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 profile s, surface radiation temperature, and eddy correlation fluxes of momentum a nd heat over a grass field in order to obtain a better parameterisation of exchange processes over such a surface. The study was performed under summe r 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 rel ations of d and z(ou) would be closer to the traditional ones if a mean can opy height, which is about half of the height used here, had been used in t he analysis. The estimation of the quantity kB(-1) required accurate determ ination of the surface temperature. The radiometric surface temperature was measured by two different radiometers, one collecting thermal radiation at angles 0-45 degrees and measuring effectively at 30 degrees from nadir, an d the other having a field of view of 15 degrees and being tilted at 45 deg rees from nadir. The detected directional variations in the surface radiati on temperature were within +/- 1K, and the former radiometer, which was jud ged to be more accurate, was used for further analysis. The quantity kB(-1) was expressed through the roughness-sublayer Stanton number and drag coeff icient. Methodologically, the most accurate determination of the Stanton nu mber was to plot the air-surface temperature difference versus the temperat ure scale multiplied by any other relevant parameter. The quantity kB(-1) s howed 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 wi th theory, considering the complexity of natural surfaces and the simplific ations used in the theoretical deductions.