Flow Field within and Above a Forest Canopy: Task I: Study of Airflow in Simulated Temperate and Tropical Forest Canopies

摘要

The velocity and longitudinal turbulence intensity distributions inside and above a forest canopy along its center line were investigated. For this purpose a model forest canopy was used in a meteorological wind tunnel. The results indicate that the flow may be divided into an entrance and fully developed region followed by a short adjustment distance close to canopy end. The entrance region has a decisive effect on the flow characteristics through the canopy. The velocity and turbulence inside the canopy are strongly affected by its structure. A similar qualitative variation for both velocity and turbulence was found in and above the canopy. Its influence stretches over more than 4 roughness heights above it. Generally, the results are in relatively reasonable agreement with field measurements. Investigation of the modified logarithmic law for describing the velocity profile above the canopy revealed that both flow parameters, i.e., friction velocity and roughness length, are not local constants. On the contrary, they vary drastically with height. It is suspected that this is due to the fact the assumption of constant shear stress throughout the boundary layer or significant portions of it is not satisfied. (Author)