Aerosol-cloud-radiation interactions in global climate models

Abstract: Clouds can reflect, absorb and re-emit radiation, thereby inducing a cooling or warming effect on the climate. However, the response of clouds to a changing climate is highly uncertain and the representation of clouds in state-of-the-art climate models remains a key challenge for future climate projections. Factors contributing to this uncertainty include processes on the microphysical scale involving aerosol particles with the size of just a few nanometers to micrometers. This thesis focuses on the representation of aerosol-cloud-radiation interactions in global climate models. Using idealized experiments from a model-intercomparison project with different anthropogenic aerosol forcings, it was found that both sulfate and non-sulfate aerosols yield an increase in cloud albedo in five regions of subtropical marine stratocumulus clouds. The changes in cloud albedo in the models were driven by changes in the cloud droplet number concentration and liquid water content. Further, it was found that the microphysical coupling of underlying aerosol-cloud interactions in models seems to dominate on the monthly timescale in subtropical marine stratocumulus regions, which can not be confirmed in observations. Quantifying the effect of aerosols on cloud properties in observations remains challenging. In addition, comparisons with satellite retrievals and the global climate model NorESM showed that this model is not able to capture elevated aerosol above cloud, seen in observations in two regions of marine stratocumulus clouds. Sensitivity experiments revealed that the model is most sensitive to the aerosol emissions, convection and wet scavenging in terms of the vertical aerosol distribution. Finally, the representation of aerosol absorption in global climate models was investigated. It was found that most of the models underestimate absorption by aerosols in a focus domain in Asia. Sensitivity studies with NorESM give rise to variations that lie within the large inter-model diversity.