Radiation exchanges above West African moist savannas: Seasonal patterns and comparison with a GCM simulation

Abstract

During a field campaign of the Savannas on the Long Term (SALT) Global Change and Terrestrial Ecosystems/International Geosphere Biosphere Program dealing with the energy budget of a West African moist savanna, particular attention was paid to the radiation budget because it is the driving force for all other surface fluxes. All components of the radiation budget were measured on the ground during one and a half year on a test site in Ivory Coast. The seasonal variations of the observed radiation at the surface and satellite data were compared to the values simulated for the 1978–1988 period by a general circulation model coupled to a land surface parameterization scheme, both developed by the Laboratoire de Météorologie Dynamique (LMD). Problems which appear when point observations of surface fluxes are compared with results from general circulation model simulations are addressed. Seasonal variations of the prescribed savanna albedo in the model differed significantly from the observed variations due to bush fire influence. However, the major discrepancy between model simulations and surface observations resulted from the overestimation of the incoming shortwave radiation at the surface. The major sources of discrepancy were found to be the simulated clear‐sky atmospheric absorption and/or scattering of the shortwave radiation and the simulation of cloud and/or cloud‐radiation interactions which lead to an overestimation of the absorbed energy at the surface by the model. It is concluded that improvement of the simulated energy budget in the humid tropics depends critically on a good simulation of the incoming shortwave radiation at the surface.