Effect of three-dimensional canopy architecture on thermal infrared exitance

We present a theoretical study of the effects of three- dimensional canopy structure on directional thermal infrared exitance. A physics-based model employing steady-state energy budget formula- tions is used to compute scene element temperatures. Two approaches are then used to combine soil and vegetation contributions to the com- posite scene response. One method uses a plane-parallel abstraction of canopy architecture to estimate canopy view factors for weighting of soil and vegetation emission terms. The second approach employs computer graphics and rendering techniques to estimate 3-D canopy view factors and scene shadows. Both approaches are applied to a test agricultural scene and compared with available measurements. The models cor- rectly estimate hemispherically averaged thermal infrared exitance to within experimental error with root-mean-square errors of 15.3 W m 22 for the 1-D model and 12.5 W m 22 for the 3-D model. However, the 1-D model systematically underestimates exitance at high sun angles. Ex- plicit modeling of canopy 3-D row structure indicates potential directional anisotropy in brightness temperature of up to 14°C.