This coupling scheme is presented here for completeness but to the knowledge of the authors it is not in use in any GCM because of numerical stability problems. Its main difference to the semi-implicit scheme is that when the surface energy balance is solved the fluxes computed for the next time-step by the vertical diffusion are not used. The ones of the previous time-step are applied. Just as for the semi-implicit scheme energy conservation may be difficult to fulfill if in the surface energy balance the fluxes are modified. The open-explicit coupling becomes an option for mesoscale models, because they use a time-step that is significantly smaller than GCM time steps (e.g. ISBA in Meso-NH). Due to the fact that the PBL and the surface energy balance are solved separately they are allowed to diverge during the time-step.