Source approach for estimating soil and vegetation energy fluxes in observations of directional radiometric surface temperature

Abstract

A two-layer model of turbulent exchange that includes the view geometry associated with directional radiometric surface temperature is developed and evaluated by comparison of model predictions with field measurements. Required model inputs are directional brightness temperature and its angle of view, fractional vegetation cover or leaf area index, vegetation height and approximate leaf size, net radiation, and air temperature and wind speed. One advantage of the approach described in this paper is that directional brightness temperatures are considered so that the model should have wider applicability than single-layer models, and it opens the possibility of a simple solution if directional measurements are available from two substantially different view angles. Comparisons with several hundred measurements from two large-scale field experiments were performed. One study was conducted in a semiarid rangeland environment in Southern Arizona (Monsoon '90) while the other was conducted in a subhumid environment, namely the tall grass prairie in Eastern Kansas (FIFE). For the Monsoon '90 site, root-mean-square-differences (RMSD) between model predictions and measurements were between 35 and 60 W m-2 for soil, sensible and latent heat flux. With the FIFE site data RMSD values were between 50 and 60 W m-2. The larger scatter with the FIFE data was mainly caused by the model having difficulty reproducing the fluxes for the observation period with dormant vegetation. Considerations of the expected variability associated with flux measurements over complex surfaces suggests that model-derived fluxes were in acceptable agreement with the observations. However refinements in formulations of soil heat flux probably would improve agreement between model predictions and measurements. © 1995.