Medium-range Plume Dispersion Calculated Directly from Surface-derived Shear and Turbulence Estimates

Classical plume dispersion models estimate lateral and vertical dispersion of individual pollution plumes primarily in terms of empirically-derived Pasquill-Gifford stability categories. Here we describe a more scientifically fundamental method for inferring the lateral plume dispersion based on the assumption that plume dispersion is governed by a combination of random, small-scale turbulent dispersion effects enhanced by lateral wind shear. Unfortunately, wind shear is difficult to accurately measure in the turbulent Planetary Boundary Layer (PBL), however, the vertical shear of the horizontal winds can be estimated from surface-based measurements of turbulence and larger-scale temperature gradients. It is proposed that shearing motions of horizontal winds in the planetary boundary layer result from a superposition of two effects: 1) Ekman-layer induced shear (proportional to boundary layer depth and turbulent intensity); and 2) Baroclinic (i. e. thermal wind) effects (proportional to horizontal temperature gradients). Both of these effects can be inferred from surface measurements. Here estimates of turbulence, wind shear and lateral plume dispersion derived only from surface-based measurements are compared with shear and lateral dispersion calculated using winds directly sampled using radar profilers in the Hudson Valley in upstate New York. It is found that lateral dispersion of plumes is dominated by shearing motions at distances greater than several kilometers from the release point. In contrast, plume dispersion close to release is dominated by turbulent diffusion coefficients that can be inferred from wind speed measurements and similarity-theory descriptions of turbulence in the lower PBL. The magnitude of shear in the PBL can be estimated using thermal wind theory coupled with Ekman-layer physics if several surface temperature measurements are available for assessing horizontal temperature gradients. Therefore it is proposed that the minimum surface observations required in order to accurately infer plume dispersion must include wind velocities at one site, and surface temperature measurements at three or more sites surrounding the wind measurement site in order to estimate lateral temperature gradients. Current guideline plume models require only wind measurements at one site to estimate dispersion, and thus the very important shearing effects on plume dispersion cannot be accurately accounted for.