bei Google ScholarTitelangaben
Thomas, Christoph ; Foken, Thomas:
Organised motion in a tall spruce canopy: temporal scales, structure spacing and terrain effects.
In: Boundary-Layer Meteorology.
Bd. 122
(2007)
Heft 1
.
- S. 123-147.
ISSN 1573-1472
DOI: https://doi.org/10.1007/s10546-006-9087-z
Abstract
This study investigates the organised motion near the canopy-atmosphereinterface 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 usingsonic 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 theorganised motion from time series and to derive its temporal scales. Spectral Fourieranalysis was deployed to compute power spectra and phase spectra. Profiles of temporalscales of ramp-like coherent structures in the vertical and longitudinal windcomponents showed a reversed variation with height and were of similar size withinthe canopy. Temporal scales of scalar fields were comparable to those of the longitudinalwind component suggesting that the lateral scalar transport dominates. Theexistence of a – 1 power law in the longitudinal power spectra was confirmed for afew cases only, with a majority showing a clear 5/3 decay. The variation of effectivescales of organised motion in the longitudinal velocity and temperature were foundto vary with atmospheric stability, suggesting that both Kelvin-Helmholtz instabilitiesand attached eddies dominate the flow with increasing convectional forcing. Thecanopy mixing-layer analogy was observed to be applicable for ramp-like coherentstructures in the vertical wind component for selected wind directions only. Departuresfrom the prediction of m = wL−1s = 8–10 (where w is the streamwise spacingof 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 theterrain as well as the vertical structure of the canopy. The occurrence of linear gravity.