Spectral short-circuiting and wake production within the canopy trunk space of an alpine hardwood forest

Using synchronous multi-level high frequency velocity measurements, the turbulence spectra within the trunk space of an alpine hardwood forest were analysed. The spectral short-circuiting of the energy cascade for each velocity component was well reproduced by a simplified spectral model that retained return-to-isotropy and component-wise work done by turbulence against the drag and wake production. However, the use of an anisotropic drag coefficient was necessary to reproduce these measured component-wise spectra. The degree of anisotropy in the vertical drag was shown to vary with the element Reynolds number. The wake production frequency in the measured spectra was shown to be consistent with the vortex shedding frequency at constant Strouhal number given by f (vs) = 0.21 (U) over bar /d, where d can be related to the stem diameter at breast height (dbh) and (U) over bar is the local mean velocity. The energetic scales, determined from the inflection point instability at the canopy-atmosphere interface, appear to persist into the trunk space when C(du)a(cr)h(c)/beta >> 1, where C (du) is the longitudinal drag coefficient, a (cr) is the crown-layer leaf area density, h (c) is the canopy height, and beta is the dimensionless momentum absorption at the canopy top.