Abstract
Abstract : The diffusion of passive scalar pollutants in the atmosphere can constitute serious environment hazards. Recent studies have pointed to the increased danger associated with extremely high instantaneous point concentrations. We analyzed the local turbulence structure underlying statistical descriptions of turbulent diffusion. Specifically, we analyzed the entrainment of passive scalar across a buoyancy-driven convective boundary layer (CBL) capping inversion using 963 and 1283 large-eddy simulation (LES). In the simulations, a uniform concentration of passive scalar was released in the stable layer above a CBL with a strong capping inversion (963 LES) and a CBL with a weak capping inversion (1283 LES). The local turbulence structure in and around the entrainment layers were analyzed using graphical imaging, single-point statistical measures and conditional statistics. Among other things we found that we found the entrainment of top-down passive scalar to be more rapid with the weaker capping inversion suggesting that entrainment from above is affected by boundary layer growth, and that entrainment was concentrated in low vorticity downdraft regions which transport scalar directly through the capping inversion.
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