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Global impact of aerosols on mixed-phase clouds

Mixed-phase clouds are clouds that occur at temperatures between 0 and -40ºC. They consist of cloud droplets and ice crystals. Aerosols such as mineral dust particles play an important role in the formation process of these clouds. They are on one hand crucial for the initialization of freezing via heterogeneous nucleation in mixed-phase clouds, where aerosols act as ice nuclei (IN) and reduce the activation energy for the phase change.

However, the interactions of aerosols and mixed-phase clouds are not very well understood yet, especially in the Arctic, where mixed-phase clouds are the dominant cloud type. Many models have difficulties to accurately estimate mixed-phase clouds in the Arctic. This might be a result of the simplified parameterization of the freezing process and the aerosol processing, which may lead to a poor representation of mixed-phase clouds. As the composition of clouds has an impact on the microphysical processes within the cloud, precipitation and the climate this issue has to be solved to improve climate predictions.

The freezing mechanisms and aerosol-cloud interactions in mixed-phase clouds are investigated with the global climate model ECHAM HAM. For that purpose a new parameterization of freezing processes in mixed-phase clouds will be implemented into ECHAM HAM, which is based on the Classical Nucleation Theory.  This new parameterization scheme makes it feasible to incorporate different lab measurements.

The impact of different freezing parameterization schemes and inclusion of aersol processing on the representation of mixed-phase clouds will then be investigated focusing on the Arctic.

 

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© 2014 ETH Zurich | Imprint | Disclaimer | 19 February 2014
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