Reflectivities of uniform and broken layered clouds

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Abstract
1 km AVHRR imagery data from the NOAA satellites, collected during the FIRE Marine Stratocumulus Intensive Field Observations, were used to determine differences between the reflectivities of uniform, layered clouds and those of broken clouds taken from the same layers. Observations for the 0.63 µm channel indicated that regardless of the viewing geometry obtained with the polar orbiters, the average reflectivities of broken clouds were approximately 80–85% of the reflectivities of the same clouds when they formed uniform layers. Furthermore, the anisotropy of radiation reflected by uniform clouds appeared to be no different than that reflected by the broken clouds. Consequently, if the uniform, layered clouds observed in this study reflect according to plane-parallel theory, then the reflectivities of broken clouds could be approximated by simply reducing the values obtained with plane-parallel theory. For observations in the 3.7 µm channel, the average reflectivities of the broken clouds were generally larger than the reflectivities of the uniform, layered clouds. Furthermore, the anisotropy of radiation reflected by broken clouds differed from that of the radiation reflected by uniform clouds. The reduced reflectivity at visible wavelengths for broken clouds may explain the 10% discrepancy between pre-satellite era estimates of the earth's albedo and satellite estimates. Furthermore, while the differences in reflectivities between uniform and broken clouds are qualitatively consistent with Monte Carlo simulations of finite cloud effects, they are also consistent with plane-parallel radiative transfer models in which the central portions of clouds contain relatively large concentrations of liquid water distributed among relatively large droplets, while the edges of clouds contain relatively low concentrations of liquid water distributed among relatively small droplets. DOI: 10.1034/j.1600-0889.1991.t01-3-00008.x