A MULTIPLE-SOURCE LAND-SURFACE ENERGY-BALANCE MODEL FOR USE IN GENERAL-CIRCULATION MODELS

A multiple-source surface energy balance model for use in general circulation models (GCMs) is developed. The model recognizes three biome architectures: closed canopies (e.g. forest), biomes with two distinct canopies (e.g. savannah) and agricultural crops. The model is based on the Penman-Monteith approach and solves for latent and sensible heat flux without the need to specify a surface temperature or humidity. The two canopies and soil layer are allowed to interact through the specification of a within-canopy saturation deficit and temperature. One or two canopy surface resistances and a soil surface resistance can be specified. The model is calibrated for tropical rainforest, tropical savannah and an agricultural crop. Good agreement is obtained for both hourly and long term evaporation. Two responses to soil moisture were identified for the savannah: a long term one and a short term one. The latter relates to the switching of evaporation from a demand controlled to a supply controlled process. Some problems in the specification of roughness lengths for sparse crops lead to errors in the calculation of momentum exchange. Suggestions are made for improvement. Possible applications of the model are discussed.