Active turbulence and scalar transport near the forest-atmosphere interface

Turbulent velocity, temperature, water vapor concentration, and other scala rs were measured at the canopy-atmosphere interface of a 13-14-m-tall unifo rm pine forest and a 33-m-tall nonuniform hardwood forest. These measuremen ts were used to investigate whether the mixing layer (ML) analogy of Raupac h et al. predicts eddy sizes and flow characteristics responsible for much of the turbulent stresses and vertical scalar fluxes. For this purpose, wav elet spectra and cospectra were derived and analyzed. It was found that the ML analogy predicts well vertical velocity variances and integral timescal es. However, at low wavenumbers, inactive eddy motion signatures were prese nt in horizontal velocity wavelet spectra, suggesting that ML may not be su itable for scaling horizontal velocity perturbations. Momentum and scalar w avelet cospectra of turbulent stresses and scalar fluxes demonstrated that active eddy motion, which was shown by Raupach et ai. to be the main energy contributor to vertical velocity (w) spectral energy (E-w), is also the ma in scalar flux-transporting eddy motion. Predictions using ML of the peak E -w frequency are in excellent agreement with measured wavelet cospectral pe aks of vertical fluxes (Kh = 1.5, where K is wavenumber and h is canopy hei ght). Using Lorentz wavelet thresholding of vertical velocity time series, wavelet coefficients associated with active turbulence were identified. It was demonstrated that detection frequency of organized structures, as predi cted from Lorentz wavelet filtering, relate to the arrival frequency [U]/h and integral timescale,where [U] is the mean horizontal velocity at height z = h. The newly proposed wavelet thresholding approach, which relies on a "global" wavelet threshold formulation for the energy in w, provides simult aneous energy-covariance;preserving characterization of "active" turbulence at the canopy-atmosphere interface.