AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

5. Terrain Preprocessor (AERMAP)

CTDMPLUS (Perry, 1992), an EPA regulatory model for complex terrain, uses the dividing streamline concept with terrain characterized as individual idealized terrain features. As such, the interaction of plume material with the idealized hill (i.e., plume mass partitioning above and below the dividing streamline height, Hc ) is considered directly in the calculation of concentration at any receptor defined to be on the hill. Since it is particularly difficult to both represent actual complex terrain as a collection of idealized terrain features and associate each receptor with a unique hill, AERMAP (the terrain preprocessor for AERMOD), operating from a receptor’s point of view, samples the landscape around each receptor to objectively specify a representative “hill” height associated with that receptor.

Like the CTDMPLUS terrain preprocessor, AERMAP is also designed to provide the terrain information necessary to calculate Hc (the dividing streamline height and its use in the model is described in section 6.a.). The AERMAP method defines a "height scale" (hc ) that represents the terrain that dominates the flow in the vicinity of the receptor (representative hill height). In other words, hc can be thought of as the height of the terrain surrounding the receptor that will most influence the flow in stable conditions. This height, hc , is not necessarily the highest elevation in the modeling domain nor is it necessarily the actual peak of any individual terrain feature. Use of the height scale (instead of an actual terrain feature height, selected by the user as in CTDMPLUS) to calculate Hc , provides a reasonable and more objective method for calculating the weighting factor, f, in eq (59).

In defining hc for a given receptor, all terrain elevations within the user defined modeling domain and the distances of those elevations from the receptor are considered. Therefore each receptor may have a unique height scale. Consider a domain of interest, and a receptor at (xr ,yr ,zr) for which an associated terrain height scale is needed. The inherent assumption in this objective scheme is that 1) the effect of surrounding terrain on the flow near the receptor decreases with increasing distance and 2) the effect increases with increasing elevation of that terrain. In other words, the "effective elevation", heff , of surrounding terrain is a function of its actual elevation and its distance from the receptor.

The terrain height scale (hc ) is determined for each receptor location ( xr , yr ) by use of the following procedure: The weighted effective height surface (heff ) is calculated for each terrain point ( xt , yt ) in the domain of interest as follows:

Equation (55)
Equation (56)
Equation (57)

For a given receptor, heff is calculated for all terrain points within the modeling domain, thereby creating an effective height surface. This is why it is very desirable to have the terrain information already digitized or in gridded form. The height scale for each receptor is then related to the maximum effective value. Figure 10 gives an example of how this effective height surface is determined for a specific receptor.

Figure 10

Figure 10: Finding hc for a specific receptor ( xr , yr , zr ). For simplicity this figure presents only one direction within the domain of interest. To calculate hc for an actual domain this procedure would have to be performed in all directions about the receptor.

Once the effective height surface is defined through eq. (55), the height scale for a given receptor is defined as the actual height of the terrain point having the largest effective height (terrain with the greatest effect on the receptor). That is, hc is the actual terrain elevation at the location with the maximum heff .

Figure 11

Figure 11: Determination of hc for a single hill and gently sloping terrain

Figure 11 provides an example of how hc is determined for two different cases: 1) a single hill, and 2) gently slopping terrain. These cases demonstrate that this procedure produces a height scale that is consistent with the critical dividing streamline height . Figure 11 shows that for the single hill hc is the hill height; what one would expect for the Hc in this case. For a gentle slope one would expect hc to be close to the height of the receptor. Figure 11 demonstrates that for the gentle slope the height scale is essentially equal to the receptor height.

The height scale is computed by solving eq. (55) at the terrain point associated with the maximum heff such that:

Equation (58)