Model: 5Scale

Description

A geometric-optical forest canopy model named "5-Scale" (Leblanc and Chen) was developed for LAI and FPAR algorithms refinement and for parameterizing radiative transfer sub-model in BEPS. 5-Scale is a combination of the "4-Scale" (Chen and Leblanc, 1997) and LIBERTY (Dawson et al., 1999) models.

What is a radiative transfer model?

A radiative transfer model simulates radiation transfer processes in certain media, such as vegetation and atmosphere. For vegetation, it computes the interaction between solar radiation and plants. Solar radiation reflected from the Earth's surfaces and measured by satellites depends strongly on the angles of the sun and the satellite in relation to the surface.

This bi-directional behaviour is quantified using the Bi-directional Reflectance Distribution Function (BRDF).

5-Scale is a radiative transfer model that simulates the BRDF according to structural and biochemical properties of the vegetation.

1. backward scattering, where the sun and the viewer are on the same side, hiding most of the shadows;
2. nadir view, where a maximum of the background can be seen; and
3. forward scattering where the sun and the viewer are on the opposite side.

Why our model is named "5-Scale"?

In 5-Scale, the solar radiation interaction with the foliage is considered at five different scales: within groups of trees, within individual crowns, within branches, within shoots, and within needles (leaves for deciduous).

1. 1-scale: turbid media;
2. 2-scale: randomly distributed discrete objects containing turbid media;
3. 3-scale: non-random discrete objects containing turbid media; and
4. 4-scale: non-random discrete objects with internal structures (such as branches and shoots).
5. 5-scale: non-random discrete objects with internal structures (such as branches and shoots or leaves) and biochemical properties used to simulate foliage transittance and reflectivity.

5-Scale was developed with emphasis on the structural composition of forest canopies at these scales and is a step forward from existing 2-scale geometrical-optical models. It employs the following modeling methodologies:

• The non-random spatial distribution of trees is simulated using the Neyman type A distribution (Neyman, 1939) that creates patches of a forest stand. The model simulates tree crowns as discrete geometrical objects: cone and cylinder for conifers, spheroid for deciduous species. The size of the crowns decreases when the trees are found in large clusters, and the tree locations are also subject to the repulsion effects to better represent the competition for light.

• Inside the crowns, a branch architecture defined by a single inclination angle is included to improve the calculation of light penetration from the geometric-optical model of Li and Strahler (1992) with the assumption of random leaf distribution inside tree crowns. A branch is in turn composed of foliage elements (individual leaves in deciduous and shoots in conifer canopies) with a given angle distribution pattern.

• The hotspot, where the view zenith and solar zenith angles coincide, is computed both on the ground and for the foliage with gap size distributions between and inside the crowns, respectively.

• The crown is treated as a complex medium where shadowed foliage can be observed on the sunlit side, and sunlit foliage can be seen from the shaded side.

• A multiple scattering scheme, based on view factors, is used to compute the amount of light reaching the shaded foliage and background.

• The LIBERTY model is used to simulate the foliage reflectivity and transmittance

Reference