Underworld

Pole

Tropics troposphere at mid- and high latitudes. Associated with these vertical motions is a poleward mass flux.

Brewer [ 14-7] and Dobson [150] envisioned the flux into the stratosphere along path A as slow, large-scale upwelling through the tropical tropopause, It was later argued that the tropical tropopause was generally too warm to explain the extremely dry stratosphere [151 [. Based on this, the region where air slowly ascends into the overworld was further narrowed to those regions and times in which the tropical tropopause is much colder than average: in the western tropical Pacific, northern Australia, and Indonesia during the November to March period and over the Bay of Bengal and India during the monsoon. This temporally and spatially limited region has become known as the "stratospheric fountain". Recent measurements of water vapor 1152) and CO, [153] show that air enters the stratosphere throughout the entire year; this has persuaded many that the temporal restrictions of the "stratospheric fountain" theory are likely wrong. An analysis using improved data sets re-examined the spatial restriction and found that it too did not stand up upon further investigation [154], At the present time, it is unclear if there are any preferred locations for transferring mass from the troposphere to the overworld.

General objections to the slow-ascent theories of troposphere-to-overworld transport have also been raised. It was argued that large-scale uplift would cause thick cirrus decks to form in the upwelling regions, which is not the case [ 155). However, the recent observations of widespread subvisible cirrus clouds near the tropopause [156-158] have lessened this criticism. Another problem is that no theory can definitively explain how air slowly ascending into the overworld can be dehydrated to the very low humidity observed in the stratosphere [ 159,160],

An alternative to slow ascent is that energetic convection, originating at the surface, overshoots the tropopause and mixes with stratospheric air, thereby transferring mass from the troposphere into the overworld (see Danielson [161], and the STEP collection of papers [162] and the references therein). During the injection process, the air mass is dehydrated to stratospheric abundances. While theories of the dehydration process exist [163], there is currently not enough data to validate these theories with any certainty.

It should be noted that both slow-ascent and direct convective injection are almost certainly both occurring. The real question is the relative importance of these two processes. At the present time the answer to this question is unknown.

Path B is isentropic transport between the troposphere and the lowermost stratosphere. Ordinarily, air parcels arc prevented from crossing the tropopause on an isentrope by the large gradient in potential vorticity (PV) at the tropopause ยก148], Synoptic-scale waves, however, can transport material from the tropical troposphere to the mid-latitude stratosphere and vice versa by means of wave breaking. Path C is mid- and high-latitude convective transport of air from the troposphere to the lowermost stratosphere.

Much of our understanding of the relative contributions of the different paths is provided by measurements of water vapor. As an air parcel cools below the frost point (the temperature at which the relative humidity with respect to ice is 100%), condensation of water vapor and subsequent sedimentation of particles dehydrates the air mass. If the temperature of the air parcel subsequently rises, the water vapor volume mixing ratio (VMR) remains unchanged, and continues to reflect the previously encountered minimum temperature.

This last fact, combined with the fact that the minimum saturation VMR is most often encountered at the tropopause, makes the water vapor abundance a useful indicator of where air crosses the tropopause. Figure 5.5 shows the distribution of temperature, pressure, and saturation water vapor abundance along the tropopause. Air following path A, which goes through the tropical tropopause, typically encounters temperatures of 195 K or below, and therefore carries only a few parts per million by volume of water into the stratosphere. Paths B and C, however, which transit the extratropical tropopause, encounter far warmer temperatures (-210225 K) than the tropical tropopause, and can carry tens of parts per million by volume of water vapor into the stratosphere.

Latitude

Figure 5.5 Temperature, pressure, and saturation water vapor abundance of the tropopause. Data are zonal and monthly averaged for January 1994.

Latitude

Figure 5.5 Temperature, pressure, and saturation water vapor abundance of the tropopause. Data are zonal and monthly averaged for January 1994.

Figure 5.6 shows several mid-latitude profiles of water vapor covering the region between the upper troposphere and overworld. The abundance of water vapor in the overworld is a few parts per million by volume. Only at the tropical tropopause are the temperatures low enough to dehydrate air to such a low abundance (Figure 5.5). Additional support for the tropical tropopause its the entry point of air into the over-world comes from observations of a correspondence between the seasonal cycle of the abundance of water entering the stratosphere and the seasonal cycle of tropical tropopause layer temperature [152], as well as measurements of other tracers [165|. As a result, it is generally believed that all of the air in the overworld transited the tropical tropopause.

Measurements suggest that, on an annual average, air enters the overworld with -3.85 ppmv of H20 [166], 1.7 ppmv of CH4 [167], and 0.5 ppmv of H2 [114,168,169], Once in the stratosphere, the CH, and H, are slowly oxidized to form

Temperature (K)

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