Low-frequency wind fluctuations within an irregular forest

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A three-dimensional array of five Gill propeller anemometers was placed in an irregular forest setting consisting mostly of birch and aspen ∼10-15 m tall interspersed among several oak trees ∼25 m in height. Each propeller was sampled at 5-s intervals for a period of 150 min during each of six trials when the mean wind speed was < 1 s-1 in the array. Analysis of the data indicates that a secondary re-circulation, analogous to that observed in cavities in laboratory experiments, develops below the tops of the birch and aspen during gusts. This circulation drifts with speeds less than the mean wind speed with a component opposite to the mean wind. Fluctuations in both the magnitude of the curl and the divergence lag fluctuations in wind speed and the divergence lags the curl. Phase velocities show dispersion, increasing linearly to 60 cycles h-1 with a possible maximum near 150 cycles h-1. In addition, the wind gusts are reflected from the ground or trees with velocities somewhat smaller than the mean wind speed. These reflections produce a ripple in the power spectrum of the wind speed. These results and related data are used to describe the structure of the three-dimensional flow within the forest. © 1990.

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Agricultural and Forest Meteorology