Volume 1 - The Atmospheric Studies Group at TRC

σ = .02 u ( z > z )
(3-44)
ws
0
zi
i
The total turbulence velocity is obtained from the sum of these components:
w
2
wb
2
ws
σ = σ + σ
(3-45)
Minimum Turbulence Velocities
CALPUFF accepts minimum lateral turbulence velocities σ v as a function of stability
class (6 values). A minimum value establishes a floor, so that any computed lateral
turbulence less than the minimum is replaced. The default value for each stability
class had been set to 0.5m/s. An additional set of minimum values for overwater
cells has been added, so now there are 12 values, two for each stability class. The
original set of six is now used for overland cells, and the default of 0.5m/s is retained.
The AERMOD minimum lateral variance, 0.25 m 2 /s 2 , is equivalent. The new set of
six used for overwater cells is set to a default of 0.37 m/s, a value originally used in
the OCD model for overwater dispersion, and one that performed well in the model
evaluations with the offshore tracer data sets.
Similarly, a set of overland and overwater minimum vertical turbulence velocities σ w
are also accepted, two for each stability class (12 values). The default values for
overwater cells are equal to those for overland cells.
---------- LAND ---------- --------- WATER ----------
Stability Class : A B C D E F A B C D E F
--- --- --- --- --- --- --- --- --- --- --- ---
Default SVMIN : .50, .50, .50, .50, .50, .50, .37, .37, .37, .37, .37, .37
Default SWMIN : .20, .12, .08, .06, .03, .016, .20, .12, .08, .06, .03, .016
Lateral Puff Timescale Diagnostic Option
For steady homogeneous dispersion, Taylor’s (1921) original expression for lateral
plume spread σ y as a function of time is:
( t −τ
+ τ exp( − τ ))
σ 2 = 2σ
2 τ
t /
(3-46)
y
v
where τ is the Lagrangian timescale and σ v is the lateral turbulence intensity. This is
typically approximated as:
( 1 / τ )
σ
y
= σ vt / + t 2
(3-47)
which has the same limits as Equation 3-46 for small and large t/τ. Lateral growth in
CALPUFF uses Draxler’s (1976) expression, which is equal to Equation 3-47 when
τ=617.3s.
Final Report Vol.1 18