A new parameterization scheme for the optical properties of ice crystals for use in general circulation models of the atmosphere

We present a new parameterization scheme for the optical properties of ice crystals, for use in general circulation models of the atmosphere (GCMs). The scheme consists of separate parameterizations for five different ice crystal shapes: hexagonal columns, hexagonal plates, bullet rosettes, spheres and planar polycrystals. The optical properties are specified as functions of the mean maximum dimension of the ice crystal size distribution using polynomial fits to a more elaborate scheme developed by the last author. The present scheme, based on 10th order fits, gives excellent accuracy and is efficient enough for use in GCMs. First, the scheme is tested in 1-dimensional models, showing a large sensitivity of albedo and emissivity to both ice crystal sizes and shapes. Also, long- and shortwave cloud radiative forcings at the top of the atmosphere are quite sensitive to those two parameters, in such a manner that the two effects do not cancel. It is shown that this may have a significant impact on climate sensitivity, through variations in net cloud forcing. (C) 1999 Elsevier Science Ltd. All rights reserved.