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 gen eral circulation model (GCM) Emeraude. ISBA represents surface physica l processes including the variability of soil hydrological properties and the influence of vegetation cover on the exchanges between the soi l-cover system and the atmosphere. The main characteristics of the sch eme, and the more recent improvements concerning the tropical rain for est, 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 spe cific 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 temper ature, mainly owing to an increased surface albedo, and an intensifica tion of the hydrological cycle. A more striking response to the inclus ion of ISBA is the improved partitioning of the surface energy budget, which then agrees moderately well with the observations over differen t systems of land cover. A comparison with previous GCM experiments wi th new land surface parametrizations shows that the models with the ne w schemes are in better agreement with each other and with the observa tions over the Amazon region.