Transcript Turbulent fluxes
Turbulent fluxes
Vertical turbulent flux of pollutant mass
Turbulent stress Vertical fluxes of momentum, heat, water vapor, and pollutant
Gradient-transport (K-theory)
K m , K h , and K z
coefficients
are called the
eddy diffusivities
or
exchange
of momentum, heat, and mss, respectively. (K m is also called eddy or turbulent viscosity).
The gradient-transport (K-theory) relations are not based on any rigorous theory, but only on an intuitive analogy between molecular and turbulent exchange processes.
Prandtl’s mixing length theory From dimensional analysis, we recognizing that eddy diffusivity must be a product of appropriate length and velocity scales. In the surface layer (constant flux layer) k= 0.40 is the
von Karman constant
is
the friction velocity
that is related to
surface stress
Finally, we have
In the surface layer, under neutral condition, we have: We can define a surface roughness parameter such that at
, .
Therefore, we can obtain the well-known
logarithmic velocity profile
law, that is
FIRST-ORDER PARAMETERIZATION OF TURBULENT FLUX
•
Observed mean turbulent dispersion of pollutants is near Gaussian
e
parameterize it by analogy with molecular diffusion: Instantaneous plume Time-averaged envelope
z
Near-Gaussian profile Source
Turbulent flux =
K z n a
C
z
Turbulent diffusion coefficient
•
Typical values of
K z
: 10 2 cm 2 s -1 (very stable) to 10 mean value for troposphere is ~ 10 5 cm 2 s -1 7 cm 2 s -1 (very unstable);
•
Same parameterization (with different
K x , K y
) is also applicable in horizontal direction but is less important (mean winds are stronger)
Mass conservation and diffusion equation
If
U
=0, the diffusion equation can be simplified to For an instantaneous point source, the solution of the above equation is Q is the total mass of pollutant in the puff
TYPICAL TIME SCALES FOR VERTICAL MIXING
•
Estimate time
D
t
to travel
D
z
by turbulent diffusion:
2
with
K z
2
K z
2 -1
tropopause (10 km) 10 years “planetary 2 km boundary layer” 0 km 5 km 1 week 1 month 1 day