Three-dimensional multifractal cloud model for radiative transfer calculations in the remote sensing of cloud properties

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Abstract
Previous studies have shown that the albedo of clouds with inhomogeneous liquid water fields is lower than that of homogeneous clouds with the same average liquid water content. This can lead to biases in the retrieval of cloud properties from satellite images with pixel sizes significantly greater than the photon mean free path length. In this work we present a three-dimensional multifractal cloud model for use in radiative transfer calculations. The model is based on aircraft measurements of liquid water content taken during 98 flights over Tasmania, Australia. Monte Carlo radiative transfer is used to calculate the optical properties of clouds that were constructed according to this model. The reflectance of the cloud not only varies with the fractal parameters and mean liquid water content, but also with the area size over which it is averaged, i.e. the pixel size used. An "effective optical depth" is defined as the optical depth of a homogeneous cloud with the same reflectance as the 3D-multifractal cloud, and is parameterized as a function of the mean optical depth and the pixel size. This parameterization allows for fast radiation calculations in the remote sensing of cloud properties, by the replacement of an inhomogeneous cloud with a plane-parallel homogeneous one.

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