where and F

voσ { } [σ ]

2, the surface value of the lateral turbulence, isvm im vo2z=MIN2 ; 0.25m2s−2equal to 3.6

u*

2. This linear variation of F2

vmwithzis consistent with field observations (e.g.,Brost et al. (1982)). In the SBL the total lateral turbulence contains only a mechanical portion

and it is given by eq. (40).

Above the mixed layer, lateral turbulence is expected to maintain a modest residual level.

Hanna (1983) analyzed ambient measurements of lateral turbulence in stable conditions. He

found that even in the lightest wind conditions, the measurements of F

vcwere typically 0.5 m s-1,but were observed to be as low as 0.2 m s-1. AERMOD adopts the lower limit of 0.2 m s-1 for F

vcin near-surface conditions, as discussed below, but uses the more typical value of 0.5 m s-1 for

the residual lateral turbulence above the mixed layer. Above the height of the CBL, the model

linearly decreases F

vc

2from Fvc

2{zic} to 0.25 at 1.2zicand holds Fvc

2constant above 1.2zic.However, if F

vc

2{zic} < 0.25 m2 s-2, then Fvc

2{zic} is persisted upward fromzic. Furthermore, itwas found that a value of the order F

vc

2= 0.25 m2 s-2 provided consistently good modelperformance (for plumes commonly above

zim) during the developmental evaluation (Paine et al.2001) supporting the presence of residual lateral turbulence in this layer.

Figure 9 shows how the vertical profile of lateral mechanical turbulence changes over a

range of mechanical mixing heights, and related friction velocities. The values of

u*used toproduce these curves are consistent with the relationship between

zimandu*which is found in eq.(24). For the SBL Figure 9 represents profiles of the total lateral turbulence. In the CBL these

curves depict only the mechanical portion of the total lateral variance. Note that for

zim =300 mand 100 m the values F

vo

2are less than 0.25 m2 s-2. Therefore the profiles are constant withheight.

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