On the coupling of canopy flow to ambient flow for a variety of vegetation types and densities

A canopy flow coupling parameter is defined from earlier canopy flow research to describe the degree of coupling of air flow in vegetation to ambient flow of the surface boundary layer. This ratio concept employs an exponential wind-height relationship in the canopy referenced to the logarithmic wind-height relationship of the ambient air in close proximity to the vegetation interface. Qualitatively, the coupling ratio decreases as the index of canopy flow increases. Numerical criteria are derived to quantify the coupling upwind of the canopy, at the leading edge, through the transition zone, through the zone of established flow, at the trailing edge, and downwind from the canopy domain. It was found that coupling was relatively independent of element density for the more dense arrays, but increased rapidly as densities became more sparsely arrayed. A high degree of coupling existed both upwind and downwind of well-defined domains, while a degeneration of coupling is clearly evident through the zone of established flow. A seasonal contrast in coupling was also discerned. Gravity and slope flows contributed to a higher degree of coupling.