Dispersion Of Air Tracers Into And Within A Forested Area 2

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).

The cited reports cover Federally-funded research on the use of tracers to study lower atmospheric air pollution movements. The tracers used include sulfur hexafluoride, krypton 85, carbon 14, and other radioactive isotopes. The studies cite the results and techniques used, tracer movement from nuclear power plants, industrial stacks, urban areas, and the detectors used in their measurement. (Contains 55 abstracts).

The effects of meteorological phenomena upon forest produc tivity and forestry operations have been of concern for many years. With the evolution of system-level studies of forest eco system structure and function in the International Biological Program and elsewhere, more fundamental interactions between forest ecosystems and the atmosphere received scientific atten tion but the emphasis on meteorological and climatological effects on forest processes remained. More recently, as recogni tion has developed of potential and actual problems associated with the atmospheric transport, dispersion, and deposition of airborne pollutants, the effects of forest canopies upon boundary-layer meteorological phenomena has come under scientific scrutiny. Looking to the future, with rising atmospheric con centrations of C02 and increasing competition for the finite fresh-water resources of the earth, interest in the role of forests in global C02 and water balances can also be expected to intensify. Thus, the nature of forest canopy-atmosphere interac tions, that is to say, the meteorological phenomena occurring in and above forest canopies, are of importance to a wide variety of scientific and social-issues. Demands for forest meteorological information currently exceed levels of knowledge and given the economic constraints of science in general and environmental sciences in particular, chances for major improvements in scien tific support in the near future are slim. Unfortunately, studies of environmental phenomena in and above forests are costly and logistically difficult. Trees, the ecological dominants of forest ecosystems, are the largest of all terrestrial organisms.