Modeling and Parameterization of Near-Source Diffusion in Weak Winds

A critical assessment is made of several different approaches that can be used for modeling near-source diffusion in weak winds, including the calm condition. For the convective boundary layer, the statistical theory approach is used in conjunction with the mixed-layer similarity scaling to compute mean concentrations in the near field. The maximum ground-level concentration and its location relative to a continuous point source are given as functions of the source height and wind speed, all made dimensionless through the use of the mixed-layer similarity scaling. The modal and the suggested parameterizations for turbulence and diffusion are only valid for the convective boundary layer over a flat and homogeneous terrain. For near-neutral and stable conditions, the gradient-transport approach is used. Some analytical models for the near-field concentration under the calm condition are discussed. Recommendations are made for the formulation of more sophisticated numerical models of near-source diffusion in weak winds. Also needed are the systematic experiments in the field and the laboratory for the purpose of testing and validating the near-source, weak-wind dispersion models.