HR: 17:15h
AN: H14B-05    [Abstracts]
TI: Organised Coherent Motion in Atmospheric Boundary Layer Flow in the Proximity to Tall Plant Canopies as Detected in Acoustic Doppler Profiler and Tower-based Observations
AU: Foken, T
EM: thomas.foken@uni-bayreuth.de
AF: Department of Micrometeorology, University of Bayreuth, Universitaetsstr. 30, Bayreuth, 95440, Germany
AU: * Thomas, C K
EM: christoph.thomas@oregonstate.edu
AF: Department of Micrometeorology, University of Bayreuth, Universitaetsstr. 30, Bayreuth, 95440, Germany
AU: * Thomas, C K
EM: christoph.thomas@oregonstate.edu
AF: Department of Forest Science, Oregon State University, 321 Richardson Hall, Corvallis, OR 97330, United States
AB: We investigated coherent structures above and in a tall plant canopy during a field campaign at a mountainous site in Germany (WALDATEM-2003). Data from a remote sensing acoustic Doppler system in concert with in-situ point measurements of turbulence in flow velocity and scalars deployed on towers yielded continuous observations from the forest ground to 200 m above the ground with a vertical resolution of 10 m at a sampling frequency of 0.4 and 20 Hz respectively. Coherent structures were extracted from time series utilizing wavelet transform techniques allowing for single structure analysis and averaged statistics of detected events. In addition to their spatiotemporal scales, we focused on the identification of generating mechanisms and surface parameters affecting coherent structures. Time scales were on the order of 20 to 36 s depending on the upstream topography and canopy morphology. Lateral transport dominated scalar coherent exchange. Vertical profiles of time scales in longitudinal and vertical velocities were mirror images showing an increase/ decrease, respectively, with height. Time scales in scalars were nearly height-constant. The ratio of the contribution of coherent structures to total vertical exchange was 0.2 for momentum and 0.25 to 0.4 for sensible heat. Analysis of power spectra confirmed an interaction between inactive eddies of atmospheric boundary layer scale and the horizontal flow in 4 % of all studied cases only, mainly under near-neutral stratification. Evaluation of the Mixing-Layer Analogy suggested that vertical shear caused by the immense canopy drag was the dominant generating mechanism. However, daytime coherent structures were found to be a superposition of shear generated events and convectional eddies. The latter led to an increase of vertical coherency in the flow around noon. At night, terrain induced linear gravity waves showed similar time scales as coherent structures emphasizing the need to differentiate between these two fundamentally different phenomena. Topography in combination with canopy morphology was found to affect coherent structures through modifying characteristic shear length scales. Based on how deep coherent structures penetrate the plant canopy, a classification of exchange regimes was proposed to qualitatively describe coupling and decoupling in tall canopies.
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 1855 Remote sensing (1640)
DE: 1872 Time series analysis (3270, 4277, 4475)
DE: 3307 Boundary layer processes
DE: 3379 Turbulence (4490)
SC: Hydrology [H]
MN: 2007 Fall Meeting