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We present a new parameterization of the optical properties of ice crystal clouds which is suitable for use in climate models. Five spectral intervals in the shortwave and five intervals in the infrared are employed, with the ice cloud optical properties parameterized in terms of ice water path (IWP) and the effective radius ( r e ) of the ice crystal size distribution. The parameterization thus allows the flexibility of varying the ice water path and effective radius independently of each other. The parameterized optical properties are used to calculate the bulk reflectivity, transmissivity, and emissivity for cirrus clouds with realistic ranges of IWP and r e . For a given change in cloud optical depth a change in r e alone is more effective than a change in IWP alone in altering the shortwave reflectivity and therefore in altering the strength of the cloud albedo feedback.
Ebert et al. (Fri,) studied this question.