Piers J. Sellers

Canopy reflectance, photosynthesis and transpiration

Abstract A two-stream approximation model of radiative transfer is used to calculate values of hemispheric canopy reflectance in the visible and near-infrared wavelength intervals. Simple leaf models of photosynthesis and stomatal resistance are integrated over leaf orientation and canopy depth to obtain estimates of canopy photosynthesis and bulk stomatal or canopy resistance. The ratio of near-infrared and visible reflectances is predicted to be a near linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass.

Satellite remote sensing of primary production

Abstract Leaf structure and function are shown to result in distinctive variations in the absorption and reflection of solar radiation from plant canopies. The leaf properties that determine the radiation-interception characteristics of plant canopies are directly linked to photosynthesis, stomatal resistance and evapotran-spiration and can be inferred from measurements of reflected solar energy. The effects of off-nadir viewing and atmospheric constituents, coupled with the need to measure changing surface conditions, emphasize the need for multitemporal measurements of reflected radiation if primary production is to be estimated

The interpretation of spectral vegetation indexes

Empirical studies report several plausible correlations between transforms of spectral reflectance, called vegetation indexes, and parameters descriptive of vegetation leaf area, biomass and physiological functioning. However, most indexes can be generalized to show a derivative of surface reflectance with respect to wavelength. This derivative is a function of the optical properties of leaves and soil particles. In the case of optically dense vegetation, the spectral derivative, and thus the indexes, can be rigorously shown to be indicative of the abundance and activity of the absorbers in the leaves. Therefore, the widely used broad-band &near-infrared vegetation indexes are a measure of chlorophyll abundance and energy absorption.

A Revised Land Surface Parameterization (SiB2) for Atmospheric GCMS. Part II: The Generation of Global Fields of Terrestrial Biophysical Parameters from Satellite Data

Abstract The global parameter fields used in the revised Simple Biosphere Model (SiB2) of Sellers et al. are reviewed. The most important innovation over the earlier SiB1 parameter set of Dorman and Sellers is the use of satellite data to specify the time-varying phonological properties of FPAR, leaf area index. and canopy greenness fraction. This was done by processing a monthly 1° by 1° normalized difference vegetation index (NDVI) dataset obtained farm Advanced Very High Resolution Radiometer red and near-infrared data. Corrections were applied to the source NDVI dataset to account for (i) obvious anomalies in the data time series, (ii) the effect of variations in solar zenith angle, (iii) data dropouts in cold regions where a temperature threshold procedure designed to screen for clouds also eliminated cold land surface points, and (iv) persistent cloud cover in the Tropics. An outline of the procedures for calculating the land surface parameters from the corrected NDVI dataset is given, and a brief d...

Canopy reflectance, photosynthesis, and transpiration. III - A reanalysis using improved leaf models and a new canopy integration scheme

Abstract The theoretical analyses of Sellers (1985; 1987), which linked canopy spectral reflectance properties to (unstressed) photosynthetic rates and conductances, are critically reviewed and significant shortcomings are identified. These are addressed in this article principally through the incorporation of a more sophisticated and realistic treatment of leaf physiological processes within a new canopy integration scheme. It is assumed, based on ecophysiological observations and arguments, that leaf physiological properties vary throughout the plant canopy in response to the radiation-weighted time-mean profile of photosynthetically active radiation (PAR). These modifications yield a simpler and more robust theoretical relationship between canopy biophysical rates (photosynthesis, conductance) and spectral vegetation indices (SVI). The results indicate that area-averaged SVI, as obtained from coarse resolution satellite sensors, may give good estimates of the area-integrals of photosynthesis and conductance even for spatially heterogenous (though physiologically uniform) vegetation covers.

Amazonian Deforestation and Regional Climate Change

Abstract Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspirat...

Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview

The Multi-angle Imaging SpectroRadiometer (MISR) instrument is scheduled for launch aboard the first of the Earth Observing System (EOS) spacecraft, EOS-AM1. MISR will provide global, radiometrically calibrated, georectified, and spatially coregistered imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. Algorithms specifically developed to capitalize on this measurement strategy will be used to retrieve geophysical products for studies of clouds, aerosols, and surface radiation. This paper provides an overview of the as-built instrument characteristics and the application of MISR to remote sensing of the Earth.

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.

The Boreal Ecosystem–Atmosphere Study (BOREAS): An Overview and Early Results from the 1994 Field Year

Abstract The Boreal Ecosystem Atmosphere Study (BOREAS) is large-scale international field experiment that has the goal of improving our understanding of the exchanges of radiative energy, heat water, CO2, and trace gases between the boreal forest and the lower atmosphere. An important objective of BORES is collect the data needed to improve computer simulation models of the processes controlling these exchanges so that scientists can anticipate the effects of global change. From August 1993 through September 1994, a continuous set of monitoring measurements—meteorology, hydrology, and satellite remote sensing—were gathered over the 1000 × 1000 km BOREAS study region that covers most of Saskatchewan and Manitoba, Canada. This monitoring program was punctuated by six campaigns that saw the deployment of some 300 scientists and aircrew into the field, supported by 11 research aircraft. The participants were drawn primarily from U.S. and Canadian agencies and universities, although there were also important ...

BOREAS in 1997: Experiment overview, scientific results, and future directions

The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.