P. J. Sellers

A Revised Land Surface Parameterization (SiB2) for Atmospheric GCMS. Part I: Model Formulation

Abstract The formulation of a revised land surface parameterization for use within atmospheric general circulation models (GCMs) is presented. The model (SiB2) incorporates several significant improvements over the first version of the Simple Biosphere model (SiB) described in Sellers et al. The improvements can be summarized as follows: (i) incorporation of a realistic canopy photosynthesis–conductance model to describe the simultaneous transfer of CO2 and water vapor into and out of the vegetation, respectively; (ii) use of satellite data, as described in a companion paper, Part II, to describe the vegetation phonology; (iii) modification of the hydrological submodel to give better descriptions of baseflows and a more reliable calculation of interlayer exchanges within the soil profile; (iv) incorporation of a “patchy” snowmelt treatment, which prevents rapid thermal and surface reflectance transitions when the area-averaged snow cover is low and decreasing. To accommodate the changes in (i) and (ii) ab...

Carbon 13 exchanges between the atmosphere and biosphere

We present a detailed investigation of the gross 12C and 13C exchanges between the atmosphere and biosphere and their influence on the δ13C variations in the atmosphere. The photosynthetic discrimination Δ against 13C is derived from a biophysical model coupled to a general circulation model [Sellers et al., 1996a], where stomatal conductance and carbon assimilation are determined simultaneously with the ambient climate. The δ13C of the respired carbon is calculated by a biogeochemical model [Potter et al., 1993; Randerson et al., 1996] as the sum of the contributions from compartments with varying ages. The global flux-weighted mean photosynthetic discrimination is 12–16‰, which is lower than previous estimates. Factors that lower the discrimination are reduced stomatal conductance and C4 photosynthesis. The decreasing atmospheric δ13C causes an isotopic disequilibrium between the outgoing and incoming fluxes; the disequilibrium is ∼0.33‰ for 1988. The disequilibrium is higher than previous estimates because it accounts for the lifetime of trees and for the ages rather than turnover times of the biospheric pools. The atmospheric δ13C signature resulting from the biospheric fluxes is investigated using a three-dimensional atmospheric tracer model. The isotopic disequilibrium alone produces a hemispheric difference of ∼0.02‰ in atmospheric δ13C, comparable to the signal from a hypothetical carbon sink of 0.5 Gt C yr−1 into the midlatitude northern hemisphere biosphere. However, the rectifier effect, due to the seasonal covariation of CO2 fluxes and height of the atmospheric boundary layer, yields a background δ13C gradient of the opposite sign. These effects nearly cancel thus favoring a stronger net biospheric uptake than without the background CO2 gradient. Our analysis of the globally averaged carbon budget for the decade of the 1980s indicates that the biospheric uptake of fossil fuel CO2 is likely to be greater than the oceanic uptake; the relative proportions of the sinks cannot be uniquely determined using 12C and 13C alone. The land-ocean sink partitioning requires, in addition, information about the land use source, isotopic disequilibrium associated with gross oceanic exchanges, as well as the fractions of C3 and C4 vegetation involved in the biospheric uptake.

A mechanism for the influence of vegetation on the response of the diurnal temperature range to changing climate

We propose a new mechanism that could contribute to The highly interactive nature of general circulation models the observed decrease in the diurnal temperature range (DTR) (GCMs) as well as weaknesses in their parameterizations make it over the last century: the physiological behavior of vegetation in difficult to identify the mechanisms underlying predicted changes response to climate. Using a physiologically based land surface in the DTR. The purpose of this study is to examine how the model, we analyze the influence of vegetation on the response of diurnal temperature cycle of vegetated land surfaces responds to the D TR to perturbations in the state of the climate and changes in external forcing and the biophysical state of the vegetation. Increasing down-welling long wave radiation and vegetation. We use the SiB2 land surface model (Sellers et al., surface air temperature together, conditions that could occur as a 1996a) in an off-line mode with prescribed meteorology for a result of doubling of atmospheric CO2, produced little change in number of scenarios highlighting the impact of vegetation on the DTR. Changes in the state of the vegetation (i.e. amount, DTR. Off-line simulations do not account for feedback between physiological capacity, stress) produce changes in the DTR of the order or larger than observed. Results emphasize that DTR modeling studies need to consider vegetation responses and suggest that recently reported increases in vegetation over the last decade could contribute to the observed decreases in the DTR.

Interactions between Vegetation and Climate: Radiative and Physiological Effects of Doubled Atmospheric CO2

Abstract The radiative and physiological effects of doubled atmospheric carbon dioxide (CO2) on climate are investigated using a coupled biosphere–atmosphere model. Five 30-yr climate simulations, designed to assess the radiative and physiological effects of doubled CO2, were compared to a 30-yr control run. When the CO2 concentration was doubled for the vegetation physiological calculations only assuming no changes in vegetation biochemistry, the mean temperature increase over land was rather small (0.3 K) and was associated with a slight decrease in precipitation (−0.3%). In a second case, the vegetation was assumed to have adapted its biochemistry to a doubled CO2 (2 × CO2) atmosphere and this down regulation caused a 35% decrease in stomatal conductance and a 0.7-K increase in land surface temperature. The response of the terrestrial biosphere to radiative forcing alone—that is, a conventional greenhouse warming effect—revealed important interactions between the climate and the vegetation. Although th...

Modeling of Energy, Water, and CO2 Flux in a Temperate Grassland Ecosystem with SiB2: May–October 1987

Abstract The Simple Biosphere Model, version 2 (SiB2), was designed for use within atmospheric general circulation models as a soil–vegetation–atmosphere transfer scheme that includes CO2 flux prediction. A stand-alone version of SiB2 was used to simulate a grassland at Station 16 of the First ISLSCP Field Experiment (FIFE) located near Manhattan, Kansas, for a period of 142 days of the 1987 growing season. Modeled values of soil temperature and moisture were initialized, using field measurements from the soil profile, and thereafter updated solely by model calculations. The model was driven by half-hourly atmospheric observations and regular observations of canopy biophysics. This arrangement was intended to mimic model forcing in a GCM. Three model versions are compared: (i) a Control run using parameter values taken from look-up tables used for running the Colorado State University GCM; (ii) a Tuned run with many adjustments to optimize SiB2 to this ecosystem; and (iii) a Calibrated run, which calibrat...