The Absorption of Solar Radiation by Cloud Droplets: An Application of Anomalous Diffraction Theory

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Abstract
In this paper we demonstrate that the anomalous diffraction theory of van de Hulst with some modifications, provides a reasonable approximation of the volume extinction and absorption coefficients. We also show how the shortwave radiative properties of a water cloud can be derived in terms of 1) cloud liquid water path, 2) the effective radius of the droplet distribution, and 3) the balk absorption coefficient of water. With the aid of the approximate diffraction theory we describe how cloud albedo and shortwave absorption depend on the droplet size. We demonstrate this dependence to be somewhat complex and show that the variation of absorption with variation of droplet size depends also on the cloud liquid water path. For “deep” or semi-infinite clouds, absorption increases monotonically with increasing effective radius, but the reverse dependence is established for thin clouds. The implications of these results to the so-called absorption paradox and to the possibility of determining droplet size information from remotely measured reflectance spectra are also discussed.

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