Stratocumulus Cloud Field Reflected Fluxes: The Effect of Cloud Shape

Reflected fluxes are calculated for stratocumulus cloud fields as a function of sky cover, cloud aspect ratio, and cloud shape. Cloud liquid water volume is held invariant as cloud shape is varied so that the results can be utilized more effectively by general circulation models (GCM) and climate models. The magnitude of the reflected flux differences between broken and plane-parallel cloudiness is of particular significance. On the basis of required accuracy in the Earth Radiation Budget Experiment (ERBE) program, an order of magnitude value of 10 W m−2 is used to estimate “significant differences” between plane-parallel and broken cloudiness. This limit is exceeded for cloud covers between 10% and 90%, indicating that plane-parallel calculations are not satisfactory at most values of cloud cover. The choice of cloud shape also leads to large differences in reflected fluxes. These differences may be traced to the anisotropic intensity pattern out the cloud sides, to the size and shape of the “holes” between clouds, and to variations in cloud area as viewed from the solar direction. An empirical relationship for effective cloud cover is given at solar zenith angle of θ = 60°. This relationship allows for the relatively accurate (ΔF = 10–15 W m−2) computation of broken cloud field reflected fluxes from plane-parallel calculations. Although the present parameterization is limited to solar zenith angles near θ = 60°, this is an indication that further work may lead to reasonably accurate estimates of broken cloud field radiative properties using modified plane-parallel calculations, irrespective of assumed cloud shape.