Modeling directional forest reflectance with the hybrid type forest reflectance model FRT

- Directional forest reflectance model FRT is compared against airborne BRF data.
- Substantial modifications of the model FRT are introduced.
- The modified model FRT reproduces the actual angular distribution of forest BRF.

The directional multispectral hybrid type forest radiative transfer model FRT by Kuusk and Nilson (2000) was compared using directional reflectance factor of high angular resolution measured over three mature hemiboreal forest stands in Estonia. The model systematically underestimated backward scattering even in the case of the best available input data, especially in coniferous stands. The measured BRF distribution could be reproduced if the spherical leaf angle distribution was replaced by erectophile in order to better model the angular dependence of gap fraction, and the Ross-Nilson area scattering phase function was replaced by significantly more asymmetric Henyey-Greenstein phase function. The limited amount of available data does not allow deciding how universal are the applied asymmetry parameter values. The asymmetry of the phase function may depend on the species composition, stand age, and site type.