Comparison of turbulence statistics within three boreal forest canopies

Abstract

Three-dimensional sonic anemometers were used to measure velocities and temperatures within three natural boreal forest canopies. Vertical profiles of atmospheric turbulence statistics for a black spruce forest, a jack pine forest, and a trembling aspen forest, all located in southeastern Manitoba, were plotted and compared. The canopy structures were quite different, with total leaf-area indices of 2, 4 and 10, for the pine, aspen, and spruce forests, respectively. The profiles of the first and second moments differed among the canopies, where velocities decreased more rapidly in the top portions of the denser canopies. The velocity distributions were skewed and kurtotic within all canopies, and showed some differences among the canopies. Eulerian time scale profiles were generally similar among the canopies, and the vertical and streamwise time scale profiles were almost mirror images of each other. Eulerian length scale profiles showed some differences among canopies caused by differences in the velocity profiles. Ratios of vertical-to-horizontal time and length scales had a maximum in mid-canopy. Shear stress profiles were similar in the top parts of all canopies, and upward momentum fluxes were occasionally observed within the canopy trunk space. Countergradient heat fluxes were also observed sometimes. The countergradient fluxes and the skewed, kurtotic velocity distributions indicate the contribution of intermittent, large-scale eddies that are important for energy and mass transfer within canopies.