Simple parameterizations of the radiative properties of cloud layers: a review

Current atmospheric general circulation models parameterize cloud-radiation effects through relationships that are based on the assumption that a model grid is partly clear and the remaining area is filled with a homogeneous cloud layer. The optical properties of this homogeneous layer are related to cloud microphysical quantities such as the liquid water path and effective drop radius which in turn are diagnosed from model variables. In the future it will be necessary to consider distributions of liquid water paths within a grid and relate the effective radius to the type of cloud formed since the grid mean radiative properties are very sensitive to these parameters. There is also the need for a computationally efficient technique to model overlapping liquid and vapor absorption in the solar near infrared.