Analytical land atmosphere radiometer model (ALARM) applied to a dense canopy

Directional radiometric surface temperature (θr), obtained from remote sensing, can be useful in estimating the energy fluxes from the land surface, especially the sensible heat flux into the atmosphere, H. Since plant canopies are often anisothermal, θr is view-angle dependent, such that the “surface” temperature needed to estimate H may be different from a measured θr. This study used the analytical land atmosphere radiometer model (ALARM) to target such a problem on a relatively dense canopy (leaf area index of 4.0), and to compare ALARM performance with models developed by Massman and by Lhomme et al. Data from the Southern Great Plains 1997 (SGP97) experiment were chosen for this study. The root mean square difference (RMSD) between measured and simulated H at the 0° view angle was smaller for ALARM (46.9 W/m2), than for the Lhomme (53.0 W/m2) and Massman (70.9 W/m2) models. At the 45° view angle, RMSDs for the ALARM (59.1 W/m2) and Lhomme (58.6 W/m2) models were somewhat smaller than for the Massman (85.4 W/m2) model. All three models did better at a view angle of 0° than at one of 45°. In conclusion, ALARM used surface temperatures at view angles of 0 and 45° to estimate H for a dense canopy, with errors comparable to or smaller than other available models.