The effect of streamwise diffusion on ground-level concentrations

This paper examines the effect of streamwise diffusion on the distribution of mean scalar concentrations associated with surface releases in the atmospheric surface layer. We use a two-dimensional (2D) (streamwise-vertical) Lagrangian stochastic model to simulate dispersion. Before applying the model, we test its validity by comparing model results of mean concentration and streamwise mass flux density with wind tunnel data reported by Raupach and Legg (1983, Journal of Fluid Mechanics 136, 111–137). The results from simulations conducted with the 2D stochastic model indicated that: (1) inclusion of streamwise dispersion in a model provides a plausible explanation for the “−2” asymptote observed in Project Prairie Grass for the fall-off of ground-level concentrations under unstable conditions, and (2) streamwise diffusion plays a minor role in dispersion under stable conditions.