Dispersion Parameters over Forested Terrain

Abstract

A unique set of micrometeorological data was obtained during a 1967?70 multidisciplinary environmental field program in a tropical forest environment. The program was under the sponsorship of the Advanced Research Project Agency (ARPA) and was known as Project TREND (Tropical Environmental Data). Wind variability parameters, which characterize the dispersion process within and above the forest canopy and over a nearby clearing under different stability conditions, were derived. Based on relations between σy and σz, it was found that in neutral condition, σz ≈ 2Cd over the observed forest surface. This is in agreement with ?universal? ratios in other studies over relatively smooth terrain. Over the clearing, σz must be increased to account for ?memory? of greater roughness upstream. Ratios of the values of σz found in unstable conditions to those in neutral conditions agree with other studies. Values of turbulence intensity based on wind speed measurements support the above findings and show that dispersion rates within the forest canopy would be lager, relative to the mean transport velocity, than in the free air. A useful correspondence was found between Pasquill stability categories and the Richardson number or Monin-Obukhov z/L parameter. The ranges of those parameters corresponding to the Pasquill categories were found to be much narrower than those reported from studies over smoother terrain.