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| Thomas, C; Foken, T: Organised motion in a tall spruce canopy: temporal scales, structure spacing and terrain effects, Boundary-Layer Meteorology, 122, 123-147 (2007), doi:10.1007/s10546-006-9087-z | |
| Key words: exchange,forest,coherent structur, turbulent flux |
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 = wL−1
s = 8–10 (where w is the streamwise spacing
of coherent structures in the vertical wind w and Ls 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. |
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