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Albedo and Reflected Radiance of Horizontally Inhomogeneous Clouds

Peter WendlingMeteorologisches Institut der Universität München, West Germany

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Abstract

The Monte Carlo method has been applied to the transfer of solar radiation through clouds with horizontal inhomogeneities which are assumed to consist of horizontally periodic striations. At a wave- length of γ = 0.55 µm the cloud albedo and reflected radiance are calculated as functions of cloud drop size distribution, optical thickness, solar geometry and type of cloud striations. The resulting cloud albedo is shown to be lower for striated clouds than for a plane parallel cloud with the same mean optical thickness. The largest differences of about 20% occur for a striated cloud with deep striations when the sun is in the zenith. A change in the cloud drop size distribution has nearly the same influence on the albedo of striated clouds as a change in the type of striations, the optical thickness remaining unchanged. A comparable influence results for the radiance reflected from striated clouds, although in this case it depends upon solar geometry and angle of the emerging radiance. It is shown that the radiance emerging from striated clouds can reach even larger values than that reflected by a plane parallel cloud with a thickness equal to the largest thickness of the striated cloud. This effect is due to the backscattering from the vertical walls of the cloud columns occuring at intermediate sun elevation angles in the antisolar direction. On the other hand, the reflected radiance of striated clouds is shown to be reduced compared to that of a plant parallel cloud by the effect of shadows. The magnitude of this effect is mainly determined by the optical thickness.

Abstract

The Monte Carlo method has been applied to the transfer of solar radiation through clouds with horizontal inhomogeneities which are assumed to consist of horizontally periodic striations. At a wave- length of γ = 0.55 µm the cloud albedo and reflected radiance are calculated as functions of cloud drop size distribution, optical thickness, solar geometry and type of cloud striations. The resulting cloud albedo is shown to be lower for striated clouds than for a plane parallel cloud with the same mean optical thickness. The largest differences of about 20% occur for a striated cloud with deep striations when the sun is in the zenith. A change in the cloud drop size distribution has nearly the same influence on the albedo of striated clouds as a change in the type of striations, the optical thickness remaining unchanged. A comparable influence results for the radiance reflected from striated clouds, although in this case it depends upon solar geometry and angle of the emerging radiance. It is shown that the radiance emerging from striated clouds can reach even larger values than that reflected by a plane parallel cloud with a thickness equal to the largest thickness of the striated cloud. This effect is due to the backscattering from the vertical walls of the cloud columns occuring at intermediate sun elevation angles in the antisolar direction. On the other hand, the reflected radiance of striated clouds is shown to be reduced compared to that of a plant parallel cloud by the effect of shadows. The magnitude of this effect is mainly determined by the optical thickness.

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