G-function and HOt SpoT (GHOST) reflectance model - Application to multi-scale airborne POLDER measurements

The complex architecture of conifer species affects the radiation scatterin g processes that influence the bidirectional reflectance distribution funct ion (BRDF) characteristics. An interesting BRDF signature is the hot spot e ffect that yields a maximum of reflectance in the sun direction. It has bee n shown that the hot spot phenomenon is sensitive to the foliage structure. The present work introduces the G-function and HOt SpoT (GHOST) model that simulates the BRDF of boreal forest in accounting for the arrangement and the structure of the stems. The model represents the medium by a distributi on of conical crowns whose size and shape are accounted for in the G-functi on expression. GHOST compares quite well with airborne POLarization and Dir ectionality of Earth Reflectances (POLDER) reflectances acquired during the BOREAS experiment at various scale resolutions ranging from 2 (helicopter) up to 35 m (aircraft). The inversion procedure exploits the multidirection ality of measurements with emphasis on the hot spot observations and the an gular range of dominant influence of each parameter The retrieved parameter s translate optical and structural properties of the canopy. They are used to calculate a new architectural parameter X that allows to get the clumpin g index of the patch vegetation. (C) 2001 Elsevier Science Inc. All rights reserved.