TURBULENCE SPECTRA AND EDDY DIFFUSIVITY OVER FORESTS

The main objectives of this observational study are to examine the sta bility dependence of velocity and air temperature spectra and to emplo y the spectral quantities to establish relations for eddy diffusivity over forests. The datasets chosen for the analysis were collected abov e the Browns River forest and the Camp Borden forest over a wide range of stability conditions. Under neutral and unstable conditions the no ndimensional dissipation rate of turbulent kinetic energy (TKE) over t he forests is lower than that from its Monin-Obukhov similarity (MOS) function for the smooth-wall surface layer. The agreement is somewhat better under stable conditions but a large scatter is evident. When th e frequency is made nondimensional by the height of the stand (h) and the longitudinal velocity at this height (u(h)), the Kaimal spectral m odel for neutral air describes the observations very well. The eddy di ffusivity formulation K = c sigma(w)(4)/epsilon provides a promising a lternative to the MOS approach, where sigma(w) is the standard deviati on of the vertical velocity and epsilon TKE dissipation rate. Current datasets yield a constant of 0.43 for c for sensible heat in neutral a nd stable air, a value very close to that for the smooth-wall surface layer. It is postulated that c is a conservative parameter for sensibl e heat in the unstable air, its value probably falling between 0.41 an d 0.54. In the absence of epsilon data, it is possible to estimate K f rom measurements of the local mean wind u and air stability. As a spec ial case, it is shown that K = 0.27(uh/u(h))sigma(w) under neutral sta bility. This relation is then used to establish a profile model for wi nd speed and scalar concentration in the roughness sublayer. The analy sis points out that u(h) and h are important scaling parameters in att empts to formulate quantitative relations for turbulence over tall veg etation.