Turbulence spectra, shear stress and turbulent kinetic energy budgets above two beech forest sites in Denmark

Abstract

The focus of this study is the combined influence of the roughness sublayer (RSL) found above tall vegetation and the internal boundary layer (IBL) on the near-neutral flow above two forest sites. Measurements of the 3-D wind field from masts about twice the forest height were analysed. For both sites, influence from upwind conditions was detected for a short-fetch sector. For one of the sites, an additional long-fetch sector without significant IBL influence is presented. Spectral analysis, dissipation length scale analysis and evaluation of the most important terms in the turbulent kinetic energy and shear stress budgets, were performed. For all selected sectors, RSL influence was detected close to the canopy top: the dissipation length scale was greater than the height above the displacement height of the forest, and the turbulent transport terms were significant. For the short-fetch sectors, the spectral analysis of measurements taken in the RSL and in the IBL (above the RSL) showed that scaling by fixed length and velocity scales resulted in a good collapse of the spectral peaks. For the long-fetch sector, the RSL influence disappeared at greater heights, and the flow is nearly adjusted with the new surface.