Surface Delays for Gases Dispersing in the Atmosphere

• Published in: Journal of applied meteorology (1988) 2001-08, Vol.40 (8), p.1422-1430
• Main Authors: Wilson, John D., Flesch, Thomas K., d’Amours, Real
• Format: Article
• Language: English
• Subjects: Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth, ocean, space; Exact sciences and technology; External geophysics; Geophysics. Techniques, methods, instrumentation and models; Meteorology; Travel time
• ISSN: 0894-8763; 1520-0450
• DOI: 10.1175/1520-0450(2001)040<1422:SDFGDI>2.0.CO;2

When a particle descends beneath the (nominal) lower boundary of the atmosphere, it may remain there for some timeτbefore it reemerges into the (resolved) flow. In particle trajectory models,τis the random duration of unresolved trajectory segments, below the heightzr at which an artificial reflection boundary condition is applied. By computing such paths, for realistic near-ground flows, it was found that the mean delay per reflection isτ ̄≈ 2.5zr/σw whereσw is the standard deviation of the vertical velocity atzr . The corresponding mean alongwind displacement per reflection, due to the mean horizontal windū(z) belowzr , isδ̄≈ 〈ū | zr 〉τ ̄, where 〈ū | zr 〉 is the height average ofūin the waiting layer. The fluctuating component of the horizontal wind causes no mean drift but upon each reflection contributes a random drift whose root-mean-square value isσδ ≈ 2zr . From simulations on the continental scale, with a lower boundary placed atzr ≈ 25 m, it was found that a typical particle suffered about 15 reflections per day, resulting in a net delay on the order of 30 min per day.