5.2

Concentration Predictions in the CBLIn AERMOD, the dispersion formulation for the convective boundary layer (CBL) represents

one of the more significant model advances by comparison with existing regulatory models. One

assumes that plume sections are emitted into a traveling train of convective elements - updrafts

and downdrafts - that move with the mean wind. The vertical and lateral velocities in each

element are assumed to be random variables and characterized by their probability density

functions (pdf). The mean concentration is found from the pdf of the position of source-emitted

“particles”; this position pdf in turn is derived from the pdf of the lateral and vertical velocities as

described by Weil et al. (1997); also see Misra (1982), Venkatram (1983), and Weil (1988a).

In the CBL, the pdf of the vertical velocity (

w) is positively skewed and results in a non-Gaussian vertical concentration distribution,

Fz(Lamb 1982). The positive skewness is consistentwith the higher frequency of occurrence of downdrafts than updrafts; for an elevated non-buoyant

source the skewness also leads to the decent of the plume centerline, as defined by the locus of

maximum concentration (Lamb 1982; Weil 1988a). Figure 13 presents a schematic

representation of an instantaneous plume in a convective boundary layer and its corresponding

ensemble average. The base concentration prediction in AERMOD is representative of a one

hour average. Notice that since a larger percentage of the instantaneous plume is effected by

downdrafts, the ensemble average has a general downward trend. Since downdrafts are more

prevalent the average velocity of the downdrafts is correspondingly weaker than the average

updraft velocity to insure that mass is conserved. In AERMOD, a skewed vertical velocity pdf is

modeled using a bi-Gaussian distribution, which has been shown to be a good approximation to

laboratory convection tank data (Baerentsen and Berkowicz 1984). In contrast to the vertical

component, the lateral velocity pdf is approximately Gaussian (Lamb 1982), and this pdf and the

resulting concentration distribution,

Fy, are assumed to be Gaussian.

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castellano: DISPER CUSTIC DESCAR RADIA italiano:

castellano: DIS CUS DES RAD english: DIS CUS DES RAD

português: DIS CUS DES RAD italiano: DIS CUS DES RAD