A Simplified Biosphere Model for Global Climate Studies

MARCH 1991 Center for 0cean—Land—Atmosphere Interactions, Department of Meteorology, University of Maryland, College Park, Maryland Y. XUE, P. J. SELLERS, J. L. KINTER AND J. SHUKLA A Simpliﬁed Biosphere Model for Global Climate Studies Y. XUE, P. J. SELLERS, J. L. KINTER AND J. SHUKLA (Manuscript received 8 February 1990, in ﬁnal form 7 November 1990) ABSTRACT The Simple Biosphere Model (SiB) as described in Sellers et al. is a bio—physically based model of land surface—atmosphere interaction. For some general circulation model (GCM) climate studies, further simpliﬁ- cations are desirable to have greater computation efficiency, and more important, to consolidate the parametric representation. Three major reductions in the complexity of SiB have been achieved in the present study. The diurnal variation of surface albedo is computed in SiB by means of a comprehensive yet complex calculation. Since the diurnal cycle is quite regular for each vegetation type, this calculation can be simpliﬁed considerably. The effect of root zone soil moisture on stomatal resistance is substantial, but the computation in SiB is complicated and expensive. We have developed approximations, which simulate the elfects of reduced soil moisture more simply, keeping the essence of the biophysical concepts used in SiB. The surface stress and the ﬂuxes of heat and moisture between the top of the vegetation canopy and an atmospheric reference level have been parameterized in an off-line version of SiB based upon the studies by Businger et al. and Paulson. We have developed a linear relationship between Richardson number and aero- dynamic resistance. Finally, the second vegetation layer of the original model does not appear explicitly after simpliﬁcation. Compared to the model of Sellers et al., we have reduced the number of input parameters from 44 to 21. A comparison of results using the reduced parameter biosphere with those from the original formulation in a GCM and a zero-dimensional model shows the simpliﬁed version to reproduce the original results quite 345 closely. After simpliﬁcation, the computational requirement of SiB was reduced by about 55%. 1 . Introduction Since Chamey’s ( 1975) pioneering study, several experiments have shown that variations in land surface characteristics can have a signiﬁcant impact on the cli- mate. The atmosphere is sensitive to the surface albedo, soil moisture, roughness, and other surface character- istics on many time scales ( Chamey et al. 1977; Shukla and Mintz 1982; Rind 1984; Sud et al. 1988). In order to understand these interactions, not only qualitatively but also quantitatively, more realistic sur- face parameterizations than those used in the above studies are required. Since the 1970s, considerable progress in understanding surface micrometeorology has been achieved through theoretical work and ob- servations from ﬁeld experiments. The results of these studies have been incorporated in simple models of the biosphere which have then been coupled to general circulation models (GCM) of the Earth’s atmosphere (Dickinson et al. 1986; Sellers et al. 1986). These models are more physically and biologically realistic than the preexisting land surface parameterizations used in GCMs. Using these models, some experiments have been carried out to investigate Corresponding author address: Dr. Yong-Kang Xue, Center for Ocean-l.and—Atmosphere Interactions, 2213 Comp. & Space Science Bldg. , College Park, MD 20742-2425. © 1991 American Meteorological Society Amazon deforestation (Dickinson and Henderson- Sellers 1988; Shukla et al. 1990), and African deser- tiﬁcation (Xue et al. 1990). These studies show that the vegetation canopy is an important factor for de- termining the surface energy budget. Some conclusions, for example, the surface warming due to deforestation or desertiﬁcation, were not found using less realistic models. The Simple Biosphere Model (SiB) of Sellers et al. (1986) is intended to realistically simulate the con- trolling biophysical processes. It includes three soil lay- ers and two vegetation layers. The model attempts to provide a more accurate diumally varying description of the surface energy partition into sensible heat and latent heat. Vegetation canopy processes include the resistances to evapotranspiration and heat ﬂux, and the effect on the interception loss. The simple biosphere is linked to the GCM atmo- sphere through ﬂuxes of radiation, sensible and latent heat, and momentum. By comparison with most of other GCMs, SiB is more complex in the treatment of the surface albedo, surface energy and soil moisture and requires many more input parameters. The spec- iﬁcation and calibration of these parameters and the sensitivity of the calculated surface energy balance to these have been presented in Sellers and Dorrnan ( 1987) and in Sellers et al. ( 1989), respectively. It was found that SiB produced generally good simulations of the observed time series of latent, sensible and