The dependence of retrieved cirrus ice-crystal effective dimension on assumed ice-crystal geometry and size-distribution function at solar wavelengths

The dependence of retrieved ice-crystal effective dimension on assumed ice-crystal shape and size-distribution function is investigated at solar wavelengths (0.87, 1.6 and 3.7 mum). The dual viewing Along Track Scanning Radiometer is used to retrieve ice-crystal effective dimension using a method of Optimal Estimation over semi-transparent cirrus located in the tropics and mid-latitudes. The ice-crystal effective dimension is defined in terms of the effective diameter and it is retrieved assuming roughened hexagonal ice aggregates, pristine hexagonal ice columns, and four-branched bullet rosettes. It is shown that if a phase function that well represents the scattering properties of cirrus is applied to each crystal type then the retrieved ice-crystal effective diameter is only weakly dependent on ice-crystal shape and size-distribution function. Absolute differences between retrieved ice-crystal effective diameter assuming hexagonal ice columns and ice aggregates combined with a representative phase function are generally well within +/-5 mum. The findings of this paper have the important implication that, in climate models, simulation of cirrus cloudy radiances at solar wavelengths might be made to be independent of assumed ice-crystal shape and size-distribution function.