Aerodynamic roughness of a plant canopy: A numerical experiment

Abstract

A numerical model based on second-order closure principles was used to evaluate the response of the logarithmic wind profile parameters, the roughness length, z 0 and the displacement height, d, to changes in the density and vertical structure of an underlying canopy of vegetation. The profile parameters were calculated by forcing the logarithmic wind equation to match the computed wind profile over three successive grid points used in the numerical model. Both z 0 and d calculated in this manner were functions of height but the displacement height calculated from the wind profile at twice the canopy height was a good approximation to the center-of-pressure within the canopy over a wide range of densities. The displacement height increased monotonically with plant density and with the height of the center of gravity of the vegetation. The roughness length was a unimodal function of density, increasing with density in sparse canopies but decreasing with density in dense canopies. Vertical structure was important. The roughest canopies were those with a high center of gravity at low plant densities but those with a low center of gravity at high densities. At high densities, a single linear relationship between z 0 and d was evident, irrespective of density or structure. Evidence based on the sensitivity of the profile parameters to an arbitrarily set length scale suggests that a second-order closure model is superior to the traditional gradient-diffusion model in the proximity of a plant canopy.