Investigation of directional reflectance in boreal forests with an improved four-scale model and airborne POLDER data

Airborne Polarization and Directional Earth Radiation (POLDER) data acquired during the boreal ecosystem-atmosphere study (BOREAS) and the four-scale model of Chen and Leblanc (1997) are used to investigate radiative transfer in boreal forest. The four-scale model is based on forest canopy architecture at different scales. New aspects are incorporated into the model to improve the physical representation of each canopy, as follows: 1) Elaborate branch architecture is added. 2) Different crown shapes are used for conifer and deciduous forests. 3) Bilayer version of the model is introduced for forest canopies with an important understory. 4) Natural repulsion effect is considered in the tree distribution statistics. Ground measurements from BOREAS sites are used as input parameters by the model to simulate measurements of bidirectional reflectance distribution function (BRDF) from four forest canopies (old black spruce, old aspen, and old and young jack pine) acquired by the POLDER instrument from May-July 1994. The model is able to reproduce with great accuracy the BRDF of the four forests. The importance of the branch architecture and the self-shadowing of the foliage is emphasized.