| Summary: | Clouds are important elements in the atmosphere and the whole climate system as they transport water and impact incoming and outgoing radiation. They may consist of liquid water, ice or a mixture of both. Among other factors the relation between liquid water and ice is governing how much longwave radiation and thereby heat a cloud traps between itself and the surface. These effects are not only relevant in the Arctic, but especially important there, as shortwave radiation is absent from the Arctic during large parts of the year. The first part of this thesis uses both long-term lidar measurements and shorter-term intensive in-situ observations to characterize cold clouds in the Norwegian Arctic. In the second part it aims to better represent Arctic mixed-phase clouds in a regional weather model. This is achieved by using observationally-constrained aerosol concentrations and adding processes for so-called secondary ice production, i.e. mechanisms that create new ice particles based on existing ice. Finally, the optimized model is used to study how Arctic mixed-phase clouds may change in a warmer climate.
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