On the land surface-atmospheric coupling and its impact in single-column atmospheric model

Three versions of the single-column European Centre for Medium-Range Weather Forecasts-Hamburg (ECHAM4) climate model are compared that differ either in the technique of the numerical coupling between land surface and atmosphere or the physical parameterization of the land surface processes. The standard ECHAM4 model utilizes a semi-implicit coupling technique between land surface and atmosphere in a way in which energy at the land surface-atmosphere interface is not conserved. This is a major deficiency. Two new model versions were developed: ECHAM4/IMPL and ECHAM4/SECHIBA. They incorporate an implicit coupling technique that conserves energy. ECHAM4 and ECHAM4/IMPL are identical with respect to all physical parameterizations they apply; the only difference is the coupling. In ECHAM4/SECHIBA, the ECHAM land surface scheme was replaced by SECHIBA (Schématisation des Echanges Hydriques à l'Interface entre la Biosphère et l'Atmosphère). The intercomparison of one-dimensional versions of these three models shows that the energy residual term in ECHAM4 is not negligibly small, but it is rather of the order of the physical fluxes. Biases of more than 1300 W m^-2 are found due to the coupling technique. These are avoided in ECHAM4/IMPL, which results in a more pronounced diurnal cycle of surface temperature and generally higher temperature maxima during a warming phase. In an offline intercomparison of the three model versions, using an observational atmospheric forcing dataset, an important impact of the coupling technique on the simulated surface energy cycle is found as well. The turbulent heat fluxes in ECHAM4 tend to be underestimated; their rise in the morning and decrease in the afternoon are delayed. Because of the improved coupling, the turbulent heat fluxes of the implicit models are in better agreement with the observations, especially regarding the phases of their diurnal cycles. Differences between ECHAM4/IMPL and ECHAM4/SECHIBA are mainly found for the simulated surface temperature, which gets closer to the observed radiative temperature for the latter model. Furthermore, the diurnal amplitude of the ground heat flux is increased in ECHAM4/SECHIBA in agreement with the observations.