Priority 4: Developing Community Models
CCSM and IPCC
A high priority of NCAR research is the ongoing development and improvement of a climate modeling system that is at the forefront of international efforts to understand and predict the behavior of the Earth's climate. This includes the Community Climate System Model (CCSM) as well as its component models. The CCSM is run on some of the world's most powerful supercomputers and simulates the many interconnected events that drive Earth's climate. These include changes in the atmosphere and oceans, the ebb and flow of sea ice, and the subtle impacts of forests and rivers. CCSM is unique among powerful models in that it belongs to the entire community of climate scientists, rather than to a single institution. Hundreds of specialists from across the United States and overseas collaborate on improvements to CCSM. The model's underlying computer code is freely available on the Web. As a result, scientists throughout the world can use CCSM for their climate experiments. The most recent version, CCSM-3, was released in 2004 and represents a major advance over earlier versions of the model because it contains far more information about Earth's physical processes such as rivers and ice cover. CCSM is constantly being updated and improved, and the release of the next version of the model will likely be in 2008.
A major project that involved NCAR staff was participation in the development of the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). This is an international assessment of the state of the science regarding climate change and is produced every four years. It is an extremely important report that provides the scientific foundation for policy discussions and decisions on climate change. NCAR scientists served as convening lead authors, lead authors, and contributing authors and also reviewed various chapters of the report and contributed to the Technical summary and Summary for Policy Makers.
The CCSM project played a major role in the IPCC AR4 through an extensive series of modeling experiments designed to study the effects on various aspects of the climate system from the release of differing amounts of chemicals into the atmosphere. The suite of CCSM-3 experiments is the most extensive ensemble of any of the international global coupled models run for the IPCC AR4. The resulting large data sets are freely available to the climate research and education community via the Earth System Grid. The CCSM data are also part of the Climate Model Evaluation Project (CMEP), which includes over 200 researchers from around the world who analyzed the multi-model data set for the IPCC AR4.
One accomplishment this past year was the production of a coupled integration that is as good as that from the CCSM-3, but using the Finite Volume (FV) dynamical core in the atmosphere component. Previous attempts using the FV core had produced a too cold Arctic climate and too much sea ice, especially in the Labrador Sea. This was corrected by changing the horizontal viscosity parameterization in the ocean component, which produced a much more realistic, warmer climate in the central Labrador Sea. The sea ice distributions from the new FV version and the released CCSM-3 version are very comparable (Figure 1). The new coupled run using the FV core was run out for 410 years, and this version of the CCSM will now form the basis for future model development. Using the FV core is very important when chemistry is added to the model, for example, because of its conservation properties.CCSM is constantly being updated and improved, and the release of the next version of the model will likely be in 2008.
The current long-term plan of the CCSM project is to develop and freeze the next version of the model, CCSM-4, by the end of 2008. In addition to several other improvements, this version will most likely have new components for the carbon cycle and interactive atmospheric chemistry. This will enable a whole new range of scientific questions to be asked of, and answered by, the CCSM including those related to complex interactions among biota, chemical processes, and the climate system. In addition, the CCSM-4 will be the model used to contribute to the next IPCC report.