Cirrus clouds are clouds in the upper troposphere/lowermost stratosphere (UTLS) region consisting purely of ice crystals. As all other cloud types cirrus clouds affect the radiation budget of the Earth-Atmosphere system via two different effects
Up to now the net effect of cirrus clouds on the radiation budget is uncertain; for thin cirrus clouds a net warming of the Earth-Atmosphere system is assumed but not confirmed. This uncertainty is mainly due to the complexity of microphysics in cirrus clouds, e.g. different nucleation mechanisms for ice crystals involving solution droplets and so-called ice nuclei (IN).
Since the global net effect of cirrus clouds is highly uncertain, the representation of cirrus clouds in the ECHAM5 GCM is a focus of our work. We are working on the impact of aerosols on cirrus clouds in the ECHAM5 GCM for a better representation of cirrus clouds. In cirrus clouds different nucleation processes can compete and change the microphysical and radiative properties of the developing cirrus cloud.
Stratospheric aerosol loadings can be enhanced by natural processes like
volcanic eruptions or by anthropogenic intervention to counteract global
warming, termed Climate-engineering. In both cases sulphate is injected
into the stratosphere. Aerosols injected into these heights can sediment to lower levels into
the upper troposphere, modify the aerosol composition and thus might
impact cirrus clouds. Current global simulations show that the microphysical properties of
cirrus clouds are indeed modified due to stratospheric sulphate
emissions in both cases. The results suggest that via a rather complex
chain of cause and effect including feedbacks on the dynamics cirrus
clouds are impacted significantly.
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