AERMOD Tech Guide
4.1.6 Lateral Turbulence Calculated by the Interface
In the CBL the total lateral turbulence , 2vT , is computed as a combination of a mechanical and convective portion (as was adopted for 2vT ) such that
In the SBL the total lateral turbulence contains only a mechanical portion.
Mechanical Portion of the Lateral Turbulence
The variation with height of the mechanical portion of the lateral turbulence is bounded by its value at the surface and an assumed residual value at the top of the mechanical mixed layer. The variation between these two limits is assumed to be linear. Based on observations from numerous field studies, Panofsky and Dutton (1984) report that, in purely mechanical turbulence, the lateral variance near the surface has the form
where the constant, C, ranges between 3 and 5 with an average value of approximately 3.6. Hicks (1985) supports the form of eq. (41) and the value of 3.6 for C.
Above the mechanically mixed layer, we expect the lateral turbulence to maintain a modest residual level. Hanna (1983) has analyzed ambient measurements of lateral turbulence in stable conditions. He has found that even in the lightest wind conditions, the measurements of v were typically 0.5 m/sec, but were observed to be as low as 0.2 m/sec. AERMOD adopts the lower limit of 0.2 m/sec for v for near-surface conditions (eq. (44)), but uses the more typical value of 0.5 m/sec for the residual v above the mixed layer. Furthermore, we found that a value of the v2= 0.25m2s-2 order provided consistently good model performance (for plumes commonly above zim ) during the developmental evaluation thus supporting the presence of residual lateral turbulence in this layer.
Between the near-surface and the top of the mechanically mixed layer, we assume the v2 varies linearly as.
The linear variation of 2vm with z is consistent with field observations (e.g., Brost et al 1982).
Figure 8 shows how the vertical profile of lateral turbulence changes over a range of mechanical mixing heights. The values of u* used to produce these curves are consistent with the relationship between zim and u* which is found in eq. (12). In the SBL Figure 8 represents the im * total lateral turbulence. In the CBL these curves depict only the mechanical portion of the total lateral variance. This in conjunction with the convective portion eq. (45) constitute the turbulence as expressed in eq. (40)
In very light wind conditions, u* may also be quite small and vo from eq. (43) may be unrealistically small. Based in part on model performance comparisons with data during the developmental evaluation, vm is bounded (when used in calculations of y and thus concentration) as follows
The wind speed u, in eq. (44) is evaluated at the same height as vm .
Convective Portion of the Lateral Turbulence
The convective portion of the lateral turbulence is calculated in AERMOD from:
This constant value of 
in the convective mixed layer is supported by the Minnesota data (Reading et al., 1974 and Kaimal et al., 1976) and by data collected at Ashchunch England (Canghey and Palmer, 1979).
For z > zi , the model linearly decreases vc2 from vc2 { zic } to 0.25 at 1.2 zic and holds constant above 1.2 z . However, if vc2 {zic } < .25 m2s-2 , then vc2 {zic } is persisted upward from zic.
If observations of v are available below zic , then the value at the lowest level is assumed to v ic persist down to the surface and at the highest level up to zic . If the highest observed v is above 1.2 zic then that value is persisted up. For observations which extend to a level between zic and 1.2 zic we linearly extrapolate the highest observed value to 1.2 zic , based on the slope of the reference profile. Above 1.2 zic we persist the value v at 1.2 zuc .
In the SBL the total lateral turbulence contains only a mechanical portion and it is given by eqs. (42) thru (44). Use, by AERMOD, of the same vm expression in the CBL and SBL is done vm to maintain continuity of vm in the limit of neutral stability. vm.40