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P. J. Sellers and J. L. Dorman

Abstract

The Simple Biosphere model (SiB) of Sellers et al. (1986) was designed for use within General Circulation Models (GCMs) of the earth's atmosphere. The main objective of SiB is to provide a biophysically realistic description of those processes which control the transfer of radiation, sensible heat, latent heat and momentum between the terrestrial surface and the atmosphere. As a result, SiB is more complex and has a larger input parameter set than most equivalent formulations used in GCMs. Prior to implementing SiB in a GCM, it is essential that its components and its functioning as a whole, be thoroughly tested. Additionally, it is highly desirable that the model's response to errors or uncertainties in the input parameter set be explored. This paper discusses investigations that were directed at addressing then two issues.

Micrometeorological and biophysical measurements from surface experiments conducted over arable crops in West Germany and the United States and a forested site in the United Kingdom were used to test the operation of SiB. Observed values of the downward radiative fluxes, wind speed, air temperature and water vapor pressure recorded above the surface were used as the boundary forcing for the SiB model. The predicted partitioning of the absorbed radiation into the sensible and latent heat fluxes compares well with observations and the various subcomponents of the model appear to operate realistically. The sensitivity of the model's energy balance calculations to changes in the various model parameters and the soil moisture initialization is examined. It is estimated that the model will generate uncertainties of the order of ±7% in the calculated net radiation, and up to ±25% in the calculated evapotranspiration rate, with typical values of ±15%.

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J. G. Lockwood and P. J. Sellers

Abstract

A multilayer crop model is used to investigate interception loss from oak, pine, wheat and grass canopies. It is shown that the evaporative properties of the full oak canopy are similar to those of the evergreen tropical rain forest. Evaporation from all the wet canopies is shown to be similar at low wind speeds but the loss from the tree canopies increases rapidly with increasing wind speed. In the low-wind-speed equatorial environment it would seem likely that changing vegetation type would cause little difference in interception loss and therefore runoff. Equatorial observations suggest that this is not so and the reasons for this are discussed. Possible hydrometeorological consequences of the deforestation of the Amazon basin are also considered.

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J. L. Dorman and P. J. Sellers

Abstract

Components of the Simple Biosphere Model (SiB) of Sellers et al. were used to generate global monthly fields of surface albedo (0.4–4.0 μm), roughness length and minimum surface (stomatal) resistance.

SiB consists of three submodels which describe the roles of radiative transfer, turbulent transfer and surface resistance in determining the energy balance of the vegetated land surface. These three submodels were detached from SiB and used on the SiB parameter set (total and green leaf area index, leaf angle orientation, canopy dimensions, etc.) to calculate global monthly fields of albedo, roughness length and minimum stomatal resistance at 1° × 1° resolution. Time series of various parameters are also displayed for each vegetation type for specified grid points. The SiB results compare reasonably well with appropriate measurements obtained from the literature and have the additional merit of being mutually consistent; the three submodels use many common parameters, which ensures that, for each grid area, the calculated surface properties are closely interrelated as is the case in nature.

The derived fields provide a check on the operation of the submodels and the correctness of the parameter set. They can also be used as prescribed fields for GCMs that do not have biophysically based land surface parameterizations.

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Y. Xue, P. J. Sellers, J. L. Kinter, and J. Shukla

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 simplifications 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 effects 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 closely. After simplification, the computational requirement of SiB was reduced by about 55%.

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P.J. Sellers, S.I. Rasool, and H.-J. Bolle

Satellite observations are essential for the global monitoring of climatologically significant interactions between the earth's atmosphere and land surface. In practice, however, interpretation of remote-sensing data requires the use of algorithms—specialized, semiempirical relationships that connect observed radiances with the actual physical variables needed for climate studies and modeling. At issue is the physical/empirical basis for these algorithms, their effectiveness and shortcomings, and the scope for further improvement.

The International Satellite Land-Surface Climatology Project (ISLSCP) Satellite Data Algorithms Workshop, conducted 5–8 January 1987 at the Jet Propulsion Laboratory in Pasadena, California, was organized to address these questions. The introduction to this paper describes ISLSCP and presents an overview of the scientific topics covered in the workshop.

