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 acquire d during the boreal ecosystem-atmosphere study (BOREAS) and the four-scale model of Chen and Leblanc [10] are used to investigate radiative transfer i n boreal forest, The four-scale model is based on forest canopy architectur e at different scales. New aspects are incorporated into the model to impro ve 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 i mportant understory, 4) Natural repulsion effect is considered in the tree distribution statisti cs. Ground measurements from BOREAS sites are used as input parameters by t he model to simulate measurements of bidirectional reflectance distribution function (BRDF) from four forest canopies (old black spruce, old aspen, an d 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 fo ur forests. The importance of the branch architecture and the self-shadowin g of the foliage is emphasized.