analysis, but the human resources have generally not kept pace with the rapid growth in model complexity.

The development and use of comprehensive climate models in the United States requires a large number of talented individuals in the following areas:

•  scientists engaged in understanding the climate system, leading to the development of new parameterizations and other model improvements (distinct cadres of scientists are often needed for various model components, such as the ocean or terrestrial ecosystem models);

•  scientists engaged in using the models for well-designed numerical experiments and conducting extensive diagnostics of the models to better understand their behavior, ultimately leading both to model products and to scientific insights that provide the impetus and context for model improvements;

•  scientists studying the regional details provided by the archived results from global model simulations and related downscaling efforts, and how these vary across various models;

•  support scientists and programmers to conduct extensive sets of numerical simulations in support of various scientific programs and to ensure their scientific integrity;

•  software engineers to create efficient and portable underlying codes, including the development and use of common software infrastructures;

•  software engineers and scientists to facilitate easy and open access to model output through modern networking technologies;

•  hardware engineers to maintain the high-end computing facilities that underpin the modeling enterprise; and

•  climate interpreters to translate climate model output for decision makers.

The U.S. institutional and funding system has addressed some of these areas better than others; in particular, the U.S. scientific effort on model diagnostics and regionspecific analyses has kept up better than the effort devoted to model improvement. The result is that many climate modeling efforts are subcritical in some aspects. In particular, there are longstanding problems in the simulation of the atmosphere-oceanland-ice system that are not yet solved, and yet these have been somewhat neglected in the desire to add additional complexity into models. One example of a longstanding problem is the tendency for virtually all climate models to simulate an unrealistic structure of the Intertropical Convergence Zone in the eastern Tropical Pacific. Another such example is the tendency for virtually all models to simulate sea-surface temperatures in the equatorial Atlantic that increase from west to east, instead of the observed increase from east to west. These errors in the simulation of the basic state of the tropical climate can distort the overall simulation of the climate system.

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