Appendix 6. Odour Buffer Assessment [As Published] · 132007.2399

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              Odour Buffer Assessment
              StarCo Developments


5.3 Stability

An important aspect of plume dispersion is the level of turbulence in the atmosphere near the
ground. Turbulence acts to dilute or diffuse a plume by increasing the cross-sectional area of the
plume due to random motions. As turbulence increases, the rate of plume dilution or diffusion
increases. Weak turbulence limits plume diffusion and is a critical factor in causing high plume
concentrations downwind of a source, particularly when combined with very low wind speed.

Turbulence is related to the vertical temperature gradient, which determines the condition known
as stability, or thermal stability. For traditional dispersion modelling using Gaussian plume models,
categories of atmospheric stability are used in conjunction with other meteorological data to
describe the dispersion conditions in the atmosphere.

The most well-known stability classification is the Pasquill-Gifford scheme (Pasquill, 1974), which
denotes stability classes from A to F. Class A is described as highly unstable and occurs in
association with strong surface heating and light winds, leading to intense convective turbulence
and much enhanced plume dilution. At the other extreme, class F denotes very stable conditions
associated with strong temperature inversions and light winds, such as commonly occur under
clear skies at night and in the early morning. Under these conditions plumes can remain relatively
undiluted for considerable distances downwind. Intermediate stability classes grade from
moderately unstable (B), through neutral (D) to slightly stable (E). Whilst classes A and F are
strongly associated with clear skies, class D is linked to windy and/or cloudy weather, and short
periods around sunset and sunrise when surface heating or cooling is small.

As a general rule, unstable (or convective) conditions dominate during the daytime and stable
flows are dominant at night. This diurnal pattern is most pronounced when there is relatively little
cloud cover and light to moderate winds.

The frequency distribution of estimated stability classes in the meteorological file is presented in
Figure 5.2. The data show a high proportion of class F conditions (38% of hours), and a total of
47% of hours with either E or F class This is consistent with the expected occurrence of stable
conditions at such a location.

Given the location of the areas of interest to the southwest of the STP, the critical conditions are
associated with northeast winds, which occur predominantly during the afternoon and early
evening, when stability classes C and D are expected to dominate. The intervening ridge between
the STP and the areas of interest also precludes an important role of stable conditions, when air
movement over obstacles is impeded.




1898a report final.doc                                                    PAE Job Number 1898a