Given the importance of a quantitative understanding of the way in which water and energy are moved from place to place and from component to component of the earth's climate system, it is necessary to obtain reliable estimates of the hydrologic and energy cycles in the global atmosphere. While a number of observing platforms designed to address this problem are anticipated in the coming decade, the theoretical and modeling concepts required to interpret the observations have not yet been well formulated. Therefore, it will be necessary to lay the groundwork for making a reasonable estimate of the global hydrologic and energy cycles on time scales of 1 month to several years. A theoretical and modeling framework must be established in which the observations taken during the Global Energy and Water Cycle Experiment (GEWEX), the Tropical Rainfall Measuring Mission (TRMM), and during the Earth Observing System Experiment (Eos) may be utilized and understood.

The major thrust of such a framework will be to validate the global general circulation models (GCMs) which must be used to predict and understand the mechanisms of global change. Future predictions of climate from these models will be reliable only if they can simulate the observed water and energy cycles, and only then can future predictions of water and energy processes also be considered accurate.

The calculations of the seasonal cycle of water and energy fluxes between atmosphere and ocean, and between atmosphere and land may be carried out in four ways. First, and least expensive, the existing operational analyses of atmospheric data from the National Meteorological Center (NMC) and the European Centre for Medium-Range Weather Forecasts (ECMWF) for the most recent, and hence most reliable, years of record may be used. Second, a set of reanalyzed data must be created from the historical record to broaden the database and to evolve an internally consistent, homogeneous, and multivariate time series of climate observations. Third, a long integration of the most realistic, high-resolution GCM available should be made for comparison with the first two datasets in order to validate the model and identify and eliminate sources of systematic error. And, finally, when they become available, the calculations should be repeated based on observations taken during the GEWEX, TRMM, and Eos missions.

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