Wind Tunnel Analysis of the 3D Adjustment of a Passive Scalar Across and Downwind of a Forest Edge

Turbulent transfer is the principal mode of interaction between the surface of the earth and the lower atmosphere and so plays an important role in many aspects of meteorology, climatology and ecology. The nature of turbulent transfer within and above vegetation canopies has been investigated over many decades commonly under the ideal conditions of uniform ground cover and flat terrain (e.g. Finnigan, 2000; Finnigan et al., 2009). However, very few parts of the world display ideal conditions. More common is fragmented and patchwork land use superimposed on terrain with varying degrees of complexity. Consequently there is interest in understanding turbulent transfer of momentum and scalars under these non-ideal conditions. In this paper, we investigate a simple example of complexity – the adjustment of a passive scalar to a sharp change in roughness from a low “grassland” surface to a tall “forest” canopy. In particular, we are interested in characterising the spatial detail of scalar adjustment downwind of the forest edge under different scalar source distributions.

It is difficult and expensive to undertake scalar adjustment studies under field conditions especially at the spatial resolution needed to provide information about the underlying physical processes acting through the adjustment. Consequently, we used a well characterised wind tunnel model that mimics a tall vegetation canopy (Raupach et al., 1986) to investigate these issues. The ‘Black Tombstones' (BT) wind tunnel model canopy has been constructed to extend our investigations into canopy-boundary layer exchange including studies of the impacts of atmospheric stability, complex terrain and the distribution of sources and sinks on the exchange of momentum and scalars. The BT canopy is an ideal environment in which to study the adjustment of the boundary layer and embedded scalars to a rapid roughness change typical of a grassland-forest interface. Mean wind and turbulent characteristics were measured using 3D Laser Doppler Velocimetry (LDV). Mean scalar concentration (passive heat was used as the scalar) was measured using a thermistor bead mounted close to, but not interacting with, the LDV measurement volume. A novel measuring technique is also used to observe the scalar eddy flux using the LDV and a co-located cold wire. Together these three measurements allow a full analysis of the scalar mass balance within and above the model canopy.

We present observations of the flow field, scalar concentration and fluxes from a dense observation grid spanning a range of 2 canopy heights (h) in the vertical and from 10h upstream to 40h downstream of the edge. In addition, a detailed spatial analysis taken at 50h downstream will be shown to illustrate the mechanisms by which scalar adjustment occurs. Two scalar vertical source-sink distributions will also be considered to illustrate how scalars with different source distributions are influenced by the same forest transition. We compare the BT results with other field, wind tunnel and modelling studies and attempt to develop an improved understanding of the physical processes driving scalar adjustment over vegetation canopies following a change in roughness within the theoretical framework advanced by Raupach (1987) and Harman and Finnigan (2008). We will also illustrate how these results shed light on tower observations of scalar exchange in the field.

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