Fetch requirements near a forest edge

Measurements on forest-atmosphere exchange should be executed some distance from the forest boundaries to be representative for this forest. In this study the minimum fetch at measuring height is estimated by analysing measurements just above a mixed forest stand near the edge. The objective was to gain insight in the development of the Equilibrium Layer (EL) after a transition in vegetation height; only wind and shear stress were considered. The variations in u*/u and wind speed gradients above the canopy, as a function of wind direction and fetch were analysed. In this canopy roughness layer, stability corrections are small due to the large roughness of the forest. An adapted version of the model of Klaassen (1992) was used to simulate fluxes as well as gradients above the forest.It appeared that the disturbances from equilibrium arise not only from the forest edges, but also from the local changes in tree height between patches within the forest itself. A fetch:height ratio of 36 for the EL seemed appropriate for specific fetch directions at the site under study. This is in agreement with previous studies.