Jp. Gastelluetchegorry et al., MODELING RADIATIVE-TRANSFER IN HETEROGENEOUS 3-D VEGETATION CANOPIES, Remote sensing of environment, 58(2), 1996, pp. 131-156
The DART (discrete anisotropic radiative transfer) model simulates rad
iative transfer in heterogeneous 3-D scenes that may comprise differen
t landscape features; i.e., leaves, grass, trunks, water, soil. The sc
ene is divided into a rectangular cell matrix, i.e., building block fo
r simulating lar er scenes. Cells are parallelipipedic. Their optical
properties are represented by individual scattering phase functions th
at are directly input into the model or are computed with optical and
structural characteristics of elements within the cell. Radiation scat
tering and propagation are simulated with the exact kernel and discret
e 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 dis
tinguished: 1) Cell illumination with direct sun radiation: Within cel
l multiple scattering is accurately simulated. 2) Interception and sca
ttering of previously scattered radiation: Atmospheric radiation, poss
ibly anisotropic, is input at this stage. Multiple scattering is store
d as spherical harmonics expansions, for reducing computer memory cons
traints. The model iterates on step 2, for all cells, and stops with t
he energetic equilibrium. Two simple accelerating techniques can be us
ed: 1) Gauss Seidel method, i.e., simulation of scattering with radiat
ion 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-D canopies. Radiation componen
ts associated with leaf volume and surface mechanisms are distinguishe
d. It gives also the radiation regime within canopies, for further det
ermination of 3-D photosynthesis rates and primary production. Accurat
e modeling of multiple scattering within cells, combined with the fact
that cells can have different x,y,z dimensions, is well adapted to re
mote sensing based studies, i.e., scenes with large dimensions. The mo
del was successfully tested with homogeneous covers. Preliminary compa
risons of simulated reflectance images with remotely acquired spectral
images of a 3-D heterogeneous forest cover stressed the usefulness of
the DART model for conducting studies with remotely acquired informat
ion. (C) Elsevier Science Inc., 1996.