Ideally, such a model would cross-fertilize parameterization development between the weather and climate communities and naturally lead to parameterization approaches that work well across a range of space and time scales. The committee acknowledges the challenges and risks in such an approach. It requires a clear national-level mandate, strong and skillful leadership, and substantial new resources that recognize that this should be a research effort that can successfully involve a broad scientific community. In particular, these conditions have not been met in the past. No current U.S. modeling center has the resources and capacity to realize this vision on its own. While NCEP’s GFS/CFS has taken important steps toward this unified modeling vision, its further development is subject to both operational constraints and resource limitations. Past experience suggests that partnerships between centers can succeed only if the incentives to work together are strong, sustained, and offer clear scientific opportunities that attract talented scientists and software engineers.

A further management challenge is harmonizing model development for weather versus climate applications. For weather applications, it is advantageous to update the modeling system whenever a proposed change has been demonstrated to improve the forecast skill, because the main application is weather forecasts with a shelf life of a few days. For climate applications, the forecast lead time can be years to decades, and the model output may be bias corrected, downscaled, or used as one step in a chain of models. For such applications, users may prefer a modeling system that remains frozen for several years before an improved version is introduced. Thus, there must be scope for separate development of weather and climate branches of the model, then a periodic, possibly challenging, reintegration of model changes into a single trunk model as at UKMO. This latter step is a defining characteristic of unified model development.

The committee recommends that the U.S. Global Change Research Program, together with the major national climate and weather modeling institutions (e.g., NCEP, GFDL, NCAR, and the Global Modeling and Assimilation Office) work toward defining a unified modeling strategy and initial implementation steps (or deciding this is not a good approach). It should take advantage of the common software infrastructure, community-wide code, and data accessibility. Its success could be judged by simultaneous improvement of forecast and climate simulation skill metrics on all time scales.

One possible benefit of unified modeling is more accurate assimilation of a broad range of observations. Hence, such a unified modeling effort could include research and development of state-of-the-art data assimilation methods, with the goal of pro-

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