Regional Roughness Parameters and Momentum Fluxes over a Complex Area

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  • 1 Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki, Japan
  • | 2 Environmental Research Center, University of Tsukuba, Tsukuba, Ibaraki, Japan
  • | 3 Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki, Japan
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Abstract

Regional surface parameters of the roughness length z0 and the displacement height d0 needed to estimate the surface fluxes of a complex region were determined by applying a profile equation to 121 reliable wind profiles measured from instruments on a 213-m tower under unstable atmospheric conditions. The derived values of z0 over the experimental area were 0.98 ± 0.08 and 1.04 ± 0.05 m for two upwind areas, while those of d0 were 6.3 ± 3.1 and 6.7 ± 2.6 m for the same areas. These values of z0 and d0 were used to calibrate the drag coefficient D of major roughness obstacles in the models of Grant and Mason and of Raupach to predict roughness parameters as a function of surface features, such as the roughness density and the mean height of major obstacles. The calibrated value of D for the experimental area was found to be on the larger side among the values determined from past experiments. The calibrated models, together with some simple expressions, were then applied to different parts of the experimental areas to predict their surface roughness parameters. These values were used to estimate friction velocity values u* from wind speed profiles measured at 29.5 m above the surface in these areas for comparison with u* values determined from the eddy correlation technique. The comparisons have shown that the surface parameters derived from the calibrated models yield a good agreement of u*, while those from the simple expressions tend to produce worse results.

Abstract

Regional surface parameters of the roughness length z0 and the displacement height d0 needed to estimate the surface fluxes of a complex region were determined by applying a profile equation to 121 reliable wind profiles measured from instruments on a 213-m tower under unstable atmospheric conditions. The derived values of z0 over the experimental area were 0.98 ± 0.08 and 1.04 ± 0.05 m for two upwind areas, while those of d0 were 6.3 ± 3.1 and 6.7 ± 2.6 m for the same areas. These values of z0 and d0 were used to calibrate the drag coefficient D of major roughness obstacles in the models of Grant and Mason and of Raupach to predict roughness parameters as a function of surface features, such as the roughness density and the mean height of major obstacles. The calibrated value of D for the experimental area was found to be on the larger side among the values determined from past experiments. The calibrated models, together with some simple expressions, were then applied to different parts of the experimental areas to predict their surface roughness parameters. These values were used to estimate friction velocity values u* from wind speed profiles measured at 29.5 m above the surface in these areas for comparison with u* values determined from the eddy correlation technique. The comparisons have shown that the surface parameters derived from the calibrated models yield a good agreement of u*, while those from the simple expressions tend to produce worse results.

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