Progress in Understanding Dispersion in the Atmospheric Boundary Layer

This paper reviews some of the major highlights in understanding dispersion in the atmospheric boundary layer (ABL) over the past several decades and is based on contributions to the AMS “100 Years of Progress in Boundary Layer Meteorology.” Prior to 1960 – 1970, dispersion modeling was limited mostly to the lowest 100 m, the so-called “tower layer” (Panofsky, 1973), where dispersion was based on eddy-diffusion methods, Lagrangian similarity theory, and statistical theories (e.g., Taylor, 1921). Major dispersion modeling developments took place in the “explosive period” of 1970 – 1990 with key achievements for convective and stable ABLs. Dispersion insights were paced by new understanding of the ABL structure and turbulence achieved through numerical simulations (e.g., Deardorff, 1972), laboratory experiments, and observations. Chief among these was convective scaling of dispersion based on convection tank experiments (Willis and Deardorff, 1976), which showed that the ABL depth and convective velocity scale were the key turbulence scales important for dispersion. The experiments inspired many others as well as confirming field observations (e.g., Briggs, 1983), a new Lagrangian particle dispersion model (LPDM) driven by large-eddy simulations (LES) (e.g., Lamb, 1978), and new simple approaches, e.g., the PDF (probability density function) model (Weil, 1988). For the stable ABL, advances came more slowly but were aided by observations and second-order closure modeling (e.g., Brost and Wyngaard, 1978), which revealed the ABL structure including a new buoyancy length scale for the turbulence. For elevated sources, statistical modeling and observations showed that dispersion followed the Taylor short-time limit (µ t), but the long-time behavior could vary between the classical diffusion and ``pancake” limits. Since about 1990, the key developments have been in the areas of: 1) use of LES in dispersion modeling, 2) further developments in LPDM’s driven either by a purely stochastic approach or by LES, 3) studies of concentration fluctuations using experiments, LES and other numerical methods, and Lagrangian two-particle dispersion models, and 4) urban dispersion.