ducing a comprehensive Earth system reanalysis for the past 50 years (or at least for the period 1980 to the present).

Hindcast Testing of U.S. Climate Models

The committee also encourages a nationally coordinated research effort of hindcast testing of all major U.S. climate models (not just the unified model). This is much easier to implement than a unified weather-climate model; each climate model can be run at its preferred grid resolution and need not have a data assimilation capability. The effort could combine several years of hindcasts on weather time scales (up to 15 days) and coupled-model hindcasts on intraseasonal to interannual time scales. Each model could use either externally initialized fields or some form of relaxation or data assimilation. A rigorous and coordinated testing process using a standardized protocol, outputs, and diagnostics would facilitate model intercomparisons and accelerate progress. Tests could include perturbed initial conditions, perturbed-parameter ensemble hindcasting capability, and perhaps ensemble Kalman filter data assimilation to guide the choice of “fast physics” parameters. Results should be made publicly available in a standard web-accessible form. The main goals would be to evaluate and improve model representations of “fast” physical processes that vary strongly on these time scales and to optimize uncertain parameters within these representations.

Recommendation 11.1: To fully exploit a multiscale approach to model advancement, the United States should nurture a unified weather-climate prediction system capable of state-of-the-art forecasts from days to decades, climate-quality data assimilation, and Earth-system reanalysis.

Recommendation 11.2: To reduce sources of uncertainty in climate simulations, the United States should pursue a coordinated research effort to use weather and/or seasonal/interannual hindcast simulations to systematically constrain uncertain parameters and to improve parameterizations in its major climate models.

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