IMPLEMENTATION OF THE ISBA PARAMETRIZATION SCHEME FOR LAND-SURFACE PROCESSES IN A GCM - AN ANNUAL CYCLE EXPERIMENT

A parametrization scheme for the Interactions between Soil, Biosphere and Atmosphere (ISBA), has been implemented in the French spectral general circulation model (GCM) Emeraude. ISBA represents surface physical processes including the variability of soil hydrological properties and the influence of vegetation cover on the exchanges between the soil-cover system and the atmosphere. The main characteristics of the scheme, and the more recent improvements concerning the tropical rain forest, are briefly described. The method used to derive the maps of the surface parameters is also presented. Two 3 year experiments with and without ISBA are compared, with special emphasis on the results at specific locations where observations are available. The general features of the model climatology are found to be independent of the inclusion of the scheme. With ISBA, the model gives a lower surface soil temperature, mainly owing to an increased surface albedo, and an intensification of the hydrological cycle. A more striking response to the inclusion of ISBA is the improved partitioning of the surface energy budget, which then agrees moderately well with the observations over different systems of land cover. A comparison with previous GCM experiments with new land surface parametrizations shows that the models with the new schemes are in better agreement with each other and with the observations over the Amazon region.