Subsidence inversion

Type of temperature inversion Subsidence, at 1000 m Radiation (near ground) at a few hundred metres... 

There is usually some descent (subsidence) of air above surface high-pressure systems. This air warms dry adiabatically as it descends, decreasing the relative humidity and dissipating any clouds in the layer. A subsidence inversion forms as a result of this sinking. Since the descending air compresses as it encounters the increased pressures lower in the atmo-... 

Fig. 17-9. Formation of a subsidence inversion in subsiding (sinking) air. Note the vertical compression of the sinking layer which is usually accompanied by horizontal divergence.

If the earth did not rotate or if it rotated much more slowly than it does, a meridional (along meridians) circulation would take place in the troposphere (Fig. 17-24). Air would rise over the tropics, move poleward, sink over the poles forming a subsidence inversion, and then stream equa-... 

Type of inversion Radiation (ground) Subsidence (overhead)... 

D4 0.460associated with subsidence inversion in anticyclonic situations. 

Inversions tend to inhibit mixing of air below the inversion with that above the inversion. The combination of the Pacific high pressure, tbpograpy and generally clear sky conditions found in the Pacific Northwest during the summer months results in multilevel inversions (e.g. subsidence, marine air, valley and canopy inversions). All of these could affect a particular site. However, the valley and canopy inversions are the most common. These, in combination with slope and valley winds, greatly affect the distribution of particle and gaseous dispersoids. 

Permeabilities in shales have been calculated based on observed pressure gradients and fluid fluxes derived from compaction rates. At constant rates of subsidence the compaction-driven flux would be nearly constant over a limited section, thus the pressure gradients is inversely proportional to the permeability. Assuming a flux of 10" mVm per s, an effective permeability of about 1 nD was calculated, based on the observed pressure gradients. This is in good agreement with the permeabilities measured on samples (Schlbmer, 1995). 

Rapid Carboniferous subsidence was followed by Late Carboniferous-Permian uplift which was part of the regional Late Variscan inversion event. Although a dominant N-S to NNW-SSE compres-sional stress field is recognized through NW Europe at this time (Coward, 1995), Kerr (1987) demonstrated evidence for local E-W compression in the North Antrim region. This resulted in E-W strike-slip faults and transtensional N-S reactivation of... 

FIGURE 16.4 Temperature profile and pollutant mixing for (a) nighttime radiation inversion and (b) subsidence inversion. 

Subsidence inversions form as the air above slowly sinks and warms. A typical temperature profile for such an inversion is shown in Figure 16.4b. Usually the air below the inversion is unstable, and pollutants mix relatively rapidly up to the inversion. The stable layer of the inversion acts as a lid and does not allow penetration. Some of the most dangerous air pollution episodes are related to subsidence inversions that persist for several days. The mixing height can be as low as a few hundred meters in such air pollution episodes. 

We can now see one of the reasons why Los Angeles is afflicted with air pollution problems. Its location on the west coast of North America in the subtropical region and on the eastern side of the Pacific anticyclone is one in which elevated inversions are frequent and strong. The lowest layer of air (the marine layer) is cooled because of its contact with the ocean. Air pollutants are trapped in the marine layer and prevented from vertically exchanging with upper-level air. Such a situation can lead to serious air pollution problems. The base of a subsidence inversion lies typically at an elevation of about 500 m, with the inversion layer extending another 500 to 1000 m upward. 

A late structural inversion defined in particular in slightly subsided marginal basins, especially on the northern flank of the Illizi Basin, caused meteoric waters to penetrate into the respective semdstones. Under the conditions of the original distribution of temperature, pressure and pore water chemistry the eogenetic and meso-genetic mineral complexes became unstable and started to react with the actual subterranean waters in an attempt to achieve a new equilibrium. These weakly mineralized waters with a near-neutral pH became able to leach the carbonate, sulfate and halite cements and generated a secondary porosity. 

Allen M.B., McDonald D.I.M., Zhao X, et al. Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai basin, northern China. Mar Petrol Geol, 1997 14 951-972. 

Another group of inversion hds is produced by parcels of air subsiding from heights in high pressure weather conditions. This type of inversion has no importance for released hazardous materials because its boundary rarely lies below 300 m. Figure 10.24 illustrate the situations described. 

In the case of plume trapping by a veiy deep inversion or by an upper air subsidence inversion when L is very large, it has been assumed that the number of reflections is reduced to one. Thus, at ground level on the centerline, the equation describing plume trapping would be... 

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