where NN = 0.7N with N given by eq. (58). N and u are evaluated initially at stack height. Once

plume rise has been computed, subsequent plume rise estimates are made (iteratively until

convergence) by averaging the u and N values at stack top with those at h h . Equation (95) s s + Δ 2

is used for downwind distances that are less than the distance to final rise (xf). Beyond xf, )hs

remains constant. The distance at which the stable plume reaches its maximum rise is given


Upon substituting eq. (96) for x in eq. (95) the maximum final rise of the stable plume

)hs{xf} reduces to:

As with eq. (95), the velocity, up, and N in eqs. (97) are evaluated initially at stack height and

then iteratively.

When the atmosphere is close to neutral, the Brunt Vaisala frequency, N, is close to zero, and

eq.(95) can predict an unrealistically large plume rise. Under, these circumstances, plume rise is

limited by atmospheric turbulence. This happens when the rate of plume rise under neutral

conditions is comparable to Fw. Under these conditions, stable plume rise (eq. (97)) is limited by

the neutral rise calculated from Weil (1985) as

where the neutral length scale L F (u u ) . n b p = *


As the wind speed approaches zero, eq. (95) again predicts unrealistic values. In these nearcalm

conditions the stable plume rise (eq. (97)) is limited by the calm rise expression that is based

on the work of Morton et al. (1956) and Briggs (1969) such that,

Finally, the stable plume rise is limited by a calculation of the unstable rise (see Section



σ σ σ y yl yo

2 = 2 + 2 (100)

5.7 Source Characterization

AERMOD gives the user the ability to characterize a source as either a point, an area, or a

volume. AERMOD additionally has the capability of characterizing irregularly shaped area




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