Diffusion in Shear Flow

Abstract

In the Green Glow program conducted near Richland, Washington, during the summer of 1959, 26 diffusion experiments were conducted during nocturnal inversions. The tracer was released near ground level. Samplers were placed at 1.5 m above ground at 533 positions on six sampling arcs, the radii of which were 200 m, 800 m, 1.6 km, 3.2 km, 12.8 km, and 25.6 km. In addition to the ground sampling network, poles or towers were erected at five points, 8 deg apart, on each of the four inner arcs. Fifteen samplers were mounted on each pole or tower, the top level increasing from 27 m on the 200 m arc to 62 m on the 1.6 km and 3.2 km arcs. The vertical measurements of tracer dosage made during Experiment No. 3 suggest that during this experiment, conducted during one of the stronger inversions, the mean wind direction changed significantly with height in the lowest 200 ft. Direct measurements of mean wind direction confirm this notion. A diffusion model is discussed in which the tracer dosage at a point downwind from the source is given by the normal frequency function of two variables (the lateral and vertical coordinates of the point) and the correlation coefficient between the lateral and vertical coordinates of the tracer particles. Experiment No. 3 is examined in terms of this model and is found to satisfy well the major conditions of the model. In addition, the model and the empirical evidence suggest that care must be exercised in the interpretation of computations of diffusion parameters made from photographs of visible tracers.