AMS Workshop on Stability Classification Schemes and Sigma Curves—Summary of Recommendations

Comparison of Observed and Predicted Sector-Averaged Air Concentrations for Elevated Releases of Fluorescein Particles

“…The PG values are based mostly on surface level releases over terrain with a surface roughness length of approximately 3 cm. 12 The BNL values are based on releases from a height of 108 m over terrain with a roughness length of approximately 1 m. 12 It is expected that both release height and surface roughness will affect the c y and o~z values at any given site, although Hubschmann and Miller 22 suggest that surface roughness may be the more important of the two. Measurements of roughness length at the Hanford site are not available, but it is assumed to be near 10-20 cm; i.e., 0.1 times the height of the "roughness elements" on the ground.…”

“…For the PG curves, as well as many other sets of dispersion parameters, variations in stability are often expressed in terms of stability categories ranging from A, most unstable, to F, most stable. The BNL values are divided into four stability classes according to "gustiness" as taken from 12 Two of the most common methods are based on (1) the temperature difference between two heights on a meteorological tower and (2) the standard deviation of the horizontal wind direction at a given height. The criteria used to classify atmospheric stability according to each of these methods for both the PG and BNL curves are given in Table I.…”

Final Review of Safety Assessment Issues at Savannah River Site, August 2011

“…The review team prefers the above formulation because, as already noted, it is functionally consistent with the EPA surface roughness adjustments to the σ A or σ E methods for estimating PG stability class (EPA 2000). Hanna et al (1977) and Irwin (1980) suggest that the exponent in the ZSCALE formulation ranges between 0.10 to 0.25, with the larger values being applicable to shorter distances and rougher surfaces. Therefore, we recommend adjusting the exponent in the ZSCALE formulation at select distances at the SRS site, as listed in Table 1, to account for the downwind dependence.…”

A Critical Review

“…(See p. 1306 of Hanna, et al 166 ) If measurements of vertical wind direction fluctuation representative of the height of released pollutants are not feasible, it is currently thought that combining vertical temperature difference with wind speed for a shallow layer of the atmosphere near the ground (where vertical temperature gradients are strongest) into a bulk Richardson number is a proper expression for inferring vertical dispersion. This is especially useful for distinguishing degrees of instability important for the elevated release.…”

“…Following the recommendations of the Stability Workshop, 166 Irwin 174 attempted to include existing theory, observations, tank experiments, and numerical experiments in the development of a scheme which will include effects of stability, roughness, height of release, and distance from the source. The scheme has since been altered in response to suggestions made during review.…”