Turbulence spectra and dissipation rates above and within a forest canopy

Three velocity components and temperature were measured using three-dimensi onal sonic anemometers/thermometers at two levels, above and within a fores t canopy, in the Changbai Mountains of northeast China. Turbulence spectral structure, local isotropy and dissipation rates above and within the fores t canopy were calculated using the eddy correlation method. Results show th at the normalized turbulent spectral curves have -2/3 slopes in the inertia l subrange. While the shapes of the spectra are in good agreement with the Kansas flat terrain results, the atmospheric turbulence is anisotropic abov e the forest canopy. Due to breaking down of large eddies by the foliage, b ranches and trunks, the spectral peak frequencies for velocity and temperat ure are higher within than above the forest canopy. Compared with measureme nts from previous studies over flat terrain, the velocity and temperature s pectra above and inside the forest canopy appear to shift toward higher fre quencies. The turbulence is approximately isotropic in the inertial subrang e within the forest canopy, and is anisotropic above the forest canopy. The turbulent kinetic energy and heat energy dissipation rates above and insid e the forest canopy are much larger than those obtained by Kaimal and Hogst rom over grassland and grazing land. The distinct features in the results o f the present experiment may be attributed to the dynamic forcing caused by the rough surface of the forest canopy.