Realisability constraints for land-surface schemes

It is well known that turbulent transfer models can sometimes produce results that cannot be realised in nature. In this paper we develop simple rules to check the physical robustness of parameterisations of land-surface turbulent transfer for heterogeneous surfaces. The methodology draws upon areal-averaging by parameter aggregation and by flux aggregation. Both techniques have been demonstrated to be valid. Combining the two techniques enables one to determine the surface potential temperature and specific humidity, averaged over the grid-box. When this is done the implied turbulent transfer is sometimes counter-gradient. The implications of this result are discussed. The methodology presented is general and provides a complementary way of evaluating the role of coupling land-surface schemes to atmospheric host models to that presented by Polcher et al. [Polcher, J., McAvaney, B., Gaertner, M.-A., Hahmann, A., Noilhan, J., Phillips, T., Pitman, A., Schlosser, A., Schulz, J.-P., Timbal, B., Verseghy, D., Viterbo, P., Xue, Y., 1998. A proposal for a general interface between land-surface schemes and general circulation models. Global Planet. Change, this issue.].