On Plume Dispersion over Roughness Elements after a Ground-Level Line Source in Crossflows*

Pollutant plume transport over urban areas is commonly observed nowadays but its parameterization for urban settings is rather limited. An analytical solution to the vertical dispersion coefficient is developed in terms of downwind distance after the source, turbulent boundary layer thickness and drag coefficient (in response to building blocks). It is then validated by wind tunnel experiments for flows over arrays of roughness elements (ribs and cubes of different sizes/separations). Water vapor is atomized by ultrasonic to simulate tracer transport after a ground-level line source in crossflows. The tracer concentrations over various rough surfaces consistently exhibit the conventional Gaussian distribution while the vertical dispersion coefficient agrees well with the newly derived analytical solution (coefficient of determination is 0.93). Comparing the fluxes in the streamwise and vertical direction helps elucidate the transport processes and the influence of surface roughness on turbulent pollutant dispersion. The analytical solution and wind tunnel result collectively suggest an improved parameterization of vertical dispersion coefficient for the air quality forecast in urban areas.