A GCM Parameterization for the Shortwave Radiative Properties of Water Clouds

TLDR

A new parameterization is presented for the shortwave radiative properties of water clouds, designed to be fast enough for inclusion in general circulation models. The parameterization employs Slingo and Schrecker relations that link optical depth, single‑scatter albedo, and asymmetry parameter to liquid‑water path and equivalent radius, then applies standard two‑stream equations for a homogeneous layer and is validated against other schemes and aircraft observations. The scheme slightly underestimates cloud absorption because water‑vapor absorption is omitted, yet it performs satisfactorily with only four spectral bands and its explicit separation of liquid‑water path and equivalent‑radius dependence is a novel, valuable feature for climate change studies.

Abstract

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.