Detection of turbulent coherent motions in a forest canopy part II: Time-scales and conditional averages
Rent the article at a discountRent now
* Final gross prices may vary according to local VAT.Get Access
Turbulent exchanges between plant canopies and the atmosphere are known to be strongly affected by intermittent coherent motions, which appear on time traces of turbulent variables as periodic, large-amplitude excursions from the mean. Detecting these features requires objective and powerful signal analysis techniques. We investigate here the possibilities offered by the recently developedwavelet transform, presented in a companion paper. For this purpose, a set of data acquired in a 13.5 m high pine forest in southwestern France was used, which provided time series of wind velocities and air temperature recorded at two levels simultaneously, under moderately unstable conditions. Firstly, a duration scale of the active part of coherent motions was estimated from the wavelet variance. Then, we focused on the detection itself of large-scale features; several wavelet functions were tested, and the results compared with those obtained from more classical conditional sampling methods such as VITA and WAG. A mean time interval Δ=1.8h/u * (h being the canopy height andu * the friction velocity) between contiguous coherent motions was obtained. The features extracted from the various traces and ensemble-averaged over 30 min periods appeared very similar throughout the four hours of data studied. They provided a dynamic description of the ejection-sweep process, readily observable at both levels. An alternate Reynolds decomposition of the instantaneous turbulent fields, using the conditionally averaged signals, allowed the relative importance of large- and small-scale contributions to momentum and heat fluxes to be estimated. The results were found to be in good agreement with comparable studies.
- Antonia, R. A., Browne, L. W. B., Bisset, D. K., and Fulachier, L.: 1987, ‘A Description of the Organized Motion in the Turbulent Far Wake of a Cylinder at Low Reynolds Number’,J. Fluid Mech. 184, 423–444.
- Antonia, R. A. and Fulachier, L.: 1989, ‘Topology of a Turbulent Boundary Layer with and without Wall Suction’,J. Fluid Mech. 198, 429–551.
- Bergström, H. and Högström, U.: 1989, ‘Turbulent Exchange above a Pine Forest, II. Organized Structures’,Boundary-Layer Meteorol. 49, 231–263.
- Bisset, D. K., Antonia, R. A., and Browne, L. W. B.: 1990, ‘Spatial Organization of Large Structures in the Turbulent Far Wake of a Cylinder’,J. Fluid Mech. 218, 439–461.
- Bogard, D. G. and Tiederman, W. G.: 1986, ‘Burst Detection with Single-Point Velocity Measurements’,J. Fluid Mech. 162, 389–413.
- Brunet, Y. and Raupach, M. R.: 1987, ‘A Simple Renewal Model for Transfer in Plant Canopies’, inFlow and Transport in the Natural Environment: Advances and Application. Poster Abstracts, International Symposium, Canberra, 31 Aug.–04 Sept. 1987, P4, 2 p.
- Collineau, S. and Brunet, Y.: 1993, ‘Detection of Coherent Motions in a Forest Canopy, Part I: Wavelet Analysis’,Boundary-Layer Meteorol. 65, 357–379.
- Finnigan, J. J.: 1979, ‘Turbulence in Waving Wheat, II. Structure of Momentum Transfer’,Boundary-Layer Meteorol. 16, 213–236.
- Gao, W., Shaw, R. H., and Paw U, K. T.: 1989, ‘Observation of Organized Structure in Turbulent Flow within and above a Forest Canopy’,Boundary-Layer Meteorol. 47, 349–377.
- Paw U, K. T., Brunet, Y., Collineau, S., Shaw, R. H., Maitani, T., Qiu, J., and Hipps, L.: 1992, ‘Evidence of Turbulent Coherent Structures in and above Agricultural Plant Canopies’,Agric. For. Meteorol. 61, 55–68.
- Phong-Anant, D., Antonia, R. A., Chambers, A. J., and Rajalopalan, S.: 1980, ‘Features of the Organized Motions in the Atmospheric Surface Layer’,J. Geophys. Res. 85, 424–432.
- Rajalopalan, S. and Antonia, R. A.: 1982, ‘Use of a Quadrant Analysis Technique to Identify Coherent Structures in a Turbulent Boundary Layer’,Phys. Fluids. 25, 949–956.
- Raupach, M. R., Finnigan, J. J., and Brunet, Y.: 1989, ‘Coherent Eddies in Vegetation Canopies’, Proc. Fourth Austral. Conf. Heat & Mass Transfer, Christchurch, N-Z, 9–12 May 1989, 75–90.
- Schols, J. L. J.: 1984, ‘The Detection and Measurement of Turbulent Structures in the Atmospheric Surface Layer’,Boundary-Layer Meteorol. 29, 39–58.
- Shaw, R. H., Tavangar, J., and Ward, D. P.: 1983, ‘Structure of the Reynolds Stress in a Canopy Layer’,J. Clim. Appl. Meteorol. 22, 1922–1931.
- Shaw, R. H., Paw U, K. T., and Gao, W.: 1989, ‘Detection of Temperature Ramps and Flow Structures at a Deciduous Forest Site’,Agr. For. Meteorol. 47, 123–138.
- Shaw, R. H. and Zhang, X. J.: 1992, ‘Evidence of Pressure-Forced Turbulent Flow in a Forest’,Boundary-Layer Meteorol. 58, 273–288.
- Subramanian, C. S., Rajalopalan, S., Antonia, R. A., and Chambers, A. J.: 1982, ‘Comparison of Conditional Sampling and Averaging Techniques in a Turbulent Boundary Layer’,J. Fluid Mech. 123, 335–362.
- Detection of turbulent coherent motions in a forest canopy part II: Time-scales and conditional averages
Volume 66, Issue 1-2 , pp 49-73
- Cover Date
- Print ISSN
- Online ISSN
- Kluwer Academic Publishers
- Additional Links
- Industry Sectors