A Radar Study of Turbulent Diffusion in the Lower Atmosphere

Abstract

A new technique for the experimental study of atmospheric turbulent diffusion is presented and applied to the study of diffusion in the lower 3 km of a cloudless summer atmosphere. This new technique makes use of Doppler radar and of radar reflecting chaff. The design and operation of a new chaff releasing device is presented. This device makes possible the creation of vertical columns of chaff through several kilometers of the atmosphere. The chaff is used to model atmospheric contaminants in diffusion measurements and as an air motion tracer in Doppler studies of turbulent air motions. The shear is observed to have a strong effect on horizontal turbulent diffusion, with the diffusion rates frequently being an order of magnitude greater in the direction of the shear than across it. The strong shear enhancement of horizontal diffusion is interpreted to imply shear-generated eddies which are oriented by the shear. Empirical equations are presented which relate the apparent eddy diffusivity, the pollutant cloud size and the time of diffusion. Three distinct periods of chaff cloud growth are seen with the first period being due to small-scale turbulence, the second to energetic shear-induced turbulence of particular orientation, and the third to slower growth as the cloud size surpasses the maximum eddy size. The observed maximum growth rates agree well with the predictions of the shear diffusion theory. The frequently used Gaussian model of turbulent diffusion is compared to the observations and is seen not to hold well in sheared atmospheric situations. The observed concentration distributions are more peaked than the Gaussian, particularly at times when the maximum turbulent eddy size would be expected to be large.