Derivation of the following key surface parameters from satellite data are discussed in detail:

Shortcomings of the various algorithms and actions required to alleviate them are discussed.

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D. S. Kimes, P. J. Sellers, and W. W. Newcomb

Abstract

The variations of spectral hemispherical reflectance (albedo) in vegetation canopies were studied as a function of solar zenith angle, leaf area index, led orientation distribution, and leaf and soil optical, properties. A three dimensional radiative transfer model was used to investigate the radiative transfers that give rise to variations in hemispherical relfectance (ρ). The results of this model were compared to those derived using an analytical two-stream approximation model which has the advantages of being simple and robust enough to use in real time applications. The hemispherical reflectance in the visible and near-infrared regions can vary as much as 60% with changes in solar zenith angles from 0° to 77°. This variation generally decreases as the leaf orientation distribution approaches a planophile distribution. The probability of gap function through the canopy and the spectral characteristics of the soil and vegetation are key factors in determining the hemispherical reflectance dynamics. The two models showed similar trends in terms of the variation of ρ with solar zenith angle, leaf area index and wavelength. Ale two-stream approximation model is the favored approach among modelers of land surface processes in climate studies because of its simplicity. In this study the model is shown to provide a reliable and robust tool in estimating hemispherical reflectance.

Many terrestrial energy budget studies require estimates of the daily reflected energy (spectral and total shortwave) of the vegetated surface. Most studies use a hemispherical reflectance value estimated near the solar noon for calculating the daily reflected energy and ignore the diurnal variation of ρ as documented in this paper. The results showed that percent errors as high as 18% can result in using this technique to calculate the daily reflected energy of vegetation canopies. Consequently, some knowledge of the daily ρ variations is required for studies requiring accuracies of daily reflected energy of less than ∼18%.

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P. J. Sellers, Y. Mintz, Y. C. Sud, and A. Dalcher

Abstract

A simple but realistic biosphere model has been developed for calculating the transfer of energy, mass and momentum between the atmosphere and the vegetated surface of the earth. The model is designed for use in atmospheric general circulation models.

The vegetation in each terrestrial model grid area is represented by two distinct layers, either or both of which may be present or absent at any given location and time. The upper vegetation layer represents the perennial canopy of trees or shrubs, while the lower layer represents the annual ground cover of grasses and other herbaceous species. The local coverage of each vegetation layer may be fractional or complete but as the individual vegetation elements are considered to be evenly spaced, their root systems are assumed to extend uniformly throughout the entire grid area. Besides the vegetation morphology, the physical and physiological properties of the vegetation layers are also prescribed. These properties determine (i) the reflection, transmission, absorption and emission of direct and diffuse radiation in the visible, near infrared and thermal wavelength intervals; (ii) the interception of rainfall and its evaporation from the leaf surfaces; (iii) the infiltration, drainage and storage of the residual rainfall in the soil; (iv) the control by the photosynthetically active radiation and the soil moisture potential, inter alia, over the stomatal functioning and thereby over the return transfer of the soil moisture to the atmosphere through the root-stem-leaf system of the vegetation; and (v) the aerodynamic transfer of water vapor, sensible heat and momentum from the vegetation and soil to a reference level within the atmospheric boundary layer.

The Simple Biosphere (SiB) has seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface); two interception water stores (one for the canopy and one for the ground cover); and three soil moisture stores (two of which can be reached by the vegetation root systems and one underlying recharge layer into and out of which moisture is transferred only by hydraulic diffusion and gravitational drainage).

