treme-scale computer architectures. Given the current pressures on human resources for model development, on making model output useful to a broad applications community, and on maintaining an adequate climate observing system, a consensus community-based process would be needed for weighing large additional investments in computing against further investments in these other key links of the climate modeling enterprise.

An NCCF must complement institutionally specific computational resources, not replace them. The NCCF would focus on the execution of cutting-edge models that are primarily developed at existing U.S. centers, but on problems exceeding their internal computational capabilities. Some types of simulations appropriate for an NCCF might include

•  the study of regional climate change and extreme weather events, including hurricanes, droughts, and floods, using atmospheric models with resolutions down to a few kilometers or less;

•  the study of the effects of small-scale processes in the ocean, including mesoscale eddies, on climate variability and change;

•  the study of biogeochemical cycles, including the carbon cycle and atmospheric chemical changes, at very high resolution to better represent ecosystem-scale effects and assess their future response to, and feedback on, climate change;

•  the study of projected changes in land-based ice sheets and their interaction with the ocean that will influence future sea-level change; and

•  the study of the interactions of ecosystems and climate change at very fine regional scales.

These simulations might involve both global and regional modeling components.

The cost of an NCCF would depend on its scope. To be transformational, it would have to offer a several-fold increase in the size or speed of computations that could be performed on institutional machines, and more useful, reliable, and stable access than is likely to be provided by national computing resources not specific to climate modeling. As discussed in federal plans for high-end computing platforms (HECRTF, 2004) and borne out over the past decade, a single leadership system is expensive, and typically costs in excess of $100 million per year to procure and operate.

Advantages of an NCCF

If the U.S. climate modeling community had stable access to such a hardware platform, it would be easier to customize or codesign software infrastructure to maximize effi-



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