25 March 2015

A Multi-scale Modelling Framework Model (Superparameterized CAM5) with a Higher-Order Turbulence Closure: Model Description and Low-Cloud Simulations

Summary

Low clouds cover a vast area of the Earth and play a critical role in regulating the energy and the hydrological cycle. Feedbacks in low clouds dominate the uncertainty in cloud feedback from climate change, while aerosol effects on low clouds are notoriously difficult to simulate in climate models due to challenges in representing aerosol effects on cloud microphysics and cloud dynamics. To improve the understanding of low-cloud interactions, a team of scientists led by a U.S. DOE researcher from Pacific Northwest National Laboratory implemented a new high-order scheme turbulence closure called Cloud Layers Unified By Binormals (CLUBB) into a numerical model for simulating aerosol-cloud interactions: the super-parameterized CAM5 (SPCAM5). Implementing CLUBB in the cloud resolving model component of SPCAM5 includes coupling with both the single-moment and double-moment cloud microphysics schemes. The team found CLUBB improves low-cloud simulations, particularly in the stratocumulus-to-cumulus transition regions. Compared to the single-moment cloud microphysics, CLUBB with two-moment microphysics produces clouds that are closer to the coast, in better agreement with observations. In the stratocumulus-to-cumulus transition regions, CLUBB, with two-moment cloud microphysics produces shortwave cloud forcing in better agreement with observations. For both cloud microphysics schemes, CLUBB substantially reduces errors in simulated shortwave cloud forcing in most regions of the Earth. Adding CLUBB into the multi-scale modeling framework provides a promising tool for examining aerosol-cloud precipitation interactions.

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Acknowledgments

This research is based on work supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research, as part of the Decadal and Regional Climate Prediction using Earth System Models (EaSM) Program and the Reduction of Tropical Cloud and Precipitation Biases in Global High Resolution Models Program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DEAC05- 76RL01830. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the US Department of Energy under Contract DE-AC05-00OR22725. V. Larson and D. Schanen acknowledge financial support under Grant AGS-0968640 from the National Science Foundation and Grant DE-SC0008659 from Office of Science (BER) of the U.S. Department of Energy. All model output is stored on the NERSC Science Gateways and can be accessed through the following link: http://portal.nersc.gov/project/m1374/MMFoutput_Wangetal2014JAMES/