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.

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