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Journal Article

Shallow cumulus cloud feedback in large eddy simulations -- Bridging the gap to storm resolving models


Radtke,  Jule
Centrum für Erdsystemforschung und Nachhaltigkeit (CEN), Universität Hamburg;
IMPRS on Earth System Modelling, MPI for Meteorology, Max Planck Society;


Hohenegger,  Cathy
Precipitating Convection, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Radtke, J., Mauritsen, T., & Hohenegger, C. (in press). Shallow cumulus cloud feedback in large eddy simulations -- Bridging the gap to storm resolving models. Atmospheric Chemistry and Physics. doi:10.5194/acp-2020-1160.

Cite as: http://hdl.handle.net/21.11116/0000-0008-0E44-D
The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty to interpretations and projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Non-hydrostatic Large Eddy Model in a control and a perturbed 4 K warmed climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened the basic state cloud fraction increases substantially, especially at cloud base, lateral mixing is weaker and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of basic state cloud fraction, thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapor availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm resolving models may exaggerate the trade wind cumulus cloud feedback.