Additional Global Climate Cooling by Clouds due to Ice Crystal Complexity

Ice crystal sub-micron structures have a large effect on the cirrus cloud radiative properties and, therefore, for the cloud radiative forcing. Theoretical calculations have shown, that compared to pristine crystals, complex ice crystals or ice crystals with surface roughness produce a flat and featureless scattering phase function with a significantly higher fraction of backscattering. Changing the radiative properties of ice crystals in general circulation models to those of roughened ice crystals could significantly affect the cloud radiative effect. Satellite measurements have indicated that natural ice crystals have a high degree of surface roughness and the latest MODIS collection 6 product has incorporated complex and roughened ice crystals. However, the use of roughened ice crystals in general circulation models and in satellite retrievals is not well justified as long as direct in-situ evidence of the degree and global coverage of ice crystal complexity in the sub-micron scale is missing.

In this contribution, the first global in-situ measurements of ice crystal sub-micron scale complexity are presented. The degree of sub-micron complexity was quantified in five airborne campaigns with a method based on analyzing the spatial intensity of the light scattering patterns of single ice crystals. These measurements were linked to measurements of the shortwave angular light scattering function. The observations showed that an overwhelming fraction of all measured ice crystals contain sub-micron deformations independent of the measurement location or cloud type. As a consequence, a quite uniform angular light scattering function with enhanced cloud back-reflection and a relatively low asymmetry parameter has been observed globally. The measured cloud angular light scattering function was parametrized in terms of the cloud asymmetry parameter and this new parameterization was tested in the ECHAM-HAM global climate model. The implications of the observations to the cloud radiative effect will be discussed.