Short-range vertical dispersion from a ground level source in a turbulent boundary layer

The short-range vertical dispersion of a passive containment in a neutrally stratified turbulent boundary layer can be usefully characterised by a number of approaches, such as a vertical diffusivity, Kz, a vertical Gaussian dispersion parameter, σz, and an entrainment velocity, we. σz and we are approximately related by we∝dσz/dt. While the Gaussian model is widely used in most air quality applications, the entrainment velocity is widely used in wind tunnel studies and models for the dispersion of hazardous gases. This paper summarises experiments in three simulated rough-wall atmospheric boundary layers (two in the US and one in the UK), which lead to the conclusion that for passive releases the entrainment coefficient α in the formula we=αu* is α=0.65±0.05. This value is consistent with a recent analysis of the Prairie Grass data (out to x=100 m, where vertical profiles were obtained) that gave α=0.63±0.05. Somewhat surprisingly we were not able to locate a theoretical analysis that could reproduce these results without recourse to some empirical input.