Parameterization of the eddy diffusivity in a dispersion model over homogeneous terrain in the atmospheric boundary layer

Abstract

Parameterizations of the vertical eddy diffusivity over homogeneous terrain in neutral, stable and unstable conditions for the entire atmospheric boundary layer are proposed using the mixing length theory. The formulation utilizes the theoretically derived local mixing length scale that is composed of three component length scales. Proposed profiles of the eddy diffusivity in various stability conditions are compared to those reported in the literature. The proposed profiles of the eddy diffusivity are used in a generalized analytical model for dispersion of pollutants released from a continuous source in the atmospheric boundary layer. The dispersion model describing the crosswind integrated concentrations is briefly presented. By considering the proposed and other commonly applied parameterizations of eddy diffusivity, the dispersion model is evaluated with the tracer observations obtained from Copenhagen diffusion experiment in unstable conditions, Prairie Grass experiment in both unstable and stable conditions and Hanford experiment in stable conditions. The dispersion model with proposed parameterizations of the eddy diffusivity is performing reasonably well with the observations and demonstrates throughout a consistent and good performance in the concentration estimation for elevated and surface releases from a continuous point source in various stability conditions. Though the magnitudes of the various eddy diffusivities are different from the proposed one, the profiles of all parameterizations in terms of the shape have almost similar characteristics. The differences in the magnitudes of diffusion produced by various parameterizations cause minor but noticeable changes in the simulation of ground level concentrations from the dispersion model. In very stable conditions, the agreement of the model evaluations with measurements is less satisfactory and none of the parameterizations used here including proposed one is adequate to describe the atmospheric dispersion process in these conditions.