Turbulence data were collected above and within the canopy of a black spruce forest in central Canada (in the southern study area of BOREAS). Streamwise (<I>u</I>), transverse (<I>v</I>) and vertical (<I>w</I>) wind speed were measured with 3-component sonic anemometers. Scalar transport within the canopy was studied using the heat fluxes from the sonic anemometers, and water vapour fluxes from two open-path infra-red gas analysers. As a result of instrumental errors, no CO<SUB>2</SUB> fluxes were obtained. Tree height (<I>h</I>) is approximately 10 m, and leaf area index is about 5. The narrow tree crowns produce a very open canopy. A zero plane displacement of 0.45 <I>h</I> was obtained from the mean momentum flux profile. This is lower than typical values, as was the attenuation coefficient, <I>a</I>, calculated from the mean wind speed profile, of 1.9. These probably result from sparse foliage in the upper canopy, allowing relatively unrestricted air movement. The roughness length was found to equal 0.11 <I>h. </I>Profiles of mean velocity statistics were constructed to describe the turbulence regime within the forest canopy. Turbulence intensities were found to be high in the lower levels of the canopy. Above the canopy, skewness and kurtosis of the three velocity components had values close to those for a Gaussian distribution. In contrast, within the canopy, skewnesses were negative for <I>w, </I>and positive for <I>u. </I>Kurtosis values were high (up to 12 close to the ground), and integral Eulerian length scales revealed eddies with dimensions of the order of canopy height at the top of the canopy. These indicate that the air flow within the canopy is characterised by intermittent gusts of air penetrating from above the canopy. The turbulence profiles, when scaled by <I>h</I> or fraction velocity, agree well with profiles from a wide range of other canopies. Non-dimensionalised variances of <I>w, </I>temperature and water vapour fluctuations, measured above the canopy, were found to vary with stability in accordance with Monin-Obukhov similarity theory. Quadrant analysis showed the transport of momentum and scalars to be intermittent, with 50% of momentum transport occurring in 3 - 8% of the time. Instantaneous events six times larger than the mean scalar flux transported 20 - 50% of the heat flux in 3 - 4% of the time, and 50 - 80% of the water vapour flux in 12 - 20% of the time. Transport mechanisms were similar for all three entities, being dominated by ejections (updraughts) above the canopy, and gusts (downdraughts) within the canopy.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:651955 |
| Date | January 1996 |
| Creators | Hale, Sophie E. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14962 |
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