CAM6 Scientific Summary

CAM6 contains substantial modifications of every atmospheric physics parameterization except for radiative transfer. The Cloud Layers Unified by Binormals (CLUBB, [GLC02] , [BGM+13] ) scheme has replaced earlier schemes for boundary layer turbulence, shallow convection and cloud macrophysics. CLUBB is a prognostic moist turbulence scheme that calculates joint higher-order moments of subgrid vertical velocity, water content and liquid water potential temperature. Equations for these moments are closed using assumed joint binormal probability density functions (PDFs) for these quantities. In addition to calculating subgrid vertical fluxes, CLUBB’s PDF closure is also used to calculate large-scale condensation and cloud fraction. An improved two-moment prognostic cloud microphysics (MG2, [GM15]) has also been introduced. The major innovation in MG2 is to carry prognostic precipitation species – rain and snow – in addition to cloud condensates. MG2 interacts with the MAM4 aerosol microphysics scheme to calculate condensate mass fractions and number concentrations. Deep convection ([ZM95]) has been significantly retuned to increase the sensitivity to convective inhibition. Both schemes to calculate subgrid orographic drag have been substantially modified. Topographic orientation (ridges) and low-level flow blocking effects have been incorporated into the orographic gravity wave scheme. The previous parameterization of boundary layer form drag – turbulent mountain stress (TMS) – has been replaced with the scheme of Beljaars et al. currently used in the European Center forecast model. In addition to these physics updates, new infrastructure for traceable generation of topographic forcing files has been developed.

Complete CAM6 scientific documentation is under development and is expected to be available by September 2018.

References

[BGM+13]

Peter A Bogenschutz, Andrew Gettelman, Hugh Morrison, Vincent E Larson, Cheryl Craig, and David P Schanen. Higher-order turbulence closure and its impact on climate simulations in the community atmosphere model. Journal of Climate, 26(23):9655–9676, 2013.

[GM15]

Andrew Gettelman and Hugh Morrison. Advanced two-moment bulk microphysics for global models. part i: off-line tests and comparison with other schemes. Journal of Climate, 28(3):1268–1287, 2015.

[GLC02]

Jean-Christophe Golaz, Vincent E Larson, and William R Cotton. A pdf-based model for boundary layer clouds. part i: method and model description. Journal of the atmospheric sciences, 59(24):3540–3551, 2002.

[ZM95]

G. J. Zhang and N. A. McFarlane. Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian Climate Centre general circulation model. Atmosphere-Ocean, 33:407–446, 1995.