Wind tunnel and field measurements of turbulent flow in forests. Part I: Uniformly thinned stands

Many forest management methods alter stand density uniformly. The effects of such a change on the wind and turbulence regimes in the forest are critical to a number of processes governing the stability of the stand and its microclimate. We measured wind speed and turbulence statistics with a Dantec tri-axial hot-film probe in model forests of various densities (31-333 trees m(-2)), created by removing whole trees in a regular pattern in a wind tunnel, and compared them with similar measurements made with propeller anemometers in similarly thinned plots (156-625 trees ha(-1)) within a Sitka spruce stand in Scotland. The results agree well, in general, with measurements made in other such studies with diverse canopy types. The systematic variations with density and vertical leaf-area distribution (which differed between wind-tunnel and field trees) in our work can explain much of the variability shown in scaled profiles of basic turbulence statistics reported in the literature. The wind tunnel and field results are shown to be in good agreement overall despite the difference in vertical leaf-area distribution. Within-canopy and isolated-tree drag coefficients in the wind tunnel show that tree-scale shelter effects increase as tree density increases. The measurements indicate that turbulence in the canopy is dominated by large-scale structures with dimensions of the same order as the height of the canopy as found in other studies but suggest that inter-tree spacing also modulates the size of these structures. These structures are associated with the sweeps that dominate momentum exchange in the canopy and it is this fact that allows the tri-axial probe to operate so well despite the relatively narrow range of angles in which the wind vector is correctly measured. The ratio of streamwise periodicity of these structures to vorticity thickness varies systematically with tree density in the range 2.7-5.1, which spans the expected range of 3.5-5 found in a laboratory mixing-layer, suggesting that tree spacing imposes another relevant length scale. This test and others show that the results are in agreement with the idea that canopy turbulence resembles that of a mixing layer even though they disagree with, and challenge the linear relationship between, streamwise periodicity and shear length scale presented recently in the literature. The measurements are also in good overall agreement with simple drag models presented recently by other researchers.