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UBC Theses and Dissertations

Atmospheric turbulence within and above a coniferous forest Lee, Xuhui


An experiment to study the exchange processes within and above an extensive coniferous forest of Douglas-fir trees was conducted on Vancouver Island during a two-week rainless period in July and August 1990. The stand, which was planted in 1962, thinned and pruned uniformly in 1988, had a (projected) leaf area index of 5.4 and a height of h = 16.7 m. The experimental site was located on a 5° gentle slope. The primary instrumentation included two eddy correlation units which were operated in the daytime to measure the fluctuations in the three velocity components, air temperature and water vapour density. One unit was mounted permanently at a height of 23.0 m (z/h = 1.38) and the other at various heights of (z/h in brackets) 2.0 (0.12), 7.0 (0.42), 10.0 (0.60), and 16.7 m (1.00) with two to three 8-hour periods of measurement at each level. Profiles of wind speed and air temperature were measured continuously during the experimental period at heights of 0.9, 2.0, 4.6, 7.0, 10.0, 12.7, 16.7 and 23.0 m using sensitive cup anemometers and fine wire thermocouples, respectively. Radiation regimes and air humidity were measured both above and beneath the overstory of the stand. The vertical structure of the stand affected, to a great extent, the vertical distributions of the velocity statistics (wind speed, variance, turbulence intensity, Reynolds stress, skewness and kurtosis), air temperature, sensible and latent heat fluxes. The effect was also evident in the quadrant representation of the fluxes of momentum, sensible heat and water vapour. Negative Reynolds stress persistently occurred at the lower heights of the stand (z/h = 0.12 and 0.42). The negative values were related to the local wind speed gradients and it is believed that the longitudinal pressure gradient due to land-sea/upslope-downslope circulations was the main factor responsible for the upward transport of the momentum at these heights. Energy budget was examined both above and beneath the overstory of the stand. The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. Beneath the overstory, the corresponding figure was 74%. On some days, energy budget closure was far better than on others. [more abstract]

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