back to space. Condensation from vapor to liquid or ice heats the atmosphere, and evaporation cools the surface. To determine the net effect of water on our climate, the full cycle of evaporation, water vapor transport, cloud formation, precipitation, and runoff must be considered as an integral system, within which these "fast climate processes" interact on relatively short time scales, thereby involving large fluxes of energy. On the basis of this scientific paradigm, the WCRP formulated (WCRP, 1987) the concept of the Global Energy and Water Budget Experiment, an integrated multidisciplinary research program to study the interactions between the fast climate processes in the atmosphere and at the land surface, and the impact of these processes on exchanges of momentum, water, and energy with the slower components of the climate system.

GEWEX is an integrated program (Table A.1) to understand, model, and predict (1) radiative processed involving cloud, aerosol, water vapor, and their impact on radiation transfer and radiation flux divergence in the atmospheric column, and (2) hydrological processes, involving the transport and release of heat in the atmosphere, precipitation, evapotranspiration and land surface exchanges, groundwater storage, and runoff. GCIP is relevant to all elements in this program. The goal of GEWEX is to study and model linkages between the energy and water cycles on all time and space scales, using optimal combinations of in situ measurements and observations from space, especially the more informative data expected from the next generation of remote sensing instruments that constitute the International Earth Observing System (IEOS).

The overall objectives of the GEWEX program were expressed by WCRP as the following:

  1. Determine the Earth's hydrologic cycle and energy fluxes using global measurements.

  2. Model the global hydrologic cycle and assess its impact on the atmosphere, oceans, and land surfaces.

  3. Develop the ability to predict variations in global and regional hydrological processes and water resources, as well as their responses to environmental change.

  4. Foster the development of observing techniques and data management and assimilation systems suitable for operational application to long-range weather forecasts, hydrology, and climate predictions.

GEWEX is not an experiment in the traditional sense; rather it is an integrated program of research, observations, and science activities ultimately leading to prediction of variations in the global and regional hydrological regimes. Because of the magnitude of the effort, GEWEX is compiling information from several ongoing studies and will initiate investigations of its own, as needed, to improve modeling accuracy and the surface-atmosphere coupling in general circulation models. In fact, GEWEX initially encouraged a suite of exploratory

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