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Next: Sub-grid scale variability Up: Tasks of a land-surface Previous: Momentum diffusion and orography

Closing the hydrological cycle

So far only the exchange of variables which intervene in the calculation of the surface turbulent fluxes and energy balance have been discussed. To allow the surface scheme to close the water balance, it needs to be provided with precipitation. The distinction between snow and ice is a delicate issue here because it touches on the parameterization of clouds within the GCM and it may affect the conservation of energy in the coupled system.

Over the last few years, GCMs have started to simulate cloud liquid water and it has been recognized that the description of the water particles forming clouds are essential in order to compute the interaction with radiation. In order to distribute correctly within the atmosphere the energy of condensation, GCMs will have to determine where in the column snow or liquid precipitation is formed. Thus it seems reasonable to assume that precipitation schemes will in the not too distant future determine if snow, ice or water reaches the surface. In the design of the interface it has to be assumed that the GCM will provide at least rainfall and snowfall.

This raises the issue of a consistent use of latent heat of evaporation and sublimation by the GCM and the land surface scheme. To conserve energy in the coupled system GCM/LSS the same physical constants have to be used. This will easily be achieved if the GCM provides the LSS with all physical constants through the interface.

With the coming of age of coupled ocean/atmosphere models, LSS will have to include a river routing scheme and thus model the river outflow. The interface has thus to include the runoff of rivers into the ocean. This field is different from the grid-box runoff produced by LSSs as it will be non zero over ocean points along the coast. The grid-box runoff is internal to the land-surface scheme.


next up previous
Next: Sub-grid scale variability Up: Tasks of a land-surface Previous: Momentum diffusion and orography

POLCHER Jan
Fri Mar 6 16:09:11 MET 1998