Spectral absorption of solar radiation in cloudy atmospheres: a 20-cm super(-) super(1) model

The spectral absorption of solar radiation in typical water clouds is determined by using a radiative transfer model based on LOWTRAN transmission functions at a 20-cm super(-) super(1) resolution and Monte-Carlo simulations of photon pathlength distributions. Relative absorption by the vapor and droplets within each cloud is obtained, and both plane-parallel and horizontally finite clouds are considered. Results indicate slightly lower absorption than found previously, with boundary layer clouds typically absorbing 9% of the extraterrestrial insolation for overhead Sun. Cloud absorption depends strongly upon the presence of water vapor above the cloud top and solar zenith angle, moderately upon cloud aspect ratio, and (provided the cloud is neither tenuous nor broken) weakly upon cloud type and thickness. The droplets, not the vapor, are shown to be the dominant absorbers within the cloud, except in the absence of water vapor above the cloud top, in which case, the vapor and droplets make similar contributions to the total cloud absorption. For many of the cases modelled, the sum of the cloud and atmospheric absorption remained invariant, permitting the net solar radiation budget at the surface to be deduced from broadband satellite measurements of albedo. An explanation for this behavior is found in the analysis of the spectral absorption by the different components.