Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
2.1.1.3.3. Stratosphere/troposphere exchange
As mentioned in Section 2.1.1.1, emissions from present aviation are injected near the tropopause. Dynamic, chemical, and radiative coupling between the
stratosphere and troposphere are among the important processes that must be understood if we are to provide an adequate description and prediction of the impact of
aviation on atmosphere and climate. Of special significance is the exchange of chemical species between the stratosphere and the troposphere. In the stratosphere,
large-scale transport takes place via the Brewer-Dobson circulation, induced by momentum deposition by planetary gravity waves. This circulation is responsible for
the observed difference between the stratospheric temperature and its radiative equilibrium value. However, this exchange involves a wide spectrum of scales ranging
from large-scale ascent and descent via synoptic scales toward transport by waves, convection, and turbulence (Brewer, 1949; Holton et al., 1995; McIntyre, 1995).
For the impact of subsonic aviation, the focus is on the exchange between the troposphere and the lowermost stratosphere (Hoskins et al., 1985). This part of the
stratosphere is strongly coupled with the troposphere and is separated from the LS by a region enclosed between the 380 K and 400 K potential temperature surfaces
(Holton et al., 1995).
For the transport, it is useful to distinguish between different regions of the globe:
In the tropics, upward transport occurs mostly through deep convection, though small-scale vertical mixing by gravity waves might also play an important role.
The tropics are the regions on Earth in which the largest net upward transport into the stratosphere occurs and which therefore directly influence the
composition of the global middle stratosphere. Mixing between the tropical and mid-latitude lower stratosphere is influenced by the subtropical barrier. Between
the tropical UT and subtropical LS, however, transport along isentropic surfaces is important (Minschwaner et al., 1996; Plumb, 1996; Volk et al., 1996).
In mid-latitudes, the exchange between the troposphere and lowermost stratosphere flows in both directions, with a somewhat larger downward component
(Siegmund et al., 1996). Most of this transport is related to the occurrence of extra-tropical cyclones and blocking anticyclones. In cyclones, polar stratospheric
air is drawn into the troposphere while subtropical tropospheric air is drawn into the stratosphere. The intermediate process of tropopause folding is followed by
re-establishment of the tropopause. There is evidence that upward and lateral mixing of tropospheric air into the stratosphere remains limited to the lowest few
kilometers of the mid-latitude LS (Dessler et al., 1995; Boering et al., 1996; Hintsa et al., 1998).
Around the polar vortices, the exchange in the stratosphere occurs along isentropic surfaces, from the polar vortex toward mid-latitudes, by filaments torn off
from the vortex boundary. Vertical transport in the vortex itself mostly takes place in the form of large-scale descent caused by radiative cooling. Horizontal
transport on the equatorward flank of the polar night jet is sharply coupled with vertical transport associated with diabatic descent caused by radiative cooling of
warm air within the descending branch of the baroclinic circulation (Pierce et al., 1993, 1994).
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