Clouds are important modulators of the Earth-Atmosphere system in terms of radiation and water vapour budget. Especially the impact of clouds in the upper troposphere consisting purely of ice crystals, so-called cirrus clouds, is not well known. In fact, not even the sign of their influence on the radiative budget is known. In large-scale models cirrus clouds are usually treated as pure layer clouds. However, they can bear a strong internal variability in terms of microphysical and radiative properties (patchiness), leading to high deviations in the actual radiative transfer. In this contribution an overview over typical features of cirrus clouds and their representation in models is given. Based on properties on different scales (small scale to large-scale) the occurring problems of representing cirrus clouds in current general circulation models are explained. Finally, some (new) concepts to overcome these problems are presented.
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