Geometric Optical Radiative Transfer Model Research Articles

Modeling photosynthesis of discontinuous plant canopies by linking the Geometric Optical Radiative Transfer model with biochemical processes

Abstract. Modeling vegetation photosynthesis is essential for understanding carbon exchanges between terrestrial ecosystems and the atmosphere. The radiative transfer process within plant canopies is one of the key drivers that regulate canopy photosynthesis. Most vegetation cover consists of discrete plant crowns, of which the physical observation departs from the underlying assumption of a homogenous and uniform medium in classic radiative transfer theory. Here we advance the Geometric Optical Radiative Transfer (GORT) model to simulate photosynthesis activities for discontinuous plant canopies. We separate radiation absorption into two components that are absorbed by sunlit and shaded leaves, and derive analytical solutions by integrating over the canopy layer. To model leaf-level and canopy-level photosynthesis, leaf light absorption is then linked to the biochemical process of gas diffusion through leaf stomata. The canopy gap probability derived from GORT differs from classic radiative transfer theory, especially when the leaf area index is high, due to leaf clumping effects. Tree characteristics such as tree density, crown shape, and canopy length affect leaf clumping and regulate radiation interception. Modeled gross primary production (GPP) for two deciduous forest stands could explain more than 80% of the variance of flux tower measurements at both near hourly and daily timescales. We demonstrate that ambient CO2 concentrations influence daytime vegetation photosynthesis, which needs to be considered in biogeochemical models. The proposed model is complementary to classic radiative transfer theory and shows promise in modeling the radiative transfer process and photosynthetic activities over discontinuous forest canopies.

Modeling the View Angle Dependence of Gap Fractions in Forest Canopies: Implications for Mapping Fractional Snow Cover Using Optical Remote Sensing

Abstract Forest canopies influence the proportion of the land surface that is visible from above, or the viewable gap fraction (VGF). The VGF limits the amount of information available in satellite data about the land surface, such as snow cover in forests. Efforts to recover fractional snow cover from the spectral mixture analysis model Moderate Resolution Imaging Spectroradiometer (MODIS) snow-covered area and grain size (MODSCAG) indicate the importance of view angle effects in forested landscapes. The VGF can be estimated using both hemispherical photos and forest canopy models. For a set of stands in the Cold Land Field Processes Experiment (CLPX) sites in the Fraser Experimental Forest in Colorado, the convergence of both measurements and models of the VGF as a function of view angle supports the idea that VGF can be characterized as a function of forest properties. A simple geometric optical (GO) model that includes only between-crown gaps can capture the basic shape of the VGF as a function of view zenith angle. However, the GO model tends to underestimate the VGF compared with estimates derived from hemispherical photos, particularly at high view angles. The use of a more complicated geometric optical–radiative transfer (GORT) model generally improves estimates of the VGF by taking into account within-crown gaps. Small footprint airborne lidar data are useful for mapping forest cover and height, which makes the parameterization of the GORT model possible over a landscape. Better knowledge of the angular distribution of gaps in forest canopies holds promise for improving remote sensing of snow cover fraction.

Mapping tree and shrub leaf area indices in an ombrotrophic peatland through multiple endmember spectral unmixing

Leaf area index (LAI) is an important parameter used by most process-oriented ecosystem models. LAI of forest ecosystems has routinely been mapped using spectral vegetation indices (SVI) derived from remote sensing imagery. The application of SVI-based approaches to map LAI in peatlands presents a challenge, mainly due to peatlands characteristic multi-layer canopy comprising shrubs and open, discontinuous tree canopies underlain by a continuous ground cover of different moss species, which reduces the greenness contrast between the canopy and the background. Our goal is to develop a methodology to map tree and shrub LAI in peatlands and similar ecosystems based on multiple endmember spectral mixture analysis (MESMA). This new mapping method is validated using LAI field measurements from a precipitation-fed (ombrotrophic) peatland near Ottawa, Ontario, Canada. We demonstrate first that three commonly applied SVI are not suitable for tree and shrub LAI mapping in ombrotrophic peatlands. Secondly, we demonstrate for a three-endmember model the limitations of traditional linear spectral mixture analysis (SMA) due to the unique and widely varying spectral characteristics of Sphagnum mosses, which are significantly different from vascular plants. Next, by using a geometric-optical radiative transfer model, we determine the nature of the equation describing the empirical relationship between shadow fraction and tree LAI using nonlinear ordinary least square (OLS) regression. We then apply this equation to describe the empirical relationships between shadow and shrub fractions obtained from mixture decomposition with SMA and MESMA, respectively, and tree and shrub LAI, respectively. Less accurate fractions obtained from SMA result in weaker relationships between shadow fraction and tree LAI ( R 2 = 0.61) and shrub fraction and shrub LAI ( R 2 = 0.49) compared to the same relationships based on fractions obtained from MESMA with R 2 = 0.75 and R 2 = 0.68, respectively. Cross-validation of tree LAI ( R 2 = 0.74; RMSE = 0.48) and shrub LAI ( R 2 = 0.68; RMSE = 0.42) maps using fractions from MESMA shows the suitability of this approach for mapping tree and shrub LAI in ombrotrophic peatlands. The ability to account for a spectrally varying, unique Sphagnum moss ground cover during mixture decomposition and a two layer canopy is particularly important.

