Experimental Determination of the Roughness Length for Temperature Over a Field of Tall Grass in Central Sweden

The aim of this study was to analyse wind, temperature and humidity profiles, surface radiation temperature, and eddy correlation fluxes of momentum and heat over a grass field in order to obtain a better parameterisation of exchange processes over such a surface. The study was performed under summer conditions over a formerly cultivated, uniform and well-watered field of tall grass in central Sweden. An unusually low displacement height, d, was found, at 0.25 times the maximum canopy height h. The roughness length for wind speed could be approximated with the relation z_ou = 0.15(h–d). The relations of d and z_ou would be closer to the traditional ones if a mean canopy height, which is about half of the height used here, had been used in the analysis. The estimation of the quantity kB^–1 required accurate determination of the surface temperature. The radiometric surface temperature was measured by two different radiometers, one collecting thermal radiation at angles 0–45° and measuring effectively at 30° from nadir, and the other having a field of view of 15° and being tilted at 45° from nadir. The detected directional variations in the surface radiation temperature were within ± 1K, and the former radiometer, which was judged to be more accurate, was used for further analysis. The quantity kB^–1 was expressed through the roughness-sublayer Stanton number and drag coefficient. Methodologically, the most accurate determination of the Stanton number was to plot the air-surface temperature difference versus the temperature scale multiplied by any other relevant parameter. The quantity kB^–1 showed a dependence on the roughness Reynolds number, Re_o, and it took the form kB^–1 = 0.08 Re_o^0.5–1.9; its value varied between 0 and 4 for the present range of Re_o. These results are judged to be in good agreement with theory, considering the complexity of natural surfaces and the simplifications used in the theoretical deductions.