4.1.6.2 Convective Portion of the Lateral Turbulence

The convective portion of the lateral turbulence within the mixed is constant and calculated

as:

This constant value of σ is supported by the Minnesota data (Readings et al. 1974; vc 2 w2 0 35 * = .

Kaimal et al. 1976) and by data collected at Ashchurch England (Caughey and Palmer 1979).

36

For z > zic , the model linearly decreases Fvc

2 from Fvc

2{ zic } to 0.25 at 1.2 zic and holds Fvc

2

constant above 1.2 zic. However, if Fvc

2{zic } < .25 m2 s-2, then Fvc

2{zic } is persisted upward from

zic.

4.2 Vertical Inhomogeneity in the Boundary Layer as Treated by the INTERFACE

AERMOD is designed to treat the effects on dispersion from vertical variations in wind and

turbulence. Consideration of the vertical variation in meteorology is important for properly

modeling releases in layers with strong gradients, for capturing the effects of meteorology in

layers into which the plume may be vertically dispersing, and to provide a mechanism (in the

CBL) by which sources that are released into or penetrate into an elevated stable layer can

eventually re-enter the mixed layer. However, AERMOD is a steady-state plume model and

therefore can use only a single value of each meteorological parameter to represent the layer

through which these parameters are varying. Thus, the model "converts" the inhomogeneous

values into equivalent effective or homogeneous values. This technique is applied to u, FvT, FwT,

M2/Mz and the Lagrangian time scale. The effective parameters are denoted by a tilde throughout

the document (e.g., effective wind speed is denoted by u~ ).

Fundamental to this approach is the concept that the primary layer of importance, relative to

receptor concentration, is the one through which plume material travels directly from source to

receptor. Figure 10 presents a schematic illustration of the approach AERMOD uses to

determine these effective parameters (" is used to generically represent these parameters). The

effective parameters are determined by averaging their values over that portion of the layer that

contains plume material between the plume centroid height, Hp {x}, (a simplified surrogate for

the height of the plume’s center of mass) and the receptor height (zr). In other words, the

averaging layer is determined by the vertical half-depth of the plume (defined as 2.15 F

z {xr}

where xr

 

 

 

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