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Impact of mesoscale dynamics and aerosols on the lifecycle of cirrus clouds

Final Activity Report Summary - IMDALCC (Impact of mesoscale dynamics and aerosols on the lifecycle of cirrus clouds)

Cirrus clouds are important modulators of the radiation budget of the Earth-Atmosphere system. They cover about 20-30% of Earth's surface and might possibly contribute to a net warming. However, it is rather difficult to provide global estimates of the radiative impact of cirrus clouds because very little is known about their life cycle and they are still not very well represented in global models. For a better understanding of cirrus clouds also the different formation mechanisms of ice in the atmosphere as well as the impact of dynamics must be taken into account. Ice formation in the upper troposphere has two main formation pathways: The probably dominant formation mechanism is homogeneous freezing of super-cooled aqueous solution droplets. This mechanism is very sensitive to changes in the ambient relative humidity with respect to ice, driven by local dynamics, in terms of formed ice crystal number concentrations. Homogeneous freezing can additionally be modified by so-called heterogeneous nucleation on insoluble aerosol particles. By forming ice crystals from such particles at lower humidities than required for homogeneous freezing, these crystals can deplete the water vapour by growth, thus changing the ambient humidity field and the following homogeneous freezing events.

In order to understand better the life cycle of cirrus clouds and the impact of (mesoscale) dynamics vs. aerosols, different model approaches (box model, cloud-resolving model, climate model) were used. In a first attempt, we concentrated on cirrus clouds and mesoscale dynamics driven by stratified airflows over topography, i.e. on so-called orographic cirrus clouds. Nevertheless, during the project it turned out that some other phenomenons of mesoscale dynamics are at least as important as orographic waves for cirrus cloud formation and evolution.

Therefore we investigated also dynamical and convective instabilities in cirrus cloud layers, triggering additional ice formation and leading to inhomogeneous structures of cirrus clouds.
The main conclusions of this work can be summarized as follows:
(1) The microphysical properties of orographic cirrus clouds were investigated successfully. The formation and evolution of cirrus clouds depends crucially on environmental conditions as temperature, humidity and flow characteristics (i.e. dynamics).
(2) Orographic cirrus clouds constitute an important contribution to the global cirrus cloud coverage. In a future climate, their microphysical and radiative properties change such that they contribute to a stronger net warming, i.e. increasing the anthropogenic greenhouse effect.
(3) Different classes of heterogeneous ice nuclei can principally change the properties of cirrus clouds in terms of modifying the dominant homogeneous freezing process. The use of more detailed and size-dependent parameterisations lead to a more pronounced change compared to simple parameterisations as used in many models.
(4) The impact of mesoscale dynamics is even more important that the impact of aerosols in terms of the lifecycle of cirrus clouds. Convective and dynamical instabilities can lead to totally different cirrus cloud structures and to pronounced inhomogeneities in cirrus clouds.