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Volume 11, issue 1 | Copyright
Geosci. Model Dev., 11, 235-255, 2018
https://doi.org/10.5194/gmd-11-235-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Model evaluation paper 17 Jan 2018

Model evaluation paper | 17 Jan 2018

The path to CAM6: coupled simulations with CAM5.4 and CAM5.5

Peter A. Bogenschutz1,2, Andrew Gettelman2, Cecile Hannay2, Vincent E. Larson3, Richard B. Neale2, Cheryl Craig2, and Chih-Chieh Chen2 Peter A. Bogenschutz et al.
  • 1Lawrence Livermore National Laboratory, Livermore, CA, USA
  • 2National Center for Atmospheric Research, Boulder, CO, USA
  • 3University of Wisconsin-Milwaukee, Milwaukee, WI, USA

Abstract. This paper documents coupled simulations of two developmental versions of the Community Atmosphere Model (CAM) towards CAM6. The configuration called CAM5.4 introduces new microphysics, aerosol, and ice nucleation changes, among others to CAM. The CAM5.5 configuration represents a more radical departure, as it uses an assumed probability density function (PDF)-based unified cloud parameterization to replace the turbulence, shallow convection, and warm cloud macrophysics in CAM. This assumed PDF method has been widely used in the last decade in atmosphere-only climate simulations but has never been documented in coupled mode. Here, we compare the simulated coupled climates of CAM5.4 and CAM5.5 and compare them to the control coupled simulation produced by CAM5.3. We find that CAM5.5 has lower cloud forcing biases when compared to the control simulations. Improvements are also seen in the simulated amplitude of the Niño-3.4 index, an improved representation of the diurnal cycle of precipitation, subtropical surface wind stresses, and double Intertropical Convergence Zone biases. Degradations are seen in Amazon precipitation as well as slightly colder sea surface temperatures and thinner Arctic sea ice. Simulation of the 20th century results in a credible simulation that ends slightly colder than the control coupled simulation. The authors find this is due to aerosol indirect effects that are slightly stronger in the new version of the model and propose a solution to ameliorate this. Overall, in these early coupled simulations, CAM5.5 produces a credible climate that is appropriate for science applications and is ready for integration into the National Center for Atmospheric Research's (NCAR's) next-generation climate model.

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This paper compares results of developmental versions of a widely used climate model. The simulations only differ in the choice of how to model the sub-grid-scale physics in the atmospheric model. This work is novel because it is the first time that a particular physics option has been tested in a fully coupled climate model. Here, we demonstrate that this physics option has the ability to produce credible coupled climate simulations, with improved metrics in certain fields.
This paper compares results of developmental versions of a widely used climate model. The...
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