The Origins of Coherent Eddy Structure in and Above Plant canopies
John Finnigan1, Roger Shaw2, Ned Patton3
1 Marine and Atmospheric Research, CSIRO
2 UC Davis
3 NCAR
2 UC Davis
3 NCAR
O 2.2 in Turbulence structure in and above forests
06.10.2009, 10:05-10:30, Kutschenhaus
In plant and some urban canopies, turbulent statistics are significantly different from those in the inertial sublayer or log layer above. In many ways the turbulence is more ‘efficient’ at transporting momentum and scalars. The differences extend above the canopy top into a Roughness Sub Layer (RSL) with particular consequences for flux measurements over forests as most such measurements are taken in the RSL. As described in Shaw et al (2009; this conference) we have educed this eddy structure by conditional sampling data from a large eddy simulation of a canopy flow. The characteristic eddy consists of an upstream Head-down, sweep-generating hairpin vortex superimposed on a downstream Head-up, ejection-generating hairpin. The conjunction of the sweep and ejection produces a pressure maximum between the hairpins and this is also the location of a coherent scalar microfront. This eddy structure matches that observed in simulations of homogeneous shear flows and channel flows by several workers and also fits with earlier field and wind tunnel measurements in canopy flows. It is significantly different from the eddy structure educed over smooth walls by conditional sampling based only on ejections as a trigger. We have developed a phenomenological model to explain both the structure of the characteristic eddy and the key differences between turbulence in the canopy/RSL and the ISL above. This model assumes that the inflected mean velocity profile at canopy top is inviscidly unstable and that this instability is ultimately responsible for the coherence of the resulting canopy eddies (the mixing layer hypothesis). However, we have extended this analysis by numerically simulating the evolution of the inviscid instability and show that the Head-up Head-down vortex pairs are generated spontaneously as the instability develops from its linear into its non-linear phase.This phenomenological model suggests a new scaling length that has been used to collapse turbulence moments over a range of vegetation canopies and diabatic stabilities.
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