The control of coherent eddies in vegetation canopies: Streamwise structure spacing, canopy shear scale and atmospheric stability

An analogy has been established between a plane mixing layer and the atmosp heric flow near the top of a vegetation canopy. It is based on a common fea ture, a strong inflection in the mean velocity profile, responsible for hyd rodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy w as tested using a small data set from thirteen experiments, all in near-neu tral conditions. It provided a good prediction of the streamwise spacing La mbda(w) of the dominant canopy eddies (evaluated from time series of vertic al velocity) that appears to depend on a shear length scale L-s = U(h)/U'(h ), where h is canopy height, U is mean velocity and U' the vertical gradien t dU/dz. The present analysis utilizes an extensive data set of approximate ly 700 thirty-minute runs, from six experiments on two forest sites and a m aize crop, with a large range of stability conditions. Lambda(w) was estima ted for each run using the wavelet transform as an objective, automated det ection method. First, the variations of Lambda(w) and L-s with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatte r whereas in stable conditions both variables decrease with increasing stab ility. It is subsequently found that Lambda(w) is directly related to L-s, in a way close to the neutral prediction Lambda(w) /h = 8.1L(s)/h. The Stro uhal number S-tr = L-s /Lambda(w) is then shown to vary with atmospheric st ability, weakly in unstable conditions, more significantly in stable condit ions. Altogether these results suggest that, to some extent, the plane mixi ng-layer analogy can be extended to non-neutral conditions. It is argued th at the primary effect of atmospheric stability, at least in stable conditio ns, is to modify the shear length scale L-s through changes in U(h) and U'( h), which in turn determines the streamwise spacing of the active, coherent motions.