Wind Tunnel simulation of diffusion in a convective boundary layer

A study of turbulent dispersion of passive tracers in unstable boundary layers, conducted in the Meteorological Wind Tunnel of the Fluid Dynamics and Diffusion Laboratory at Colorado State University, is described. The measured mean and turbulent velocities are found to be similar to those measured in atmospheric convective boundary layers. The diffusion pattern, from ground-level and elevated sources over both a smooth floor and a rough floor, is found to be the same as that measured in the water-tank experiments of Deardorff and Willis (1975) and in numerical models. The measurements show an initial rapid descent of plumes from elevated sources and a subsequent plume rise at t^* > 0.5 h/w^*. Ground-level concentrations from elevated sources are found to be larger, at certain distances from the source, than those from a ground-level source of equal strength. The measurements of the cross-wind spread _v are in agreement with the Prairie Grass measurements and confirm earlier predictions that the initial cross-wind spread for ground-level sources is larger than that for elevated sources.