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SIMPLE PDF MODELS FOR CONVECTIVELY DRIVEN VERTICAL DIFFUSION
Citation:
Zong-Kai, L. AND G. Briggs. SIMPLE PDF MODELS FOR CONVECTIVELY DRIVEN VERTICAL DIFFUSION. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-88/329 (NTIS PB89206692).
Description:
The mode of vertical velocity in convective boundary layers is usually negative and the probability distribution function(PDF) of w, pw is rarely symmetrlc. Consequently, vertical diffusion from elevated sources is usually asymmetric and exhibits a descending mode of concentration, causing higher peak surface concentration than predicted by Gaussian models. The main concentration (x) effects, we argue, can be modeled using simple PDF diffusion models, with tracers responding to Pw at the source height with straight line trajectories and simple reflection at the surface and zi, the mixing depth. he critical element is the choice of pw. Two Pw models are offered, a bi-Gaussian (BG) and a Gaussian-ramp (GR) formulation. oth have some observational support, and the resulting PDF models are mathematically tractable. nalytical solutions for key variables are given; these show some surprising contrasts between the BG and GR models, but both can approximate laboratory and numerical modeling results for Sxdy patterns. he PDF models are compared with diverse selection of atmospheric turbulence measurements. ecent lidar measurements of oil fog plumes are presented that show a large variability in Sxdy patterns, even with substantial averaging periods. imple adaptation of the BG model is recommended that fits the average peak Sxdy and distance of occurrence as observed so far.