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Water Vapor (ppmv)

Figure 5.6 Mid-latitude profiles of water vapor (solid line) and temperature (light dashed line) vs. potential temperature <K). The heavy dashed line indicates the location of the tropopause; the heavy solid line indicates the 380 K surface, the boundary between the over-world and lowermost stratosphere. (After Dessler et at. [ 1641, Figure 2.)

additional water vapor [110,112—114]. Complete oxidation of C'H ; and 11 would form an additional 2x 1.7 + 0.5 = 3.9 ppmv of water vapor. In agreement with this simple constraint, stratospheric H>Q abundances greater than 8 ppmv are never observed, with typical observed abundances between 4.5 and 7 ppmv [170].

Figure 5.7 shows typical cross-sections of H20 and CH4. As one can see, the decrease in CH4 (due to oxidation) is mirrored by an increase in H20. II2 is also

Latitude

Latitude

Figure 5.7 Latitude-height cross-sections of U ,0 and CH4 for December ¡992. Data were measured by instruments onboard the UARS; in the UARS Reference Atmosphere Project (W. J. Randel, personal communication, 1998). (See Randel etui. [ 174] for details of the plots.)

Latitude

Figure 5.7 Latitude-height cross-sections of U ,0 and CH4 for December ¡992. Data were measured by instruments onboard the UARS; in the UARS Reference Atmosphere Project (W. J. Randel, personal communication, 1998). (See Randel etui. [ 174] for details of the plots.)

oxidized to form H20, but it is also produced in the stratosphere as a by-product of CH,, oxidation. These two processes are approximately equal, so the abundance of H2 remains constant at -0.5 ppmv throughout the lower and mid-stratosphere, decreasing to lower values in the upper stratosphere 1171 J. (For more discussion on the hydrogen budget, see Dessler et al. [172] and Hurst et al. [173].)

Between the mid-latitude tropopause and the overworld lies the lowermost stratosphere. Figure 5.6 shows that in this region the abundance of water vapor is variable but can reach several tens of parts per million by volume. We know that some of the air in this region has descended from the overworld via path A as part of the Brewer-Dobson circulation. This air, however, carries 4—8 ppmv of water vapor, and so cannot be the only source of air for the lowermost stratosphere [164], It is concluded, therefore, that some of the air must have followed a path that crossed the extratropical tropopause—thereby allowing the air to carry more water vapor into the stratosphere. It is generally agreed that isentropic transport of air into the stratosphere (path B) is responsible for much of this transport [ 175,176).

While mid- and high-latitude convective transport into the stratosphere (path C) has been shown to occur [177|, it is presently unknown how important this path is for the global budget of STE. It should be noted that this pathway likely only transports air into the lowest few kilometers of the lowermost stratosphere 1177,178]. Mid- and high-latitude convection is simply not energetic enough to reach overworld potential temperatures.

To summarize, the stratosphere can be thought of as being made up of two separate but related regions. The overworld is the region of the stratosphere above 380 K potential temperature. Air enters this region exclusively through the tropical tropopause, and the water vapor abundance in this region is set by the cold temperatures encountered there. The lowermost stratosphere—the mid- and high-latitude stratosphere between 380 K and the tropopause—is made up of a mix of overworld air that has descended from above and air that has gone through the extratropical tropopause. Air crossing the extratropical tropopause experiences higher temperatures than air crossing the tropical tropopause, allowing higher water vapor in the lowermost stratosphere (tens of parts per million by volume) than in the overworld stratosphere (a few parts per million by volume).

Most of the ozone in the stratosphere resides in the overworld. As a result, the focus of this book is almost exclusively on the overworld, and we use the terms "stratosphere" and "overworld" interchangeably. This loose notation is not problematic, however, because most of the concepts presented also apply to the lowermost stratosphere.

Finally, the stratosphere also exchanges mass with the mesosphere. By almost any measure this exchange of mass is less important than exchange with the troposphere, but there are aspects of it worthy of note. Probably the most important is the transport of NOv formed in the mesosphere and thermosphere into the stratosphere. This can be a potentially important source of NO, for the upper stratosphere, especially following severe solar proton events [97].

5.2 The General Circulation

The existence of winds can be attributed to differential heating of the atmosphere— the fact that the tropics receive more solar energy per unit area, on an annual average, than the poles (see Hartmann [179], Figure 2.7). This differential heating causes a pressure gradient between the pole and equator, which drives a strong zonal (low with typical velocities in the stratosphere of 10-100 m s Dissipation of planetary waves in the stratosphere and above leads to meridional and vertical flow; this cause-and-effect relationship has become known as the "wave-driven pump" [148]. The meridional flow typically has zonally averaged velocities of tens of centimeters per second, and is therefore weaker than the zonal flow. The vertical flow typically has zonal average velocities of the order of millimeters per second, smaller than both the zonal or meridional velocities.

While flow in the atmosphere is obviously three-dimensional, strong zonal flow means that the stratosphere tends to be well mixed in the zonal direction. Thus, air at different longitudes but the same latitude and altitude tends to be similar. For this reason, we have often relied in this book on plots in which the data from the same latitude and altitude but different longitudes have been averaged together to form a "zonal average" plot. Using this same logic, two-dimensional (2D) models of the stratosphere have been developed that take advantage of this fact and represent the atmosphere in only two dimensions: latitude and height (see WMO [15], Chapter 12).

It should be remembered, however, that important longitudinal asymmetries do exist in the O, field. As Figure 1.3 shows, column O, varies with the synoptic weather pattern (i.e. the pattern of high and low pressure). This occurs because high-pressure systems push up the tropopause. This causes convergence of tropospheric air into the bottom of the column and a divergence of stratospheric air out of the upper parts of the column. Since tropospheric air has a much lower O, content than stratospheric air, this causes the total column abundance to decrease. Low-pressure systems have the opposite effect.

Despite these longitudinal asymmetries, many of the important facets of the transport of O, can be understood by examining the 2D circulation of the atmosphere. In the next section we examine this circulation. Then, we will discuss how it affects the distribution of trace gases, especially O,, in the stratosphere.

5.2.1 The 2D circulation

The 2D circulation of the stratosphere in the meridional plane can be divided into two parts: the Brewer-Dobson circulation, a mean overturning circulation, and quasi-horizontal meridional eddy transport. The Brewer-Dobson circulation was introduced earlier in this chapter in our discussion on STE. It comprises upwelling in the tropics and downwelling in the extratropics, and is shown schematically as path A in Figure 5.4. Figure 5.8 shows streamlines of the circulation during northern hemisphere winter, while Figures 5.9 and 5.10 show estimates of the horizontal and vertical components of the Brewer-Dobson circulation. Upwelling in the tropics is primarily compensated for by downwelling in the winter hemisphere [180,1811; the meridional circulation in the summer hemisphere is much weaker. Most mass entering the stratosphere rises into the lower stratosphere, is transported poleward, and descends out of the stratosphere, with only a small fraction of the mass rising to high altitudes. Finally, the Brewer-Dobson circulation in the northern hemisphere during its winter is stronger than the circulation in the southern hemisphere during its winter. This arises from differences in the layout of continents and mountain ranges in the two hemispheres [146,148].

The second part of the 2D circulation is the quasi-horizontal transport arising from longitudinally asymmetric processes such as planetary-scale wave breaking. Because there is no longitude coordinate in the 2D zonally averaged framework, these processes must be parameterized in terms of other variables, and how this parameterization is implemented remains a central problem in the implementation of a 2D framework. This class of processes is generally referred to as "eddy transport".

Eddy transport is relatively fast, transporting and mixing material on horizontal

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