CALCULATION OF THE GLOBAL LAND-SURFACE ENERGY, WATER AND CO2 FLUXES WITH AN OFF-LINE VERSION OF SIB2

Global land surface fluxes of energy and CO2 have been simulated using an off-line version of a biosphere-atmosphere model, SiB2, forced wit h analyzed or observed atmospheric boundary layer mean potential tempe rature, water vapor mixing ratio, and wind, surface downward solar and thermal radiation, and precipitation. The off-line model is called Si BDRV. Soil and vegetation boundary conditions were specified from sate llite data and other sources. The European Centre for Medium-Range Wea ther Forecasts (ECMWF) data assimilation system products were used to derive the atmospheric and radiative forcings. Precipitation was based on station observations. The SiBDRV results were compared with corres ponding simulation results produced by the Colorado State University g eneral circulation model (CSU GCM), with the ECMWF assimilation system output and with observations. Differences between the surface energy budget components and the surface climatology produced by SiBDRV and t he ECMWF assimilation system are due to differences in the land surfac e parameterizations between the two models. SiBDRV produced lower surf ace latent heat fluxes and larger sensible heat fluxes than the ECMWF data assimilation, partly due to large canopy resistent term explicitl y formulated by SiB2 and possible precipitation differences between th e SiBDRV precipitation forcing and the ECMWF data. Differences between the SiBDRV and the CSU GCM results are due to the different climates associated with the ECMWF assimilation system output, which is strongl y constrained by assimilated observations, and by the CSU GCM, which i s run in pure simulation mode. More specifically, the major reasons fo r the surface energy and CO2 budget differences between the SiBDRV and the GCM are greater incoming solar radiation in the GCM and differenc es in the precipitation patterns. The simulated global annual carbon u ptake by the terrestrial biosphere is similar in SiBDRV and the GCM. T he annual gross primary productions of SiBDRV (116 Gt) and the GCM (11 3 Ct) agree well with other studies, using either ecological process m odels or empirical regression models. SiBDRV takes up 10 and 5% more C O2 than the GCM in January and July, respectively. The seasonally vary ing land surface CO2 fluxes estimated by the SiBDRV and the GCM both c ompare reasonably well with the results of other calculations.