Measurements of Boundary Layer Flow Over a Forested Mountain Slope.
As part of a study to measure turbulent transfer processes over a forested mountain slope, wind speed, air temperature, wet bulb depression temperature profiles, wind direction and radiation balance measurements were made. The instrumentation and measurements were evaluated. Based on the evaluation, the wind speed profiles and wind direction measurements were selected for further analysis. Turbulent wind flow in the horizontal surface boundary layer has been shown to be a logarithmic function of height above the zero plane (e.g., Lumley and Panofsky, 1964; Belt, 1968). Although these surface boundary layer models have never been tested over sloping surfaces, they have recently been incorporated into mesoscale models (Mahrer and Pieklke, 1977) replacing models which didn't correctly predict the surface boundary layer flow. The measured wind profile slopes were linear and consistently less than the logarithmic profile law plotted through the highest wind level at all wind speeds, atmospheric stability conditions and wind directions. These deviations from the logarithmic profile law imply that the surface boundary layer equations for flow over horizontal terrain must be modified for flow over slopes. This conclusion was supported by the following evidence summarized from the wind profile analysis. (1) When the theoretical wind profiles were normalized to the wind speed at the highest level, the deviations between the theoretical and measured wind profiles were a function of wind direction. (2) When the wind was blowing across the slope, the deviations of the measured profiles from the theoretical profiles were at a minimum. (3) The deviations occurred at all three towers and, therefore, were not a result of local effects at any one tower. (4) The wind speed differences observed between the towers could not be used to estimate the effects of advection on forested slopes, even though advection of momentum probably does exist. The terms which should be included in the development of a theoretical model which predicts linear wind profiles over forested mountain slopes were suggested. Initially, such a model should be two dimensional, assume a steady -state, and include the mean flow advection terms, a buoyancy term and pressure gradient terms. The advection terms would permit convergence of the wind on the slope. The buoyancy term would allow a flow to develop on the slope due to thermal heating, while a pressure gradient term would provide for the dynamic effects of a non-zero wind gradient at the boundary of the model as the winds are forced up the slope. Further recommendations were made concerning instrumentation for future testing of slope flow models and for estimation of evaporation from Bowen ratio measurements. Recommendations include using 6 to 9 profile levels at heights up to 10 m above the forest. These additional profile levels would give better definition to the small profile gradients above the forest and give a better statistical fit to either the Bowen ratio estimates or to the slope flow models. Profile levels need be no closer than 0.5 m. For testing slope flow models, good estimates of tree heights near each tower are required and measurements should include vector wind direction. Detailed discussions on the use of anemometers, temperature instrumentation, evaluation of the measurements, recommendations for improving the instrumentation, averaged wind profile data and plots of this data are found in five appendices.
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- Physics: Atmospheric Science