Vertical and Horizontal Distributions of Solar Absorption in Finite Clouds

Abstract

A band absorption model is used in conjunction with a Monte Carlo scattering model to calculate the amount of solar radiation absorbed above, below, within and adjacent to cubic, finite clouds. Horizontally and vertically nonhomogeneous values of absorption within the finite cloud range from 0.16 to 6.4 times the corresponding values in horizontally infinite clouds of the same optical thickness, which were calculated using the same model. Absorption values in the regions adjoining the finite cloud on the solar and anti-solar sides, converge to clear sky values within a distance of two cloud dimensions from the side walls of the cloud. Absorption below the finite cloud ranges from 1.4 to 4.5 times that below the infinite cloud volume element. Values of absorption above the two cloud types are nearly identical when normalized to the cross-sectional area of the incident beam. If the absorption in the atmospheric column containing the finite cloud is normalized with respect to the horizontal area of the parallel radiation incident on the top plus the side of the cloud, the resulting value is within 3% of the absorption value in the column containing an element of infinite cloud. Thus, infinite cloud total column absorption values may be used to compute areal averages of absorption for a region partially covered by widely separated finite clouds, whose height to width ratio is near unity, if the fractional cloud cover is adjusted in the appropriate manner.