Dispersion Of Air Tracers Into And Within A Forested Area

Dispersion of fluorescent powders, dyes and spores from a clearcut area into and within a 45-year old Douglas-fir forest was studied under various meteorological conditions. A total of 111 rotorods samplers were used to trap the tracers within a canopy volume of 20 m height, 210 m width and 270 m length. Wind speed, direction and air temperature were monitored on five towers on a line perpendicular to the forest wall. Twenty-four multiple location releases were made during 1968 and 1969. Part 1 describes the experimental site, procedures and methods, Part II contains data collected and Part III results, discussion and conclusions. (Author).

New semi-empirical models are obtained of the mean momentum transport processes in and above forests for two contrasting micrometeorological problems: (1) the equilibrium air flow in forests far upwind of any inhomogeneity in the fetch, and (2) the nonequilibrium air flow in the transition region of the surface layer flow disturbed by the entrance into a forest. The study is confined to neutral stability conditions. Experimental data are from studies in eight forests and a wind tunnel simulation. The equilibrium mean velocity profiles in forest canopies are shown to be two-dimensional. A lateral component of flow increases downward from the top of the canopy. A semi-empirical model of mean velocity components is derived and verified for the upper 90% of the forest canopy depth. In the nonequilibrium flow in the transition region downwind of a forest wall: (a) The logarithmic velocity profile is found to be an empirical approximation for the lower part of the boundary layer, assuming that only the friction velocity varies with downstream distance. An empirical relation for the horizontal variation of the friction velocity is obtained. (b) The mean velocity profiles in a mixing zone located at midcanopy level are shown to have a low-level velocity maximum and horizontal similarity. (Author).