Analytical parameterizations of diffusion: the convective boundary layer.

Abstract

A brief review is made of data bases which have been used for developing diffusion parameterizations for the convective boundary layer (CBL). A variety of parameterizations for lateral and vertical dispersion, sigma y and sigma z, are surveyed; some of these include mechanical turbulence, source height, or buoyancy effects. Recommendations are made for choosing among these alternatives, depending on the type of source. Because observations of passive plumes indicate that the Gaussian model does a poor job of describing vertical diffusion in the CBL, alternative models for predicting dimensionless crosswind integrated ground concentration, Cy, are reviewed and compared. These include an analytical equation which closely approximates laboratory results; this equation can be applied to any source height more than 0.04zi, where 3i is the mixing depth. An analysis of a limited amount of buoyant plume data indicates that a radically different approach is needed when the dimensionless buoyancy flux, F*, exceeds 0.1. Such plumes impinge on the 'lid' of the mixing layer before ground impact occurs, and residual plume buoyancy causes enhanced lateral spreading under the lid; the observations indicate that sigma y approximates the X2/3 law that applies to buoyant plume rise when F* is more than 0.06. -from Author