where and Fvo

σ { } [σ ] 2, the surface value of the lateral turbulence, is vm im vo 2 z = MIN 2 ; 0.25m2 s2

equal to 3.6 u*

2. This linear variation of F 2

vm with z is 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 Fvc were 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 Fvc

in 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 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 } < 0.25 m2 s-2, then Fvc

2{zic } is persisted upward from zic. Furthermore, it

was found that a value of the order Fvc

2 = 0.25 m2 s-2 provided consistently good model

performance (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 to

produce these curves are consistent with the relationship between zim and u* 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 m

and 100 m the values Fvo

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






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