Directional radiometric temperature profiles within a grass canopy

The scalar roughness for sensible heat flux, z_oh, which appears in Monin and Obukhov similarity theory has been shown to exhibit substantial variability. Brutsaert and Sugita (cf. W. Brutsaert and M. Sugita, J. Atmos. Sci. 53 (1996) 209-16) derived an expression for z_oh to account for variability in z_oh as a result anisothermal vegetation skin temperature, which assumes an exponential profile of canopy skin temperature. We examined this assumption through directional radiometric measurements of temperature profiles in a relatively uniform grass canopy. The temperature gradient steepened monotonely throughout the morning and was modeled successfully with an exponential profile. The root-mean-square error (RMSE) between measured and modeled profiles was small and ranged between 0.038°C and 0.530°C. One of the most significant findings of this research is that the decay coefficient, b, of the exponential profile model was approximately constant with time for this canopy. This may provide a useful simplification in applying Brutsaert and Sugita's (loc. cit.) parameterization for z_oh.