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A. Henderson-Sellers, A. J. Pitman, P. K. Love, P. Irannejad, and T. H. Chen

The World Climate Research Programme Project for Intercomparison of Land Surface Parameterization Schemes (PILPS) is moving into its second and third phases that will exploit observational data and consider the performance of land surface schemes when coupled to their host climate models. The first stage of phase 2 will focus on an attempt to understand the large differences found during phase 1. The first site from which observations will be drawn for phase 2 intercomparisons is Cabauw, the Netherlands (51 °58′N, 4°56′E), selected specifically to try to reduce one of the causes of the divergence among the phase 1 results: the initialization of the deep soil moisture. Cabauw's deep soil is saturated throughout the year. It also offers a quality controlled set of meteorological forcing and 160 days of flux measurements. PILPS phase 2 follows the form of the phase 1 intercomparisons: simple off-line integrations and comparisons, but in phase 2 participating schemes' results will be compared against observed fluxes. Preliminary results indicate that between model variability persists (i) in better specified experiments and (ii) in comparison with data. Although median values are consistent with observations, there is a large range among models. Phase 3, in which the intercomparison of PILPS schemes as a component of global atmospheric circulation models, is being conducted jointly with the Atmospheric Model lntercomparison Project (AMIP) as diagnostic subproject number 12. Preliminary results suggest that results differ by about the same range as in the off-line experiments in phases 1 and 2. Incomplete diagnostics suggest that bucket and canopy models differ and that variability among models can be tracked to the soil moisture parameterization. This paper offers a review of the PILPS project to date and an invitation to participate in PILPS' current and future activities.

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G. D. Colello, C. Grivet, P. J. Sellers, and J. A. Berry

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 calibrated the Control version soil to the local site and incorporated two important changes from the Tuned version. Modeled fluxes of latent heat, sensible heat, soil heat, net radiation, and net site CO2 were compared to over 800 half-hourly observations; modeled surface and deep soil temperatures compared to 6500 observations; and three layers of modeled soil water content compared to 15 measurements of the soil water profile. Statistical methods were used to analyze these results. In the absence of water stress all three versions accurately simulated photosynthesis and canopy conductance. However, during episodes of drought, only the Tuned and Calibrated versions accurately simulated physiological control of canopy fluxes. The largest errors were encountered in the simulation of soil respiration. These were traced to problems predicting water content and temperature in the soil profile. These results highlight the need for improved simulation of soil biophysics to obtain accurate estimates of net CO2 balance. The accuracy of the Tuned version was improved by changes that (i) allowed water extraction by roots from all soil layers, (ii) matched the soil texture specification to the site, and (iii) calibrated the expressions used for diffusion of water and heat within the soil profile.

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M. F. Wilson, A. Henderson-Sellers, R. E. Dickinson, and P. J. Kennedy

Abstract

The soils data of Wilson and Henderson-Sellers have been incorporated into the land-surface parameterization scheme of the NCAR Community Climate Model after Dickinson. A stand-alone version of this land-surface scheme, termed the Biosphere-Atmosphere Transfer Scheme (BATS), has been tested in a series of sensitivity experiments designed to assess the sensitivity of the scheme to the inclusion of variable soil characteristics. The cases investigated were for conditions designed to represent a low-latitude, evergreen forest; a low-latitude sand desert; a high-latitude coniferous forest; high-latitude tundra; and prairie grasslands, each for a specified time of year. The tundra included spring snowmelt and the grassland incorporated snow accumulation. The sensitivity experiments included varying the soil texture from a coarse texture typical of sand through a medium texture typical of loam to a fine texture typical of clay. The sensitivity of the formulation to the specified total and upper soil column depth and the response to altering the parameterization of the soil albedo dependence upon soil wetness and snow-cover were also examined. The biosphere-atmosphere transfer scheme showed the greatest sensitivity to the soil texture variation, particularly to the associated variation in the hydraulic conductivity and diffusivity parameters. There was only a very small response to the change in the soil albedo dependence on wetness and, although the sensitivity to the snow-covered soil albedo via the response to roughness length/snow-masking depth was significant, the results were predictable. Changing the total depth of the active soil column produced a much smaller response than altering the depth of the upper soil layer, primarily because the degree of saturation of the upper layer plays an important role in the parameterized hydrology. Soil moisture responses can also be initiated by changes in vegetation characteristics such as the stomatal resistance through changed canopy interaction which modify the radiation and water budgets of the soil surface. Overall, this land-surface parameterization scheme shows considerable sensitivity to the choice of soil texture. This sensitivity seems to be at least comparable to that involving changes in vegetation characteristics and it may be more important because soil characteristics are very poorly known at a resolution appropriate for global climate models.

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