Troposphere horizontal mixing

Figure 3.30. Schematic representation of the atmospheric circulation (arrows) and associated quasi-horizontal mixing between the surface and the middle stratosphere. Mixing processes leading to stratosphere-troposphere exchanges are also represented. The heavy vertical lines denote dynamical barriers against meridional transport. Note the large-scale ascent in the tropical stratosphere above intense convective systems in the tropical troposphere, and large scale descent associated with the polar vortex during winter (WMO, 1999).

While molecular diffusivity is commonly independent of direction (isotropic, to use the correct expression), turbulent diffusivity in the horizontal direction is usually much larger than vertical diffusion. One reason is the involved spatial scales. In the troposphere (the lower part of the atmosphere) and in surface waters, the vertical distances that are available for the development of turbulent structures, that is, of eddies, are generally smaller than the horizontal distances. Thus, for pure geometrical reasons the eddies are like flat pancakes. Needless to say, they are more effective in turbulent mixing along their larger axes than along their smaller vertical extension. 

Assuming little vertical variation in the mixing ratio of total mercury in the troposphere and using the available horizontal surface-based measurements, Mason et al. (1994) estimated the total... 

In contrast, convection currents are not possible in the stratosphere, because of the positive temperature gradient. Mixing there is caused by differential solar heating of the stratospheric gas across the latitudes and is known as advection. Air heated at the equator expands and moves towards the poles. Displaced air tends to rise at the poles and comes back down to replace the sinking heated air at the equator. An important point is that air on either side of the tropopause, moves horizontally and parallel, which makes mixing across the boundary difficult. The stratospheric air mass is therefore, partially isolated from that of the troposphere. Thus, whereas effective mixing within the troposphere may take only a few days or weeks, effective mixing across the boundary may take many months. 

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