Modelling Atmospheric Dispersion Flashcards
For what is diffusivity a function?
Positions x, y, z and wind speed, u
Their values vary with position and conditions.
What value is diffusivity applied to the model?
The mean diffusivity
For which plumes are the Gaussian concentration profiles valid?
Continuous plumes and discrete clouds
What form does a Gaussian concentration profile?
y = constant * exp (- x^2/ sigma ^2)
What is sigma?
Standard deviation of the Gaussian curve
What is the definition of standard deviation?
68 % of the pollutant falls within sigma of the centre line. For 95 % within 2*sigma
What are the units of sigma?
Metres
What physical factors affect sigma?
Position and wind speed
What are the three equations for standard deviations?
- instantaneous release, ground level
- continuous release, ground level
- continuous release at elevation H
What is the relationship between sigma (y) and x?
There is a linear relationship between sigma (y) and x.
From stability classes A-F, the intercept decreases.
What is the relationship between sigma (z) and x?
There is not a necessarily a linear relationship between x and sigma (z). For stability classes A-F the intercept decreases and at the extrema, the relationship is more curved.
What do more recent models include rather than stability classes?
The Mornin-Obukhov length L. This is an indication of the respective strength of turbulent thermal and mechanical forcing that may disturb air within boundary layer. It can be estimated from wind velocity and temperature vertical profile.
