Validation of the forest radiative transfer model FRT

The components of the forest radiative transfer model FRT developed at Tartu Observatory, Estonia are compared to the results of measurements of forest canopy downward radiance under forest canopy. Measurements were performed with a hemispheric-view imaging CCD-radiometer which was specially designed for this task. A thorough study of metrological properties of the radiometer was carried out and the respective preprocessing algorithms were created. The angular distribution of view probabilities and NIR radiances for tree crowns, trunks and canopy gaps are measured and compared to the model simulations for two coniferous and a broadleaf forests. Generally, the model reproduced the angular courses of component probabilities and NIR radiances rather well, however, in some cases problems with both the absolute levels of radiance and the angular course arose.