TURBULENCE SPECTRA AND DISSIPATION RATES ABOVE AND WITHIN A FOREST CANOPY *

Abstract. Three velocity components and temperature were measured using three-dimensional sonic anemometers/thermometers at two levels, above and within a forest canopy, in the Changbai Moun tains of northeast China. Turbulence spectral structure, local isotropy and dissipation rates above and within the forest canopy were calculated using the eddy correlation method. Results show that the normalized turbulent spectral curves have-2/3 slopes in the inertial subrange. While the shapes of the spectra are in good agreement with the Kansas flat terrain results, the atmospheric turbulence is anisotropic above the forest canopy. Due to breaking down of large eddies by the foliage, branches and trunks, the spectral peak frequencies for velocity and temperature are higher within than above the forest canopy. Compared with measurements from previous studies over flat terrain, the velocity and temperature spectra above and inside the forest canopy appear to shift toward higher frequencies. The turbulence is approximately isotropic in the inertial subrange within the forest canopy, and is anisotropic above the forest canopy. The turbulent kinetic energy and heat energy dissipation rates above and inside the forest canopy are much larger than those obtained by Kaimal and Hogstrom over grassland and grazing land. The distinct features in the results of the present experiment may be attributed to the dynamic forcing caused by the rough surface of the forest canopy