Volume 1 - The Atmospheric Studies Group at TRC

adjusted to account for the effect of downwash. The plume rise equations are not
functions of h s or h s -z plat . The linear decay factor for σ z is determined as:
σ ′ z
= A σ ′ z
(3-37)
where σ ′
z
is determined from the HS downwash equations, and
⎧1
⎪
H
A = ⎨
⎪
⎪
⎩0
b
− ( H
e
2L
b
− z
plat
)
+ 1
H
b
< ( H
H
b
e
H
− z
e
+ 2L
plat
b
≤ ( H
) ≤ H
< ( H
b
e
− z
b
plat
)
+ 2L
− z
plat
b
)
(3-38)
AERMOD Turbulence Profile Option
The turbulence velocity for horizontal fluctuations is computed from contributions
from shear and buoyancy. The total turbulence velocity is obtained by summing the
component variances:
σ = σ + σ
(3-39)
v
2
vs
2
vb
The shear component is modeled with a variance that is a maximum at the ground
(σ vs 2 =3.6u * 2 ), and decreases linearly through the depth of the mechanically mixed
layer to a residual value of 0.25 m 2 /s 2 , if this residual value is less than the value at
the ground. If the residual value is larger, the value at the ground is used for all
heights. The buoyancy component is constant (σ vb 2 =0.35w * 2 ) up to z i , and decreases
linearly to the residual value at 1.2z i . Again, the value at the ground is used at all
heights if it is less than the residual value.
The turbulence velocity for vertical fluctuations is also computed from shear and
buoyancy contributions. The buoyancy component of the variance is computed as:
2
wb
2
*
3
σ = 1.6w
( z z )
2 / ( z ≤ 0.1z
)
(3-40)
2
wb
i
i
2
σ = 0.35w ( z = 0.1z
to z )
(3-41)
2
wb
*
i
i
2 −6(
z−z
) zi
σ = 0.35w
e
i ( z > z )
(3-42)
*
i
The shear component has both a surface-driven contribution (from u * ) and a residual
contribution from turbulence aloft that is assumed to have an intensity of order 2% of
the wind speed at z i :
2
ws
2
*
2
σ = 1.3u
(1 − z z ) + (0.02 u z / z ) ( z ≤ z )
(3-43)
i
zi
i
i
Final Report Vol.1 17