Stratospheric transport

The ozone balance in the stratosphere is determined through complex interactions of solar radiation, meteorological movements within the stratosphere, transport to and from the troposphere, and the concentration of species based on elements other than oxygen that enter the stratosphere by natural or artificial means (such as flight of aircraft). 

There are several important points with respect to the effects of any future HSCT emissions. First, ozone concentrations at a particular location and time depend not only on the local chemistry but on transport processes as well. In the lower stratosphere, transport processes occur on time scales comparable to the rates of ozone formation and loss so that taking into account such transport is particularly important. However, in the middle and upper stratosphere, production and removal of 03 are much faster than transport so that a steady state exists between these two processes. 

The pattern and rate of exchange of stratospheric air with the mesosphere, with the troposphere, and between hemispheres is still poorly known. Within the stratosphere a complex pattern of eddy mixing and mean motions governs the redistribution of trace constituents. In the lower stratosphere the adsorption or attachment of trace constituents to natural sulfate particles influences their subsequent transport. Some main features in stratospheric transport processes based on tracer studies are reviewed here. 

Some of the earliest proposed descriptions of stratospheric transport were based on observations of chemical species. The first observations of stratospheric water vapor densities showed that the stratosphere was extremely dry, exhibiting mixing ratios of the order of a few parts per million by volume, in marked contrast to the troposphere, where water vapor abundance reaches a few percent. Brewer (1949) suggested that the dryness of the stratosphere was determined primarily by condensation and that the water vapor content of an air parcel rising from the troposphere to the stratosphere would therefore be determined by the lowest temperature experienced by the parcel, which would normally correspond to the tropopause. He also noted that the tropopause temperatures in the tropics were low enough to yield stratospheric water vapor densities as low as those observed, while the... 

The eddy-mean flow cancellation represents a difficulty for two-dimensional descriptions of stratospheric transport. Perhaps most importantly, it suggests that in the Eulerian framework, the mean and eddy transports are intimately coupled, and that a proper representation of atmospheric transport must employ a consistent set (see, e.g., Harwood and Pyle, 1975 Rood and Schoeberl, 1983). Further, it suggests that the use of eddy diffusion coefficients to characterize all types and scales of eddy processes must be questioned, since the stratospheric eddies appear to be at least partly steady and conservative (i.e., non-diffusive in character, see e.g., Matsuno, 1980). 

Plumb, R.A., A tropical pipe model of stratospheric transport. J Geophys Res 101, 3957, 1996. 

Pyle, J. A., and C. F. Rogers, Stratospheric transport by stationary planetary waves — The importance of chemical processes. Quart J Roy Meteorol Soc 106, 421, 1980. 

Randel, W.J., J.C. Gille, A.E. Roche, J.B. Kumer, J.L. Mergenthaler, J.W. Waters, E.F. Fishbein, and W.A. Lahoz, Stratospheric transport from the tropics to middle latitudes by planetary-wave mixing. Nature 365, 533, 1993. 

Allen, D.R., R.M. Bevilacqua, G.E. Nedoluha, C.E. Randall, and G.L. Manney, Unusual stratospheric transport and mixing during the 2002 Antarctic winter,... 

That stratospheric 03 concentrations are maximum in areas far removed from those where 03 is being produced suggests that the lifetime of 03 in the stratosphere is longer than the time needed for the transport to occur. The stratospheric transport timescale from the equator to the poles is of order 3-4 months. 

The classic depiction of stratospheric transport is that material enters the stratosphere in the tropics, is transported poleward and downward, and finally exits the stratosphere at middle and high latitudes. The mean meridional stratospheric circulation, known as the Brewer-Dobson circulation, is generated by stratospheric wave forcing, with the circulation at any level being controlled by the wave forcing above that level (recall Figure 5.25). This process is also called the extratropical pump. The composition of the lowermost stratosphere varies with season, suggesting a seasonal dependence in the balance between the downward transport of stratospheric air and the horizontal transport of air of upper tropospheric character. The stratosphere and troposphere are actually coupled by more dynamically complex mechanisms than the traditional model of... 

While the division of the troposphere into two compartments, NH and SH, is reasonable in terms of estimating lifetimes of relatively long-lived tropospheric constituents, such a division is less applicable in the stratosphere. In the case of stratospheric transport and mixing, abetter compartmental division would be between the tropical and midlatitude stratosphere. With such a division, the stratosphere would actually be represented by three compartments, the tropical stratosphere and the NH and SH midlatitude to polar stratospheres. The development in this section can be extended to such a five-compartment model, if desired. 

Boering, K. A. et al. (1995) Measurements of stratospheric carbon dioxide and water vapor at northern midlatitudes implications for troposphere-to-stratosphere transport, Geophys. Res. Lett., 22, 2737-2740. 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

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