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The movement of water in its solid, liquid, and vapor forms, and the changes in phase from one form to another, are responsible for a considerable fraction of the heating and cooling of the atmosphere, which in turn drives the winds and affects the climate at each location on the Earth. The movement and transformations of water substance around the Earth are referred to as the "water cycle."
Climate models are unable to represent all of the physical laws governing the behavior of the atmosphere, oceans, and land surfaces at the level of detail necessary to describe them exactly. Many physical processes must be approximated using simplified, parameterized equations. The values of the parameters can sometimes be determined from the basic physical laws or estimated from analysis of field experiments, but not infrequently they must be adjusted based on comparisons of the climate model behavior with analyses of observational data. Some of the most important and yet most difficult of processes to represent involve precipitation. The global water cycle is a major focus area of the U. S. Global Change Research Program (USGCRP), which together with the Climate Change Research Initiative (CCRI) constitutes the U. S. Climate Change Science Program (CCSP). Scientists in the Climate and Radiation Branch are engaged in: Contact: Thomas Bell
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