UPDATED THERMAL-MODEL USING SIMPLIFIED SHORT-WAVE RADIOSITY CALCULATIONS

An extension to a forest canopy thermal radiance model is described th at computes the short-wave energy flux absorbed within the canopy by s olving simplified radiosity equations describing flux transfers betwee n canopy ensemble classes partitioned by vegetation layer and leaf slo pe. Integrated short-wave reflectance and transmittance factors obtain ed from measured leaf optical properties were found to be nearly equal for the canopy studied. Short-wave view factor matrices were approxim ated by combining the average leaf scattering coefficient with the lon g-wave view factor matrices already incorporated in the model. Both th e updated and original models were evaluated for a dense spruce-fir fo rest study site in Central Maine. Canopy short-wave absorption coeffic ients estimated from detailed Monte Carlo ray tracing calculations wer e 0.60, 0.04, and 0.03 for the top, middle, and lower canopy layers co rresponding to leaf area indices of 4.0, 1.05, and 0.25. The simplifie d radiosity technique yielded analogous absorption values of 0.55, 0.0 3, and 0.01. The resulting root mean square error in modeled versus me asured canopy temperatures for all layers was less than 1-degrees-C wi th either technique. Maximum error in predicted temperature using the simplified radiosity technique was approximately 2-degrees-C during pe ak solar heating.