Modeling radiative transfer in heterogeneous 3-D vegetation canopies

Abstract

The DART (discrete anisotropic radiative transfer) model simulates radiative transfer in heterogeneous 3-D scenes that may comprise different landscape features; i.e., leaves, grass, trunks, water, soil. The scene is divided into a rectangular cell matrix, i.e., building block for simulating larger scenes. Cells are parallelipipedic. Their optical properties are represented by individual scattering phase functions that are directly input into the model or are computed with optical and structural characteristics of elements within the cell. Radiation scattering and propagation are simulated with the exact kernel and discrete ordinate approaches; any set of discrete direction can be selected. In addition to topography and hot spot, leaf specular and first-order polarization mechanisms are modeled. Two major iterative steps are distinguished: 1) Cell illumination with direct sun radiation: Within cell multiple scattering is accurately simulated. 2) Interception and scattering of previously scattered radiation: Atmospheric radiation, possibly anisotropic, is input at this stage. Multiple scattering is stored as spherical harmonics expansions, for reducing computer memory constraints. The model iterates on step 2, for all cells, and stops with the energetic equilibrium. Two simple accelerating techniques can be used: 1) Gauss Seidel method, i.e., simulation of scattering with radiation already scattered at the iteration stage, and (2) decrease of the spherical harmonics expansion order with the iteration order. Moreover, convergence towards the energetic equilibrium is accelerated with an exponential fitting technique. This model predicts the bidirectional reflectance distribution function of 3-19 canopies. Radiation components associated with leaf volume and surface mechanisms are distin

Does the azimuth orientation of Norway spruce (Picea abies/L./Karst.) branches within sunlit crown part influence the heterogeneity of biochemical, structural and spectral characteristics of needles?

“…Canopy level radiative transfer (RT) models (e.g. Discrete Anisotropic Radiative Transfer; Gastellu-Etchegorry et al, 1996 are often parameterized by leaf or shoot measurements. This study's results propose that spruce needle parameters acquired from any azimuth orientation are representative for the whole forest stand canopy.…”

Emulation of Leaf, Canopy and Atmosphere Radiative Transfer Models for Fast Global Sensitivity Analysis

“…These type of RTMs enable the generation of complex or detailed scenes, but at the expense of a tedious computational load. In short, the following families of RTMs can be considered as non-economically invertible: (1) Monte Carlo ray tracing models (e.g., Raytran [18], FLIGHT [19] and Drat [20]); (2) voxel-based models (e.g., DART [21]) and (3) advanced integrated vegetation and atmospheric transfer models (e.g., SimSphere [22], SCOPE [23] and MODTRAN [24]). Although these advanced models serve perfectly as virtual laboratories for fundamental research on light-vegetation and atmosphere interactions (see e.g., [18,25]), their high computational cost make them impractical for applications such as inversion.…”

Investigating the Utility of Wavelet Transforms for Inverting a 3-D Radiative Transfer Model Using Hyperspectral Data to Retrieve Forest LAI

“…However, heterogeneous plant stands such as forests may contain partial cover and generally exhibit horizontal variability in their structural and optical properties, and are thus poorly represented by 1-D models. The more realistic description of reflected radiation from a forest canopy can be provided by 3-D models [11,[15][16][17][18]. The models are referred to as 3-D because the extinction and scattering coefficients that define photon interactions are explicit functions of the spatial coordinates [18].…”

“…The parameter combination that yields the closest spectrum in the database is considered to be the inversion solution [26]. The major advantage of the LUT-based approaches is that the forward modeling is divorced from the inversion procedure, and hence can be used for inversion of any complex model like DART [15,27].…”