THE EVOLUTION OF TURBULENCE ACROSS A FOREST EDGE

An experiment was set-up to investigate the adjustment of turbulence o ver a roughness transition (moorland to forest). Results from this exp eriment support the development of an internal boundary layer (IBL) at the transition, which propagates upwards by turbulent diffusion as a function of distance downwind from the transition. Spectra and length- scale results uphold the hypothesis that, over a transition to a rough surface, the variance distribution shifts towards smaller wavelengths /length scales. However, results suggest that the adjustment of stream wise velocity variance may be faster than the adjustment of the vertic al velocity variance. The concept of an equilibrium layer developing a bove the new surface is supported. Fetch requirements for equilibrium are, however, found to differ between first order and second order (fl ux) statistics, with second order statistics requiring a longer fetch. Results indicate that fetch should exceed 25 times the height of the measurement above the zero plane, which is a 2 degrees (+/-0.5) gro2th angle, for flux equilibrium.