Author Posting. © American Meteorological Society, 2011. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 24 (2011): 4973–4991, doi:10.1175/2011JCLI4083.1.
The fourth version of the Community Climate System Model (CCSM4) was recently completed and released to the climate community. This paper describes developments to all CCSM components, and documents fully coupled preindustrial control runs compared to the previous version, CCSM3. Using the standard atmosphere and land resolution of 1° results in the sea surface temperature biases in the major upwelling regions being comparable to the 1.4°-resolution CCSM3. Two changes to the deep convection scheme in the atmosphere component result in CCSM4 producing El Niño–Southern Oscillation variability with a much more realistic frequency distribution than in CCSM3, although the amplitude is too large compared to observations. These changes also improve the Madden–Julian oscillation and the frequency distribution of tropical precipitation. A new overflow parameterization in the ocean component leads to an improved simulation of the Gulf Stream path and the North Atlantic Ocean meridional overturning circulation. Changes to the CCSM4 land component lead to a much improved annual cycle of water storage, especially in the tropics. The CCSM4 sea ice component uses much more realistic albedos than CCSM3, and for several reasons the Arctic sea ice concentration is improved in CCSM4. An ensemble of twentieth-century simulations produces a good match to the observed September Arctic sea ice extent from 1979 to 2005. The CCSM4 ensemble mean increase in globally averaged surface temperature between 1850 and 2005 is larger than the observed increase by about 0.4°C. This is consistent with the fact that CCSM4 does not include a representation of the indirect effects of aerosols, although other factors may come into play. The CCSM4 still has significant biases, such as the mean precipitation distribution in the tropical Pacific Ocean, too much low cloud in the Arctic, and the latitudinal distributions of shortwave and longwave cloud forcings.
Science Foundation, which sponsors NCAR and the
CCSM Project. The project is also sponsored by the U.S.
Department of Energy (DOE). Thanks are also due to
the many other software engineers and scientists who
worked on developing CCSM4, and to the Computational
and Information Systems Laboratory at NCAR,
which provided the computing resources through the
Climate Simulation Laboratory. Hunke was supported
within theClimate, Ocean and Sea Ice Modeling project at
Los Alamos National Laboratory, which is funded by the
Biological and Environmental Research division of the
DOE Office of Science. The Los Alamos National Laboratory
is operated by theDOENationalNuclear Security
Administration under Contract DE-AC52-06NA25396.
Raschwas supported by theDOEOffice of Science, Earth
System Modeling Program, which is part of the DOE
Climate Change Research Program. The Pacific Northwest
National Laboratory is operated forDOEbyBattelle
Memorial Institute under Contract DE-AC06-76RLO
1830. Worley was supported by the Climate Change Research
Division of the Office of Biological and Environmental
Research and by the Office ofAdvanced Scientific
Computing Research, both in the DOE Office of Science,
under Contract DE-AC05-00OR22725 with UT-Batelle,
Meridional overturning circulation
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