A GCM Parameterization for the Shortwave Radiative Properties of Water Clouds

A new parameterization is presented for the shortwave radiative properties of water clouds, which is fast enough to be included in general circulation models (GCMs). It employs the simple relationships found by Slingo and Schrecker for the optical depth, single scatter albedo and asymmetry parameter of cloud drops as function of the cloud liquid water path and equivalent radius of the drop size distribution. The cloud radiative properties are then obtained from standard two-stream equations for a homogeneous layer. The effect of water vapor absorption within the cloud is ignored in this version, leading to a small underestimate of the cloud absorption. The parameterization is compared with other schemes and with aircraft observations. It performs satisfactorily even when only four spectral bands are employed. The explicit separation of the dependence of the derived cloud radiative properties on the liquid water path and equivalent radius is new, and should prove valuable for climate change investigations.