Improvements in the CCSM4 compared to the CCSM3

This talk describes improvements to all the CCSM4 components, and documents improvements in fully coupled integrations compared to CCSM3. The sea surface temperature biases in the major upwelling regions are improved compared to the CCSM3, when an atmosphere and land resolution of 1 degree is used. Two changes to the deep convection scheme in the atmosphere component result in the CCSM4 producing much more realistic El Nino/Southern Oscillation variability than the CCSM3, and mean biases in the tropical Pacific Ocean are considerably reduced, but not eliminated, in the CCSM4. A new overflow parameterization in the ocean component represents deep overflows driven by density differences, and leads to an improved simulation of the deep ocean density structure. CCSM4 land surface temperatures and Arctic sea ice concentration are improved, because of improvements in the land and sea ice components. The CCSM4 equilibrium climate sensitivity is 3.0 C, which is 10% larger than the sensitivity of the CCSM3. The CCSM4 has significant biases in Arctic low clouds and the latitudinal distributions of both short-wave and long-wave cloud forcings. An updated version of the atmosphere component, aimed at reducing these biases, and including the physics necessary to represent the cloud-aerosol indirect effects, has been finalized for inclusion in the Community Earth System Model, version 1.