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Gary A. Briggs

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, σy and σz, 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 > 0.04zi, where zi 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 σy approximates the x law that applies to buoyant plume rise when F* > 0.06. The residual buoyancy also causes a delay in downward mixing that is proportional to F *. The main consequence of these two effects is that maximum ground concentration is reduced, compared to that from passive plumes, and is independent of wind speed. For smaller F *, the observations indicate that, with an assumed plume rise Δh = 3zi F *⅗, several different Cy parameterizations give satisfactory results, including a Gaussian model.

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Gary A. Briggs

Abstract

A few minor errors in a paper by Hanna are noted and several questions are raised about apparent inconsistencies. One question is why substantial enhancement of σy, by buoyancy was noted for the Bull Run data but was unmentioned for the Kincaid data; information given in the paper suggests similar ranges of X */F * at both plants, but much smaller F * values at Kincaid, which may account for the lack of noticeable buoyancy effects there. The inconsistency between the recommended equation σy/x = 0.6 w */u and the σy/x values recommended for various stability classes (based on u/w * ranges) can be removed by multiplying this σy expression by a factor which accounts for the effect of mechanical turbulence. Finally, it is noted that an identical equation was recently reported to give a good fit to σy from passive sources released above 0.3 h, albeit to much smaller distances than reported by Hanna.

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