Shortwave Radiative Properties of Clouds: Numerical Study

Abstract

The dependence of cloud shortwave radiative properties on the details of the droplet size distribution is minimized when the effective radius is chosen as a variable characterizing the size distribution. The shortwave radiative properties of clouds (extinction coefficient, single scattering albedo, and asymmetry parameter) are determined for a given value of the effective radius with an accuracy of better than 10% for effective radii above 10 ?m. However, the shortwave radiative properties of clouds composed of small droplet size distributions with effective radii below 5 ?m can vary up to 40% (with the change of effective variance) at the given value of the effective radius. The spectral albedo of optically thick clouds at wavelengths shorter than 1 ?m is determined very accurately (within 1%) by the effective radius of the droplet size distribution. At longer wavelengths, the albedo of optically thick clouds is a sensitive function of the effective variance at all values of the effective radius and at all wavelengths. Typical variations are within 10% to 50%.