Observation of organized structure in turbulent flow within and above a forest canopy

Abstract

Ramp patterns of temperature and humidity occur coherently at several levels within and above a deciduous forest as shown by data gathered with up to seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers at an experimental site in Ontario, Canada. The ramps appear most clearly in the middle and upper portion of the forest. Time/height cross-sections of scalar contours and velocity vectors, developed from both single events and ensemble averages of several events, portray details of the flow structures associated with the scalar ramps. Near the top of the forest they are composed of a weak ejecting motion transporting warm and/or moist air out of the forest followed by strong sweeps of cool and/or dry air penetrating into the canopy. The sweep is separated from the ejecting air by a sharp scalar microfront. At approximately twice the height of the forest, ejections and sweeps are of about equal strength.In the middle and upper parts of the canopy, sweeps conduct a large proportion of the overall transfer between the forest and the lower atmosphere, with a lesser contribution from ejections. Ejections become equally important aloft. During one 30-min run, identified structures were responsible for more than 75% of the total fluxes of heat and momentum at mid-canopy height. Near the canopy top, the transition from ejection of slow moving fluid to sweep bringing fast moving air from above is very rapid but, at both higher and lower levels, brief periods of upward momentum transfer occur at or immediately before the microfront.

Coherent Momentum Exchange above and within a Scots Pine Forest

“…Another study determined a median coherent structure duration of 90-115 s with wavelet analysis and 1.3-1.8 s with quadrant analysis during stable and unstable conditions [7]. Based on visual inspection, D was reported to vary between 24 and 39 s below a deciduous forest canopy and between 20 and 23 s above the canopy during three 30-min intervals with stable, neutral and unstable conditions [2]. Furthermore, based on visual analysis, a mean duration of coherent structures above a pine forest of 33-40 s was determined, with separations of 97-124 s during two 100-min intervals under unstable conditions [49].…”

“…These structures are often referred to as coherent structures, because their characteristics appear coherently at several levels within and above plant canopies [2,3] and can be separated from weaker, small-scale fluctuations in turbulent flow [4]. Two quantitative methods, which have been most commonly applied to investigate coherent structure characteristics in data measured in the field, are quadrant analysis [5,6] and wavelet analysis [7][8][9].…”

Net ecosystem CO<sub>2</sub> exchange measurements by the closed chamber method and the eddy covariance technique and their dependence on atmospheric conditions

“…In particular, larger, low-frequency flow patterns, i.e., coherent structures (Collineau and Brunet, 1993;Gao et al, 1989;Thomas and Foken, 2007), may cause differences between chamber and EC measurement results. Another cause of flux differences can be differing atmospheric stratification.…”

“…Under stable stratification and low turbulence, the flux contribution of coherent structures to the entire flux increases (Collineau and Brunet, 1993;Gao et al, 1989;Thomas and Foken, 2007;Holmes et al, 2012). These wellorganized structures, with typical periods of 10-100 s, are caused by strong roughness or landscape heterogeneities such as tree lines, bushes and ditches.…”

Wavelet analysis of the turbulent flow over the very rough surface

“…temperature or a concentration (for example [10], [11]. The basic principle of Wavelet transformation is to convolute a signal and a so-called mother wavelet function.…”