An experimental study on atmospheric diffusion

Abstract

A method has been devised by which atmospheric diffusion can be studied over a few kilometres range with rather good precision at only moderate expense. The method which is based on smoke puff photography is especially well suited for thermally stable conditions. It is possible to study the effect of both the low frequency part and the high frequency part of the Lagrangian spectrum of turbulence on the diffusion. More than 100 complete experiments have been performed at two places in Sweden at three different levels of release.The vertical standard deviation of matter is found to be a function of the following stability parameter: (∂θ/∂z)/u2f, where ∂θ/∂z is the local vertical gradient of potential temperature, and uzf is “the free wind”, that is the wind just above the friction layer. A comprehensive formula, or rather a system of formulae, for the variation of the vertical standard deviation (sampling time about one hour) with distance from the source, stability and height above the ground of the point of release, has been developed. The formulae are believed to have a very general application, the only site parameter being necessary to know is the roughness length, z0. The formulae have been successfully tested against results from various tracer experiments described in the literature.The variance of the horizontal distribution of matter for one hour sampling time is found to be the sum of two parts: one which is independent of stability and one which is dependent on stability in much the same way as is the vertical variance.The distribution of matter from a ground level release is described by new formulae. The result is in good agreement with experimental and theoretical evidence in the literature.