Wind Tunnel Analysis of the Evolution of Turbulence over Fragmented Forest Surfaces

Current understanding of turbulent transfer above and within forest canopies has evolved from studies conducted under ideal conditions of flat terrain and homogeneous vegetative cover. However, most real land cover is fragmented and patchy and the adjustment of the atmospheric boundary-layer to these conditions remains a key topic within forest meteorology, especially for applications like mitigating wind damage. It is difficult and expensive to undertake the required momentum adjustment studies under field conditions. Consequently, we investigated flow adjustment over a series of fragmented landscapes using a well characterised wind tunnel model that mimics a tall vegetation canopy (Raupach et al., 1986). Mean wind and turbulence characteristics were measured using 3D Laser Doppler Velocimetry at high spatial resolution to capture flow adjustment processes across each fragmented block. Four representative profiles were sampled at each measurement position to allow computation of spatial averages. This paper will discuss the evolution of turbulence from a low canopy, representative of grass, to a tall canopy, representative of a forest, using several fragmented grass:forest configurations. These results will also be compared with the evolution of turbulence across a single grass:forest edge where the forest extends in excess of 100 tree heights downwind of the edge. The adjustment of the mean flow and turbulence moments depends both on the order and nature of the moments and on the distance downwind from the first grass:forest unit. We compare these results with other field, wind tunnel and modelling studies and attempt to develop an improved understanding of the physical processes driving turbulence adjustment over vegetation canopies following several repetitive changes in roughness within the theoretical framework advanced by Belcher et al. (2012). We will emphasize the relevance of these results to flux tower observations in fragmented landscapes and to managing forests to reduce wind damage risk.

Belcher, Stephen E., Ian N. Harman and John J. Finnigan (2012) The wind in the willows: Flows in forest canopies in complex terrain. Annu Review of Fluid Mechanics. 44, 479-504 Raupach, M.R., Coppin, P.A. and Legg, B.J., 1986. Experiments on scalar dispersion within a model plant canopy. Part I: The turbulence structure. Boundary-Layer Meteorol., 35: 21-52