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Turbulent fluxes

 

The vertical turbulent diffusion in the lower layers of the atmosphere is strongly coupled to the surface processes which are the main source of energy. This link is formed by the latent, sensible heat and momentum fluxes at the surface. As the surface fluxes and the turbulent diffusion are very non-linear and share similar time-scales they will have the same time-step within a GCM thus simplifying the coupling. This time-scale however is usually shorter than the typical time-step in GCMs, thus great care has to be taken when choosing a numerical scheme for the turbulent fluxes.

As surface temperature is the result of the balance of fluxes and at the same time the turbulent fluxes have a strong dependence on this same temperature they need to be determined while solving equation 1, or at least be varied with the temperature change. Temperature and relative humidity from the lowest level of the GCM are also needed to compute the turbulent fluxes in equation 1 but their variations are generally slower than the surface conditions. Thus it appears that the vertical diffusion scheme is best closed with the two turbulent fluxes provided by the LSS. Some LSS will not use a Dirichlet closure for the turbulent fluxes in equation 1, but rather a mixed closure. As this choice has strong implications for the design of the LSS, a general interface should not restrict this diversity. On the other hand most vertical diffusion schemes already use surface fluxes as lower boundary conditions. This will be discussed in detail in section 4.





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