Two-Zone Convective Scaling of Diffusion in Complex Terrain

Abstract

A “two-zone” convective scaling equation is developed that allows for a step change in scaling parameters. The results of the Lompoc Valley Diffusion Experiment, conducted at coastal site in California under variable cloud cover, are compared to the two-zone model and five other parameterizations obtained from the literature. The two-zone method is shown to unambiguously select appropriate convective scaling parameters for the partially cloud-covered cases. Its accuracy is comparable to a statistical approach that relies on in situ turbulence measurements and a Lagrangian time scale derived from the data. While the two-zone method works for maximum concentration predictions at any downwind distance, all models underpredict plume width and crosswind-integrated concentration measured at long ranges. It is speculated that the observed enhancement is due to near-surface topographic flows that spread the plume at the surface but maintain centerline concentration.