speed increases with height, Hc can be thought of as the level in the stable atmosphere where the

flow has sufficient kinetic energy to overcome the stratification and rise to the height of the

terrain. However, in determining the amount of plume material in the terrain-following state at a

receptor, it is only important to know the lowest height in the flow where the kinetic energy is

sufficient for a streamline to just maintain its height above the surface, i.e. terrain-following.

Whether it will be deflected further and reach the top of some specified hill is not important for

determining the amount of plume material in the terrain-following state for this receptor.

Venkatram et al. (2001) first proposed the idea that for real terrain, often characterized by a

number of irregularly-shaped hills, Hc should be defined in relation to a terrain-following height

at each receptor location. This is in contrast to the more classical definition where Hc is defined

in relation to the top of a single representative hill upon which may reside many receptor


In the AERMOD approach, plume height, receptor elevation, and Hc will determine how

much plume material resides in each plume state. For a receptor at elevation zt and an effective

plume at height he the height that the streamlines must reach to be in the terrain-following state is

zt+he . Therefore the terrain height of importance, hc, in determining Hc is simply equal to this

local terrain-following height. Any actual terrain above hc = zt + he is of no consequence to the

concentration at the receptor. This receptor and plume dependent approach to computing Hc

assumes that there is sufficient terrain affecting the flow near the receptor to vertically force the

streamlines to the terrain-following level. If the actual surrounding terrain does not reach the

height of the terrain-following state, hc is calculated from the highest actual terrain height in the

vicinity of the receptor. Therefore, for any receptor, hc is defined as the minimum of the highest

actual terrain and the local terrain-following height. Given hc, the dividing streamline height is

computed with the same integral formula found in the CTDMPLUS model.


where C x y z is the concentration in the absence of the hill for stable conditions. In s r r r { , , }

convective conditions, H and c p = 0 ϕ = 0.

As described by Venkatram et al. (2001), the plume state weighting factor f is given by

f ( ). When the plume is entirely below Hc the concentration p = 0.5 1+ ϕ (ϕ ) p = 1.0 and f = 1.0

is determined only by the horizontal plume. When the plume is entirely above the critical

dividing streamline height or when the atmosphere is either neutral or convective,

ϕ Therefore, during convective conditions the concentration at an elevated p = 0 and f = 0.5.

receptor is simply the average of the contributions from the two states. As plumes above Hc

encounter terrain and are deflected vertically, there is also a tendency for plume material to

approach the terrain surface and to spread out around the sides of the terrain. To simulate this

the estimated concentration is constrained to always contain a component from the horizontal

state. Therefore, under no conditions is the plume allowed to completely approach the terrain



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