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 µm). 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 µm. 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. © Crown copyright, 2004.