The radiation is the driving flux in the surface energy balance. The solar part of the spectrum is the forcing and the long-wave is in most situations a sink of energy. In the absence of moisture the surface temperature is dominated by the balance between these two radiative fluxes. Thus special attention has to be paid to them at the interface.
All radiation calculations are performed in the GCM but they need the surface conditions as a lower boundary condition. In the case of short-wave radiation the typical time-scale is faster than the variations of the boundary condition, thus a Dirichlet solution is well adapted. This is achieved by providing surface albedo to the radiation scheme which will then compute all fluxes and balance them at the surface. The LSS will expect in return the net short-wave flux from the GCM and the zenith angle. This last variable will be used to compute the next value of albedo. When, in the future, bi-directional reflectances are modeled in land-surface schemes this choice may have to be revisited. Reflection will not only depend on the zenith angle but also on the intensity of direct and diffuse light and thus will change at the time scale of the incoming flux.
As the long-wave radiation emitted from the surface is largely independent of the atmospheric conditions the Neumann and Dirichlet closures are equivalent. Thus the upward flux in the required spectral bands or an equivalent radiative temperature can be provided to the GCM . Traditionally this problem has been solved by providing the surface temperature to the radiation scheme because of practical considerations. Indeed this solution has the advantage of not requiring in the LSS any knowledge about the choice of spectral discretization made in the radiative code. For the down-welling radiation the flux is the best quantity to exchange as it is the result of the balance of fluxes computed in the radiative code at the lowest level of the GCM and it enters directly the surface energy balance equations 1.
Albedo, emissivity and surface radiative temperature are the quantities which are computed by the LSS while it will expect in return from the GCM net solar radiation, the zenith angle and down-welling long-wave radiation . A method of exchanging this information in such a way that it can be applied to any GCM is proposed in section 3.