SUMMARYEnergy dissipation rates and the eddy fluxes of sensible heat, momentum, and total kinetic energy have been measured above a non-homogeneous surface. Calculations were made from simultaneous observations of fluctuations in air temperature and the orthogonal components of wind velocity at heights of 16 m and 40 m. In all thermal stratifications, dissipation rates given by the Kolmogoroff structure function are in close agreement with direct measurements of the energy extracted from the mean flow. During stable stratification, energy losses due to work done against gravity and to the divergence of the vertical flux of turbulent kinetic energy are nearly an order of magnitude smaller than the dissipation. In unstable stratification, energy losses due to the flux divergence are of the same order as the dissipation and are approximately three times larger than the energy produced by buoyancy. Estimates ofthevertical heat flux from the Obukhov structure function compare favourably with directly-measured fluxes if the constant in the structure function is assigned a value of 0.7 in lapse conditions and a value of 1.6 in inversion conditions. The measurements also show that the ratio of the diffusivities of heat and momentum at a height of 16 m is significantly greater than unity in unstable stratification and significantly less than unity in stable stratification. Finally, the new measurements of dissipation rate are shown to be consistent with values obtained in previous studies.

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IntroductionDuring the summer of 1956, an extensive empirical investigation of atmospheric diffusion was conducted at a field site near O'Neill, Nebraska under sponsorship of the Air Force Cambridge Research Center (Project Prairie Grass). The field site was carefully selected to provide level terrain, uniform vegetation cover (maintained at an average height of about 6 cm. by mowing), and practically complete freedom from large roughness elements. Objectives of the observational program included determination of the rate of diffusion of a gas tracer, emitted from a continuous point source near ground level, as a function of meteorological parameters. As a participant in Project Prairie Grass, the Massachusetts Institute of Technology made measurements of average gas concentrations over a range of 800 m. and of certain meteorological parameters related to the structure of turbulence. The experiments were conducted in a wide variety of weather conditions (wind speed, cloud cover, thermal stratification, etc.). A preliminary analysis has been made of the diffusion measurements and of some of the meteorological observations. This paper briefly describes the conduct of the diffusion experiments; summarizes basic features of plume structure and behavior thus observed; and, discusses empirical relations between diffusion and 2 simple meteorological parameters-the standard deviation of azimuth wind direction and the stability ratio.