Parameterization of Solar Near-Infrared Radiative Properties of Cloudy Layers

Abstract

The process of finding computationally efficient methods to parameterize the effects of the radiative interactions between water vapor absorption and cloud droplet absorption is fraught with complications. Inside a cloud, scattering greatly enhances the vapor absorption, and the amount of vapor above the cloud layer influences the absorption in a cloud layer. A widely used technique used to treat water vapor and liquid absorption is through the use of the k-distribution method. In the current study, this method is used with a one- and a three-band model to produce absorptances, reflectances, and transmittances of cloudy layers in the near infrared, but unlike standard usage, the single scattering properties are assigned to individual k values from weighting with the k distribution in the limit of semi-infinite and thin clouds, as well as the square root of the co-albedo. While improvement in the accuracy of the radiative parameters is noted for the three-band model as compared to standard three-band models, the one-band model with the square root approximation is very successful in producing absorptances, reflectances, and transmittances that are shown to be on the same order of accuracy as those produced by the three-band models with average single scattering properties. This method shows promise as a useful computational tool in general circulation models since it reduces the number of times the typical two-stream computation needs to be carried out or, alternately, provides more accurate results for the same computational effort as standard models.