Experimental observations on the relationship between diffusion characteristics and spectral coherence

Abstract

The direct relationship of spectral coherence to space-time correlation is used to infer various characteristics of diffusion processes. Data gathered at three levels and at various downwind instrumental separations, along the mean wind direction are analyzed in terms of horizontal coherence of the longitudinal and lateral components of the wind. The analysis is within the framework of Davenport's ( Q. Jl R. met. Soc. 87, 194–211. 1961) empirical representation of coherence and a semi-empirical formulation of the decay parameter. The characteristics of the decay parameter are found to be consistent with its latest formulation which incorporates the relative importance of turbulent intensity and lateral diffusion. Atmospheric stability is seen to be the controlling factor in the results. The noted characteristics of the decay parameter are used to confirm its inverse relationship to Pasquill's β. This, in turn, confirms the usefulness of the determination of the latter from single point observations. Furthermore, Pasquill's nondimensional lateral dispersion function is shown to be stability dependent which must be borne in mind when calculating the lateral dispersion parameter of Gaussian models.