Rm. Killen et Rg. Ellingson, THE EFFECT OF SHAPE AND SPATIAL-DISTRIBUTION OF CUMULUS CLOUDS ON LONGWAVE IRRADIANCE, Journal of the atmospheric sciences, 51(14), 1994, pp. 2123-2136
In the longwave part of the spectrum, clouds are generally modeled in
GCMs as flat black plates. The true effective cloud cover for transmit
tance of infrared radiation may be larger or smaller than the fraction
al 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 fun
ction of zenith angle in terms of the cloud cover normal to the surfac
e 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 effe
ctive cloud cover is also sensitive to the thermal gradient between th
e cloud top and its base.