Wind Tunnel Analysis of the Joint Adjustment of a Passive Scalar and the Wind Field Across a Forest Edge

Turbulent transfer is the principal means of connection between the surface of the earth and the lower atmosphere and consequently plays an important role in many aspects of meteorology, climatology and ecology. Our understanding of turbulent transfer within and above vegetation canopies under the ideal conditions of uniform ground cover and flat terrain is well advanced (e.g. Finnigan et al. 2009). However, our understanding of the turbulent transfer under the more common occurrence of fragmented and patchwork land-use, superimposed on terrain with varying degrees of complexity is less complete. Consequently there is interest in understanding turbulent transfer of momentum and scalars under less ideal conditions and identifying any consequent implications this has for our interpretation of real world observations. Advances in our understanding also has implications for the representation of canopy turbulent transfer processes within models aimed at a range of applications including wind throw assessments, numerical weather prediction and climate research.

We investigate one such case of real world complexity – the adjustment of the wind and a passive scalar to a sharp change in roughness from a low “grassland” surface to a tall “forest” canopy – through scale modelling and theoretical analysis. In particular, we are interested in characterising the detail of scalar adjustment downwind of the forest edge under different scalar source distributions in comparison to the better understood adjustment of the wind field (e.g. Belcher et al. 2012). We use a well characterised wind tunnel model – the ‘black tombstones' - that dynamically mimics a tall vegetation canopy (Raupach et al., 1986) as the primary canopy surface. A co-located scalar source (using heat as a passive scalar) can be controlled in order to replicate primarily ground based, canopy based or mixed scalar source distributions. High frequency wind information is measured using 3D Laser Doppler Velocimetry (LDV). The mean scalar concentration (as heat) is measured using a thermistor bead mounted close to, but not interacting with, the LDV measurement sphere. A novel measuring technique is also used to observe the concurrent scalar flux using the LDV and a co-located coldwire. Together these three measurements allow a full analysis of the momentum and scalar mass balances 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, and scales upon, momentum and scalar adjustment occurs. Different scalar sources-sink distributions will also be considered to illustrate how different scalars are influenced differently by the same forest transition. We will also illustrate how these results can shed light on real world tower observations of scalar exchange.