UWIS, Boundary Layer, Chapter 14

UWIS, Boundary Layer, Chapter 14

Peter Barmet, Thomas Kuster January 29, 2007

1 Difference between relative and absolute diffu- sion

1.1 Relative diffusion

In a relative diffusion model the plume is modeled in a single realisation, fluid elements are dependent based on two-particle statistic. They are used for instantaneous concentration application e. g. for emergency response systems.

1.2 Absolute diffusion

In a absolute diffusion model the overall (mean) trubulence characteristics of the plume is modeled. Fluid elements are treated independently of each other. This type of models is used for mean concentrations application e. g.

for yearly average estimates and environmental impact assessments.

2 Necessary input for Lagrangian particle disper- sion model

The prerequisite for Lagrangian particle dispersion models is the same as for puff models. It is assumed that the turbulence and flow fields (strongly inho- mogeneous and/or non-Gaussian) are ‘given’ either from parameterisations or numerical modelling.

A ‘large’ number of particles (10

– 10

) to compute their trajectories.

3 Advantages and disadvantages of Lagrangian par- ticle dispersion models

3.1 Advantages

ˆ The Model allows a more comlete characterization of the impact of tur- bulence on the transport of air pollutants because the model calculates

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UWIS, Boundary Layer, Chapter 14 2 more accurately the advection and dispersion from various sources.

ˆ Can easily be used with measurable meteorological parameters.

3.2 Disadvantages

ˆ The Model doesn’t consider buoyancy or processes which depend on concentration values.

ˆ To obtain a continuous concentration distribution a mapping algo- rithm is needed (attribute a ‘density distribution’ to each particle rather than a ‘delta peak’).

ˆ Assumpton of homogenous and stationary meteorological condistions.