Near Infrared Scattering by Sunlit Terrestrial Clouds

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Abstract
Absolute intensity (radiance) spectra of solar radiation scattered by clouds were obtained in the 1.15-μ to 3.6-μ region as a function of cloud type, cloud altitude, and scattering angle. The angular dependence of cloud scattering was determined at a number of wavelengths between 1.15 μ and 3.6 μ From 2.6 μ to 2.9 μ, where water and ice are strongly absorbing and single scattering dominates, scattering efficiency increases nearly tenfold as the scattering angle decreases from 150° to 45°, in agreement with theory. At shorter and longer wavelengths, scattering efficiency increases by about a factor of three over the same range of scattering angles. Characteristic minima are observed in spectra of ice clouds at 1.5 μ, 2.0 μ, and 2.8 μ, which are related to absorptions in the bulk material at the same wavelengths. The 1.5-μ and 2.0-μ minima are absent in spectra of liquid-water clouds, even though the absorption characteristics of bulk water and ice are very similar. The differences in scattering behavior are explained on the basis of particle size.