| Re: gaussian air dispersion model [message #46567 is a reply to message #46529] |
Mon, 28 November 2005 20:00   |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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guillaume.drolet.1@ulaval.ca wrote:
>> If the distances in question are
>> small you can get away with
>
>
>> sigma-y = sigma-theta * x
>
>
> Are distances between 500 and 1500 m considered small?
Damn, I hoped you weren't going to ask!
I'm reaching way back into my memory banks here. The short answer is
that 500 to 1500 m is probably small enough that the above formula will
be an overestimate, but good enough to get started with. The long answer
is that the distance at which the growth of sigm-y starts to drop away
from the linear formula depends on the correlation time scales of the
cross-wind turbulent fluctuations. You can estimate the Eulerian time
scale from your anemometer. The problem is that what you really want is
the time scale for the fluctuations experienced by a Lagrangian particle
(ie one moving with the wind) and these will generally be longer. There
is a lot of info about this in the literature so you should eventually
be able to come up with reasonable values. Right now I suggest you use
the linear relation.
> Yes. Since the mean wind direction for a 30-minute period comes from
> high frequency measurements (10 Hz), I should be able to get
> sigma-theta.
Good.
> You are right. I definitely need to do some reading and maybe I posted
> in the wrong forum.
No worries. I just did a quick search on Google Scholar and came up with
the following. It's rather old and has an urban focus rather than an
agricultural one, but it might help...
Applied dispersion modelling based on meteorological scaling parameters.
Gryning, S E | Holtslag, A A M | Irwin, J S | Sivertsen, B |
Atmospheric Environment. Vol. 21, no. 1, pp. 79-89. 1987
A method for calculating the dispersion of plumes in the atmospheric
boundary layer is presented. The method is easy to use on a routine
basis. The inputs to the method are fundamental meteorological
parameters, which act as distinct scaling parameters for the turbulence.
The atmospheric boundary layer is divided into a number of regimens. For
each scaling regime the authors suggest models for the dispersion in the
vertical direction. The models directly give the crosswind-integrated
concentrations at the ground, chi sub(y), for non-buoyant releases from
a continuous point source. Generally the vertical concentration profile
is proposed to be other than Gaussian. The lateral concentration profile
is always assumed to be Gaussian, and models for determining the lateral
spread sigma sub(y) are proposed. The method is limited to horizontally
homogeneous conditions and travel distances less than 10 km. The method
is evaluated against independent tracer experiments over land. The over
land. The overall agreement between measurements and predictions is very
good and better than that found with the traditional Gaussian plume model.
Descriptors: atmosphere | boundary layers | meteorology | pollutant
dispersion; mathematical models | atmospheric conditions |
--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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