The Transfer of Visible Radiation through Clouds

R. E. Danielson Princeton University Observatory, Princeton, N.J.

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D. R. Moore Princeton University Observatory, Princeton, N.J.

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H. C. van de Hulst Sterrewacht, Leiden, The Netherlands

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Abstract

The transfer of visible radiation through terrestrial clouds has been calculated by a Monte Carlo computer program using a Henyey-Greenstein phase function which is similar to the true scattering function of water droplet clouds. From the fact that the maximum albedo of thick stratocumulus clouds is 0.7–0.8, it is deduced that 1 − ω0 is of the order of 10−3, where ω0 is the single scattering albedo. This value of 1 − ω0 is ∼104 times larger than expected from pure water. It is argued that the aerosols upon which the cloud droplets condensed are the source of the absorption. A set of simple formulae and tables are presented which give the reflection and transmission of clouds having an arbitrary phase function. For optical depths τc > 10, they are accurate to a few per cent provided that (1 − ω0)/(1 − g) < 10−2, where g = ¯cosθ and where θ is the scattering angle. Over the same range of parameters, the formulae and tables are accurate to about 1% for the Henyey-Greenstein phase function.

Abstract

The transfer of visible radiation through terrestrial clouds has been calculated by a Monte Carlo computer program using a Henyey-Greenstein phase function which is similar to the true scattering function of water droplet clouds. From the fact that the maximum albedo of thick stratocumulus clouds is 0.7–0.8, it is deduced that 1 − ω0 is of the order of 10−3, where ω0 is the single scattering albedo. This value of 1 − ω0 is ∼104 times larger than expected from pure water. It is argued that the aerosols upon which the cloud droplets condensed are the source of the absorption. A set of simple formulae and tables are presented which give the reflection and transmission of clouds having an arbitrary phase function. For optical depths τc > 10, they are accurate to a few per cent provided that (1 − ω0)/(1 − g) < 10−2, where g = ¯cosθ and where θ is the scattering angle. Over the same range of parameters, the formulae and tables are accurate to about 1% for the Henyey-Greenstein phase function.

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