Mixing, atmospheric timescales

The real atmosphere does not conform to either model assumption, but lies somewhere between them. Over the five-day timescale of Bi decay, there is too much mixing to allow for the identification and sampling of a parcel that has behaved as a closed system and far too little mixing and transport to allow the homogenization required by the steady-state assumptions. The curves are, however, similar enough to allow meaningful interpretation of data. 

The region of the atmosphere that is in direct contact with the surface (on a timescale of 1 h or less) is commonly referred to as the boundary layer or mixed layer. Technically, the boundary layer refers to the region of the atmosphere that is dynamically influenced by the surface (through friction or convection driven by surface heating). Less formally, the boundary layer is used to represent the layer of high pollutant concentrations in source regions. The top of the boundary layer in urban areas is characterized by a sudden decrease in pollutant concentrations and usually by changes in other atmospheric features (water vapor content, thermal structure, and wind speeds). 

In the centre of the north Pacific subtropical gyre, Bruland et al., on the basis of a vertical profile, emphasized a significant aeolian contribution to the dissolved Fe concentration in the surface mixed layer (surface concentration of 0.35 nM compared with the minimum of 0.02 nM at 70-100 m) (159). Taking into account the study of Hutchins et al. demonstrating that Fe assimilated by plankton in sueh oceanic waters is recycled on a timescale of days, they concluded that a substantial part of Fe entering through the atmospheric input is recycled and is retained in the oligotrophic mixed layer (160). 

We should note that the mixing time of the atmosphere is very rapid. Debris from a large accident, such as the one at the nuclear reactor at Chernobyl in 1986, can quickly be detected all over the globe. Pollutant particles from Europe and North America can be detected over China. This mixing, while distributing contaminants widely, dilutes them at the same time. By contrast, the spread of contaminants in the ocean is much slower and in the other reservoirs of the Earth takes place only over geological timescales of millions of years. 

The case of unstable or convective conditions is a special one in determining atmospheric dispersion. Since under convective conditions the most energetic eddies in the mixed layer scale with Zi, the timescale relevant to dispersion is z,/w . This timescale is therefore roughly the time needed after release for material to become well mixed through the depth of the mixed layer. 

Instead of the GCMs, it has been common to use Energy Balance Models (EBMs) to study changes in climate on orbital timescales. These types of models can be grouped into four categories (1) annual mean atmospheric models (2) seasonal atmospheric models with a mixed layer ocean (3) Northern Hemisphere ice sheet models and (4) coupled climate-ice sheet models, which in some cases include a representation of the deep ocean. 

These developments follow theoretical advances built on vorticity theorems which are based on the conservation of both potential vorticity and potential temperature (i.e., entropy) on timescales for which friction, small-scale mixing, and diabatic heating can be ignored. IPV is a valuable atmospheric tracer which therefore shows the origin and predicted motion of atmospheric parcels indeed, it is the only such dynamical (as opposed to chemical) tracer. Since heating/cooling and friction act to alter the IPV, it is clear that careful representation of these plysical processes (i.e., parameterization ) in models and theories is crucial. 

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