Water is the most effective agent in the climate system to modulate
energy transfer by radiative processes, through its exchanges of latent
heat and within cascades of chemical processes. It is the source of all
life on earth, and once convective clouds are formed, it enables large
vertical transports of momentum, heat and various atmospheric
constituents up to levels above the tropical tropopause. Water triggers
very complex processes at the earth's continental surfaces and within
the oceans. At last, water in its gaseous phase is the most important
greenhouse-gas! Numerical modelling and measurements of the state of the
present climate system needs a very thorough understanding of all these
processes and their various interactions and forcings. This is a
prerequisite for more substantial forecasts of future states in all
scales of time, from days to centuries. Therefore, the management of the
World Climate Research Programme established in 1988 the new programme
GEWEX (Global Energy and Water Cycle Experiment). GEWEX is specifically
defined to determine the energy and water transports in the fast
components of the climate system with the presently available modelling
and measurement means and to provide new capabilities for the future.
Research in GEWEX must further develop methods to determine the
influence of climatic anomalies on available water resources.