A laboratory study of dispersion from an elevated source within a modeled convective planetary boundary layer

Abstract

Dispersion from an elevated source has been studied by means of a laboratory model of the convective planetary boundary layer. Results are presented in terms of a continuous point source located within a thermally convecting field and in the presence of a simulated uniform mean wind. Quantities measured in this study are nondimensionalized through the use of free-convection similarity scaling. From a source height of z s ∼- 0.24h , where h is the height of the convectively mixed layer, the average plume concentration maximum descends to ground level at a downstream distance x ∼- 0.4h/(w ∗U) , and remains there for a comparable distance before being carried high up into the mixed layer. (The convective velocity scale w ∗ divided by the mean wind U serves as a dimensionless stability parameter.) It is shown that a Gaussian plume formulation cannot adequately describe this evolving vertical concentration distribution. However, the Gaussian formulation provides an adequate estimate of the vertically-integrated cross-wind concentration distribution. The ground-level concentration field in the X-Y plane indicates that the mean plume first reaches ground level at x ∼- 0.15h/(w ∗U) and attains its maximum concentration near x ∼- 0.4h/( ∗/U) . Power-law formulations for the Gaussian diffusion parameters are made for small downstream distances.