The effect of viewing geometry and topography on viewable gap fractions through forest canopies

AbstractThe fraction of the surface under forest canopies that is visible from above, or the viewable gap fraction (VGF), influences a number of significant physical processes, such as the longwave radiation budget of the surface and the magnitude of diffuse irradiance. In addition, it has significant implications for the remote sensing of the surface. The VGF is dependent on canopy structure, topography and viewing geometry. Although it is difficult to map VGF using current operational remote sensing systems, it is possible to estimate VGF using models based on the three‐dimensional structure of forest canopies.Results from hemispheric photographs taken in the field at Fraser Experimental Forest, Colorado, and a geometric optical (GO) model show a trend of rapid decrease in VGF as the view zenith angles diverges from nadir. Whereas there is general agreement between model estimates and the hemispheric photographs, the hemispheric photographs generally show higher VGF values for all view zenith angles. In particular, the higher values for VGF are apparent at high view zenith angles. Use of a more complicated GO radiative transfer model would add the effect of within‐crown gaps to those modelled by the GO model and will be used in future studies.VGF maps estimated using the GO model for the Fool Creek intensive study area show a significant decrease in VGF when view zenith angle is increased from 0° (nadir) to 30° viewing from the east. To produce VGF maps in mountain areas, the effect of topography must be taken into account, as changes in slope angle and azimuth are similar to changes in the view zenith angle. Hence, topography can serve either to accentuate or to minimize view zenith angle effects, depending on the slope orientation relative to the viewing position. Copyright © 2004 John Wiley & Sons, Ltd.

Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models

Net solar radiation (Q⋆) and the partitioning of Q⋆ between vegetation (Q⋆veg) and the substrate (Q⋆soil) are important quantities computed by soil‐vegetation‐atmosphere‐transfer (SVAT) models. Commonly, two‐stream models of canopy radiative transfer are used for this purpose. We examine the validity of this approach for nonuniform canopies and compare estimates of Q⋆, Q⋆veg, and Q⋆soil computed using a two‐stream model with estimates computed using a model that accounts for the effects of three‐dimensional (3‐D) structure in vegetation on radiative transfer. To accomplish these goals, a sensitivity analysis is conducted for the key input parameters to two‐stream canopy radiative transfer models. The parameters examined include leaf area index (L), leaf optical properties, solar zenith angle, leaf orientation, and background albedo. Sensitivity analyses are also conducted using a Geometric‐Optical Radiative Transfer (GORT) model. The GORT model treats vegetation canopies as being composed of 3‐D crowns and allows for both vertical variation in leaf area and horizontal variation in stem density. Results show that Q⋆ computed by the two‐stream model varies by up to 10% relative to the GORT model. Further, the partitioning of Q⋆ between the vegetation canopy and substrate computed by the two‐stream model can vary by up to 30% relative to the more realistic GORT model, even for relatively dense canopies (L = 6). These differences arise because two‐stream models use parameterizations for gap probabilities that are not realistic in discontinuous canopies. Based on this conclusion, a parameterization is proposed to include the effects of canopy heterogeneity in two‐stream models. Results from the original two‐stream model, a parameterized two‐stream model, and the GORT model are compared using data from the Boreal Ecosystem‐Atmosphere Study. These results show that the proposed parameterization captures the effect of 3‐D structure in vegetation on radiation regimes and is therefore suitable for inclusion in SVAT models. Inclusion of improved treatment for radiative fluxes in land surface parameterizations should improve modeled estimates for other flux quantities computed by SVAT models.