Mechanisms controlling turbulence development across a forest edge

In this paper we discuss the development of turbulence back from the transition from open moorland to a forest. Data from a field study and a wind-tunnel experiment are presented. These show that the variance in the streamwise velocity begins to adjust to the new surface between 2 to 4 tree heights downwind of the transition. This is sooner than either the vertical velocity variance or the shear stress, both of which begin to adjust in a zone 3 to 5 tree heights downwind of the edge. Key terms in the prognostic equations for streamwise and vertical velocity variance are evaluated in order to explain these differences. The flow distortion caused by the forest edge, which extends to 4 tree heights downwind of the forest edge, is shown to be crucial in the delayed turbulence development. Initially the shear production term, which is the dominant source for the streamwise velocity variance, is counteracted by a sink in the vertical advection term. After the flow levels out the pressure redistribution (return-to-isotropy) term becomes the main sink of streamwise velocity variance and feeds energy into the vertical velocity component. Therefore, the development of the vertical velocity variance and shear stress cannot begin until after development of an increase in the streamwise velocity variance. Results are compared with other experiments, including the flow across shelterbelts, and large-eddy simulations of forest flow.