Scalar Turbulence within the Canopy Sublayer
The exchange of scalars between the biosphere and the atmosphere has direct bearing on a large number of problems such as climate change, air and water quality, agricultural management and food security, landscape ecology, and decision making for environmental compliances and policy formulation. Near the canopy-atmosphere interface, turbulent fluctuations in scalar concentrations exhibit complex dynamic behaviour that shows some parallels with the transporting turbulent velocity field. However, the statistical properties of scalar turbulence within and just above the canopy can also be partly decoupled from those of the transporting velocity. Here, a unifying framework is proposed that accounts for vertical variations in scalar sources and sinks within the canopy volume, the lack of equilibrium between production and dissipation terms in second-order scalar budget equations, the rapid changes in turbulent kinetic energy dissipation rate inside canopies, the relative importance of ejections and sweeps, the role of thermal stratification, and the 'near-field' effects of the scalar source on gradient-diffusion theories.Ways in which these same phenomena modify the scalar spectra and scalar-vertical velocity co-spectra within the inertial subrange above the canopy and at scales finer than the wake generation region within the canopy are presented. Finally, the origin of organized eddy structure connected with surface renewal that leads to scalar ramps is briefly discussed. The work draws upon a large number of flume, wind tunnel, and field experiments, and offers novel theoretical scaling arguments as to how coherency in the flow field impacts scalar spectra and scalar-velocity co-spectra at scales smaller than the shear production scales.
