A Coupled Leaf/Canopy Turbid Medium Radiative Transfer Model

Barry Ganapol
University of Arizona

Applications of radiative transfer are commonly concerned with participating scattering and absorbing media that are geometrically well defined and have well characterized physical properties. This is not the case for radiative transfer describing reflection from a vegetation canopy in response to passive sunlight. Here one is faced with a medium under continuous morphological transition resulting from seasonal and interannual variation, biogenic and anthropologic stresses as well as the ubiquitous biodiversity of nature. Canopy reflectance modeling is a necessary component of scientific investigation that establishes diagnostic links between investigative conjecture and spectral data collected in the laboratory and field or through remote sensing from air-born or satellite detectors. Various approaches have been proposed to model canopy reflectance. Turbid canopy reflectance models, such as SAIL and THREEVEG, and leaf radiative transfer models like PROSPECT and LIBERTY, have been developed to address fundamental issues in remote sensing and ecology. In this presentation, the coupling of a within-leaf radiative transfer model (called LEAFMOD) to a canopy radiative transfer model (called CANMOD) will be described for estimating biochemical content of vegetation through spectral profiles of reflected light or for detecting background spectral signatures. The governing philosophy in the development of the coupled leaf/canopy radiative transfer model, called LCM2, is that nature averages over our ignorance of the exact geometrical canopy configuration and leaf structure and that “simple” is most appropriate here. Features of the model include a consistent radiative transfer characterization of photon scattering within a homogeneous leaf, which then directly couples leaf chemical information to the canopy reflectance. With an average leaf thickness, specific absorptivities, a scattering profile and concentrations for the major biochemical constituents, LEAFMOD (in the forward mode) provides leaf hemispherical reflectances and transmittances and the directional distribution of the radiance exiting the leaf surfaces. In this way, a leaf scattering characterization (or leaf phase function) can be constructed that will serve as input to a vegetation canopy model. This presentation will concentrate on the transport methods used in the LCM2 model. In particular, a new 1D/SN algorithm based on Romberg acceleration convergence of the inner iteration will be presented. In addition, the inclusion of linear polarization in an FN solution format will be discussed.

Presentation (PowerPoint File)

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