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Closed forest canopies


This set of experiments was suggested in order to compute the radiative flux properties, i.e., the canopy Reflectance (R), Absorption (A), and Transmission (T), of structurally homogeneous leaf canopies. The overall architecture of these test cases is reminiscent of closed deciduous forest canopies (were the foliage is assumed to be oriented in a uniform manner (equiprobable) along all directions). A total of 72 test cases are proposed. Various canopy density, soil brightness and illumination conditions are given both for the visible (VIS) (400-700nm) and near infra-red (NIR) (700-3000nm) spectral ranges. Whether you are using Look-Up-Tables, analytical radiative transfer formulations, or some other form of parameterisation to prescribe/simulate the radiative flux values for these test cases, please stick as closely as possible to the structural, spectral and illumination conditions provided below (in some look-up-table based approaches this may mean that the same estimates will be submitted for different test cases). Since all required RAMI4PILPS measurements consist of flux ratios, the actual magnitude of the incident irradiance is irrelevant.

RAMI4PILPS represents the 'closed forest canopies' as essentially infinite foliage layers (1D canopies) that are composed of a 'cloud' of dimension-less but oriented scatterers (foliage) located over a horizontal plane standing for the underlying background surface. In order to avoid hotspot-related issues - i.e., the influence of enhanced backscattering along the retro-reflection direction - it is strongly recommended to treat the foliage as a turbid medium (i.e., made up of infinitesimally small oriented scatterers) in any radiative transfer models/modules. This foliage is distributed uniformly between the background level and the top of the canopy at a height of 15m. The foliage consists of non-dimensional scatterers characterized by

  1. their specified radiative properties (reflectance, transmittance and the directionality of the scattering events), and
  2. the orientation of the normals of the scatterers which adhere to a uniform distribution function (i.e., the probability of intersecting a scatterer is the same for all directions of travel - see definitions).

A simple Lambertian scattering law describes the directionality of all scattering events (irrespective of whether they occur with the foliage elements or with the background surface. Apart from the completely absorbing (black soil) scenario, the particular values selected for these input variables represented typical plant and soil conditions within the visible and near-infrared spectral domains.

The following graphical representation aims at giving an idea of how such scenes may look like:

Overall three different foliage densities are proposed: sparse (LAI=1), medium (LAI=2) and dense (LAI=4). For each of these structural scenarios, three different soil brightnesses (black, medium ,snow) and four illumination conditions (direct illumination only at 27, 60 and 83 degree zenith, as well as isotropic diffuse illumination only) are specified both for the visible and near-infrared spectral domains. The leaf spectral properties, orientation, size and scattering properties remain identical for each one of the spectral domains. This should facilitate the batch processing of all the proposed 'closed forest canopies' test cases with a shell script using a series of for loops, for example, as follows:

Scene graphical representation
Scene graphical representation

for LAI in LAI_LIST do
for BAND in BAND_LIST do
for SOIL in SOIL_LIST do
endfor (SOIL)
endfor (BAND)
endfor (LAI)

where LAI_LIST stands for the canopy structural properties (LAI=1, 2 and 4), BAND_LIST stands for the visible (VIS) or near-infrared (NIR) spectral domain (which in turn determine the leaf spectral properties in these domains), SOIL_LIST determines the (black, medium, snow) brightness values of the background in each one of the spectral domains, and ILLUMINATION_LIST prescribes the illumination conditions (SZA=27, 60 and 83 degrees, and diffuse isotropic). Last but not least STRUCTURE relates to the structural properties of the canopy (e.g., canopy height, leaf orientation, leaf scattering law, etc.) and remains the same for all 72 test cases.

The following table provides an overview of the variables that change for the various test cases proposed in the closed forest canopy category. Individual experiment identifier tags are used to label these test cases e.g., CFC100_MED_ERE_VIS_27. They are needed in the naming of the various measurement results files (see file naming and formatting conventions). You may use the links provided in the table to directly access the detailed descriptions of certain sets of test cases.