解决的变率参数化的基础。
Cloud optical properties vary dramatically at spatial scales smaller than typical grid cells in large-scale models, which can cause a significant overestimate of cloud albedo by the model. This plane parallel homogeneous (PPH) albedo bias exist may be reduced if the mean cloud optical thickness and the amount of variability are available, but little is known about how much variability exists in nature and to what factors it is sensitive. The authors combine 1331 observations made by volunteer surface observers with satellite imagery to assess the relationships between cloud fraction, cloud optical properties, and cloud type in marine boundary layer clouds off the coast of California during summer. Estimates of cloud fraction from the two datasets are in best agreement when a reflectance threshold between 0.09 and 0.10 is used. Satellite-derived cloud fraction increases slowly with sensor resolution at spatial scales from 1 to 32 km. Cloud fraction in scenes dominated by cumulus is much more sensitive to the reflectance threshold used for cloud detection than are scenes containing stratiform clouds. The mean magnitude of the PPH bias found here, 0.025, is considerably smaller than those found in other recent studies. When fit to the observed distributions of optical thickness both log-normal and gamma distributions substantially reduce the PPH bias. The mean and dispersion of log optical thickness are related to cloud type: optical thickness increases as cloud type changes from cumuliform to stratiform, while the relative amount of variability decreases. The authors suggest a basis for the parameterization of unresolved variability in large scale models.