Directional radiometric temperature pro®les within a grass canopy

The scalar roughness for sensible heat ̄ux, z0h, 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 z0h to account for variability in z0h as a result anisothermal vegetation skin temperature, which assumes an exponential pro®le of canopy skin temperature. We examined this assumption through directional radiometric measurements of temperature pro®les in a relatively uniform grass canopy. The temperature gradient steepened monotonely throughout the morning and was modeled successfully with an exponential pro®le. The root-mean-square error (RMSE) between measured and modeled pro®les was small and ranged between 0.038°C and 0.530°C. One of the most signi®cant ®ndings of this research is that the decay coecient, b, of the exponential pro®le model was approximately constant with time for this canopy. This may provide a useful simpli®cation in applying Brutsaert and Sugita's (loc. cit.) parameterization for z0h. Ó 2000 Elsevier Science Ltd. All rights reserved.