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Abstract
Abstract. Turbulent boundary layer concepts of constant flux layers and surface roughness lengths are extended to include aerosols and the effects of gravitational settling. Interactions between aerosols and the Earth's surface are represented via a roughness length for aerosol which will generally be different from the roughness lengths for momentum, heat or water vapour. Gravitational settling will impact vertical profiles and the surface deposition of aerosols, including fog droplets. Simple profile solutions are possible in neutral and stably stratified atmospheric surface boundary layers. These profiles can be used to predict deposition velocities and to illustrate the dependence of deposition velocity on reference height, friction velocity and gravitational settling velocity.
Highlights
Within the turbulent atmospheric “surface layer”, typically 0 < z < ∼ 50 m, it is helpful to look at idealized situations where fluxes of momentum, heat or other quantities are considered to be independent of height, z, above a surface which is a source or sink of the quantity being diffused by the turbulence
Monin–Obukhov similarity theory (MOST) is based on constant-flux-layer situations in steady-state, horizontally homogeneous, turbulent atmospheric boundary layers and leads to suitably scaled, dimensionless velocity and other profiles being dependent on z/L, where z is the height above the surface and L is the Obukhov length
If we are considering sand or dust being picked up from the surface by wind, upward diffusion will be countered by downward gravitational settling, while if the source of the aerosol is above our constant flux layer, the turbulent fluxes and gravitational settling combine
Summary
Within the turbulent atmospheric “surface layer”, typically 0 < z < ∼ 50 m, it is helpful to look at idealized situations where fluxes of momentum, heat or other quantities are considered to be independent of height, z, above a surface which is a source or sink of the quantity being diffused by the turbulence. Garratt (1992, chap. 3) and Munn (1966, chap. 9) discuss this “constant flux layer” concept, and, for momentum, the paper by Calder (1939), discussing earlier work by Prandtl, Sutton and Ertel, is an early recognition of the utility of this idealized concept. Monin–Obukhov similarity theory (MOST) is based on constant-flux-layer situations in steady-state, horizontally homogeneous, turbulent atmospheric boundary layers and leads to suitably scaled, dimensionless velocity and other profiles being dependent on z/L, where z is the height above the surface and L is the Obukhov length (defined below). For fog droplets, other aerosol particles deposited to water and other surfaces, we assume Qc → 0 as z → 0 and, as a trial value, will generally use z0c = 0.01 m for illustration. This is somewhat larger than values typically assumed for water vapour or heat.
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