Hydrologic forecasting and spatial scales: an example of Meteo-France's operational models

Water is one of the most important components of the atmosphere. The water vapour represents about 5% of the atmosphere, and is one of the most active gases for the greenhouse effect. The phase changes that occur in the atmosphere (condensation, evaporation, sublimation, freezing) lead to important exchanges of energy, and are one of the key process of turbulence, and more generally, of the atmospheric circulation. Water dissolves and transports chemicals, and the scrubbing of rainfall modifies the aerosol concentration, and therefore the radiation exchanges, and can generate acid rain. Moreover, atmospheric water governs the atmospheric electricity processes such as lightning. Therefore, it is necessary to have a good representation of the water cycle to be able to simulate the atmospheric circulation at all scales, from regional to large scale. After a brief presentation of the water cycle at the global scale, this article focuses on the simulation of the precipitation in the global and regional atmospheric models, and especially in the Meteo-France models. Then, the role of the surface schemes and their links with the atmosphere and the hydrology through the simulation of the water cycle at the surface are presented. (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.