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Momentum diffusion and orography

In the case where the LSS computes the surface layer transfer coefficient for latent and sensible heat fluxes, which are part of equation 1, one may wish it to calculate the momentum flux as well. This would then ensure the internal consistency of all surface transfer calculations. Including the momentum diffusion in the LSS raises the issue of the interaction of the orography with the flow for the LSS. Depending on the treatment of gravity wave drag within the GCM the momentum diffusion may have to take into account the form drag by modifying the roughness length or the stability. As this is still a topic of research, the interface could for the moment be restricted to providing the LSS with the orography from the GCM and returning the momentum diffusion coefficient of the surface layer.

The LSS can not be permitted to use its own orography or land/sea mask because it might induce inconsistencies in the coupled model. For instance, spectral models construct their orography in such a way that it is well behaved in the spectral transforms. This elevation map is used in the physical parameterizations of the GCM and thus has to be applied to the LSS as well.



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