A REVISED LAND-SURFACE PARAMETERIZATION (SIB2) FOR ATMOSPHERIC GCMS .1. MODEL FORMULATION

The formulation of a revised land surface parameterization for use wit hin atmospheric general circulation models (GCMs) is presented. The mo del(SiB2) incorporates several significant improvements over the first version of the Simple Biosphere model (SIE) described in Sellers et a l. The improvements can be summarized as follows: (i) incorporation of a realistic canopy photosynthesis-conductance model to describe the s imultaneous transfer of CO2 and water vapor into;and out of the vegeta tion, respectively; (ii) use of satellite data, as described in a comp anion paper, Part II, to describe the vegetation phenology; (iii) modi fication of the hydrological submodel to give better descriptions of b aseflows and a more reliable calculation of interlayer exchanges withi n the soil profile; (iv) incorporation of a ''patchy'' snowmelt treatm ent, which prevents rapid thermal and surface reflectance transitions when the area-averaged snow cover is low and decreasing. To accommodat e the changes in(i) and (ii) above, the original two-layer vegetation canopy structure of SiB1 has been reduced to a single layer in SiB2. T he use of satellite data in SiB2 and the performance of SiB2 when coup led to a GCM are described in the two companion papers,Parts II and II I.