Shallow trade wind clouds cover a large fraction of the Earth, and are a key source of uncertainty in projections of the Earth's changing climate. Clouds can cause both warming and cooling. Low-level clouds tend to cool by reflecting sunlight. A positive cloud feedback means warming is enhanced. Increasing computing power opens new and exciting opportunities for scientists in climate modelling. Now models that explicitly simulate convective motions globally, often called storm-resolving models using kilometre grid spacings, are feasible. In past studies of shallow cumulus clouds and their response to a warmer climate mostly large eddy simulations (LES) have been used, that typically apply hectometre or even finer grid spacings, but these have been limited to small domains; however, they become imaginable on global scale. The authors aim to bridge the gap between findings based on these (until now) limited-area large eddy simulations and emerging global storm resolving models.
To do so they simulated a shallow trade cumulus field in a present-day and a 4 Kelvin warmed climate while degrading the horizontal resolution from 100 m to 5 km. As the resolution is coarsened, the cloud fraction increases substantially 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, acting to reflect less sunlight and therewith alone constituting a positive shortwave cloud feedback. Its strength correlates with the amount of cloud fraction and thus is stronger at coarser resolutions. However, clouds also get thicker, which reflects more sunlight. This acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to a convergence to a near-zero shallow cumulus feedback. Insofar as these findings carry over to other models, they suggest that storm-resolving models configured with a similar cloud scheme may exaggerate the trade wind cumulus cloud feedback.
Original publication:
Radtke, J., Mauritsen, T., and Hohenegger, C.: Shallow Cumulus Cloud Feedback in Large Eddy Simulations – Bridging the Gap to Storm Resolving Models, Atmos. Chem. Phys. Discuss. [preprint], doi.org/10.5194/acp-2020-1160, in review, 2020.
Contact:
Jule Radtke
Center for Earth System Research and Sustainability (CEN), Universität Hamburg,
and International Max Planck Research School on Earth System Modelling, Max Planck Institute for Meteorology
Email: jule.radtke@mpimet.mpg.de
Dr Cathy Hohenegger
Max Planck Institute for Meteorology
Email: cathy.hohenegger@mpimet.mpg.de
Prof Dr Thorsten Mauritsen
Department of Meteorology, University Stockholm
Email: thorsten.mauritsen@misu.su.se