The Effect of Shape and Spatial Distribution of Cumulus Clouds on Longwave Irradiance

Abstract

In the longwave part of the spectrum, clouds are generally modeled in GCMs as flat black plates. The true effective cloud cover for transmittance of infrared radiation may be larger or smaller than the fractional cloud cover normal to the surface because of emittance of radiation from the sides of clouds into the clear sky and because the sides may have a finite cross section normal to the view and a vertical thermal gradient. The authors have derived the effective cloud cover as a function of zenith angle in terms of the cloud cover normal to the surface for several models of cumulus clouds with measured spatial and size distributions as a function of aspect ratio (height to radius or half-width) and shape. The effective cloud cover is shown as a function of cloud shape and aspect ratio as well as spatial distribution. The effective cloud cover is also sensitive to the thermal gradient between the cloud top and its base.