Organised Motion in a Tall Spruce Canopy: Temporal Scales, Structure Spacing and Terrain Effects

Abstract

This study investigates the organised motion near the canopy-atmosphere interface of a moderately dense spruce forest in heterogeneous, complex terrain. Wind direction is used to assess differences in topography and surface properties. Observations were obtained at several heights above and within the canopy using sonic anemometers and fast-response gas analysers over the course of several weeks. Analysed variables include the three-dimensional wind vector, the sonic temperature, and the concentration of carbon dioxide. Wavelet analysis was used to extract the organised motion from time series and to derive its temporal scales. Spectral Fourier analysis was deployed to compute power spectra and phase spectra. Profiles of temporal scales of ramp-like coherent structures in the vertical and longitudinal wind components showed a reversed variation with height and were of similar size within the canopy. Temporal scales of scalar fields were comparable to those of the longitudinal wind component suggesting that the lateral scalar transport dominates. The existence of a -1 power law in the longitudinal power spectra was confirmed for a few cases only, with a majority showing a clear 5/3 decay. The variation of effective scales of organised motion in the longitudinal velocity and temperature were found to vary with atmospheric stability, suggesting that both Kelvin-Helmholtz instabilities and attached eddies dominate the flow with increasing convectional forcing. The canopy mixing-layer analogy was observed to be applicable for ramp-like coherent structures in the vertical wind component for selected wind directions only. Departures from the prediction of m = w L −1 s = 8-10 (where w is the streamwise spacing of coherent structures in the vertical wind w and L s is a canopy shear length scale) were caused by smaller shear length scales associated with large-scale changes in the terrain as well as the vertical structure of the canopy. The occurrence of linear gravity 124 Boundary-Layer Meteorol (2007) 122:123-147 waves was related to a rise in local topography and can therefore be referred to as mountain-type gravity waves. Temporal scales of wave motion and ramp-like coherent structures were observed to be comparable.

Influence of Canopy Seasonal Changes on Turbulence Parameterization within the Roughness Sublayer over an Orchard Canopy

“…Figure 1 (FrC) and force convection (FoC)] and near-neutral (NN) conditions and are less dependent under stable conditions. The reason for the insignificant phenology influence on the exchange coefficients under stable stratification could be that under stable conditions the decoupling between the canopy and the atmosphere is independent from canopy density/sparsity (Thomas and Foken 2007). Therefore, we focus our analysis on the stability range from FrC to NN.…”

Wavelet Analysis of Atmospheric Turbulence over a Coral Reef Flat

“…Because the velocimeter was acquiring data at 8 Hz, to directly compare its wavelet spectrum with that of w9 both of the datasets were block averaged down to 2 Hz. This is a feasible option because the coherent structures in w9 are expected to be below this frequency and the loss of high-frequency detached eddies should not significantly affect the results (Thomas andFoken 2005, 2007). In wavelet analysis the maximum allowable time scale must be the next power of 2 below half the length of the time series.…”

“…5 and 6). The computation of the CWT, XWT, WTC, and global wavelet variance was carried out in Matlab version 7.4a following the methods outlined by Torrence and Compo (1998), Thomas and Foken (2005), and Grinsted et al (2004) using scripts written by Torrence and Compo (1998; available online at http://paos.colorado.edu/research/ wavelets/) and Grinsted et al (2004; available online at http://www.pol.ac.uk/home/research/waveletcoherence). Determining the strength of any interference from water pressure fluctuations ( p9) on the measurement of w9 required that the individual wave forms, ranging from the 2-to 32-s scale for both time series, be summed and averaged (Figs.…”

Studying geometric structures in meso-scale flows

“…Since then, numerous studies have reported geometrical shapes of coherent structures over a variety of surfaces such as land (e.g., Barthlott et al, 2007), ice (e.g., Lykossov and Wamser, 1995), vegetation (e.g., Serafimovich et al, 2011), and water (e.g., Boppe et al, 1999). Ramp-like coherent structures under unstable conditions have been attributed mainly to surface-layer plumes or mixed-layer thermals (e.g., Kaimal and Businger, 1970) but also superposition of shear driven and thermal structures has been reported (Thomas and Foken, 2006). Coherent structures were also detected in stable flows (Kikuchi and Chiba, 1985), and they might appear without the presence of a boundary (Belušić and Mahrt, 2012).…”