In urban areas, AERMOD accounts for the dispersive nature of the “convective-like”

boundary layer that forms during nighttime conditions by enhancing the turbulence over that

which is expected in the adjacent rural, stable boundary layer. The enhanced turbulence is the

result of the urban heat flux and associated mixed layer which are estimated from the urban-rural

temperature difference as suggested by Oke (1978; 1982).

In complex terrain, AERMOD incorporates the concept of the dividing streamline (Snyder

et al., 1985) for stably-stratified conditions. Where appropriate the plume is modeled as a

combination of two limiting cases: a horizontal plume (terrain impacting) and a terrain-following

(terrain responding) plume. That is, AERMOD handles the computation of pollutant impacts in

both flat and complex terrain within the same modeling framework. Generally, in stable flows, a

two-layer structure develops in which the lower layer remains horizontal while the upper layer

tends to rise over the terrain. The concept of a two-layer flow, distinguished at the dividing

streamline height (Hc), was first suggested by theoretical arguments of Sheppard (1956) and

demonstrated through laboratory experiments, particularly those of Snyder et al. (1985). In

neutral and unstable conditions Hc = 0.

A plume embedded in the flow below Hc tends to remain horizontal; it might go around the

hill or impact on it. A plume above Hc will ride over the hill. Associated with this is a tendency

for the plume to be depressed toward the terrain surface, for the flow to speed up, and for vertical

turbulent intensities to increase. These effects in the vertical structure of the flow are accounted

for in models such as the Complex Terrain Dispersion Model (CTDMPLUS) (Perry 1992).

However, because of the model complexity, input data demands for CTDMPLUS are

considerable. EPA policy (Code of Federal Regulations 1997) requires the collection of wind



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