Tropopause, tropical

Water vapor concentrations have also been used to show that stratospheric air in the midlatitudes cannot all have originated via the tropical pump, i.e., path I in Fig. 12.3. For example, Dessler et al. (1995b) have shown that water vapor concentrations in the lowermost stratosphere at 37.4°N, 122.1°W are higher than expected for an air mass that has passed through the cold tropical tropopause. Their data are consistent with path II, although as they point out, these measurements do not exclude path III, which represents convective transport from the troposphere to the stratosphere at mid and high latitudes. Lelieveld et al. (1997) report aircraft measurements of CO, 03, and HNO-, over western Europe that suggest that tropospheric air can be mixed into the lower stratosphere. 

Studies of the dynamics of the tropical tropopause layer are of great interest for quantitative estimates of climate change and an understanding of the mechanisms... 

Figure 3.2. Zonally averaged temperature (K) from the surface to approximately 120 km altitude in January, based on Fleming et al. (1988). Note the temperature minimum (less than 200 K) at the tropical tropopause, the temperature maximum (280 K) at the summer stratopause and the temperature minimum (less than 140 K) at the summer mesopause. The height of the mesopause increases from approximately 90 km in summer to 100 km in winter.

Transport from the troposphere to the stratosphere occurs primarily in the tropics and is associated with the upward branch of the Brewer-Dobson circulation. Folkins et al. (1999) argue that the top of the tropospheric Hadley circulation in the tropics occurs at 14 km, he., well below the tropopause, and that a barrier to vertical mixing therefore exists in the tropical tropopause layer (TTL) 2 or 3 km below the thermal tropopause. Air injected above this barrier subsequently participates in generally slow vertical ascent into the stratosphere. Small-scale exchanges also take place at mid-latitudes through filamentary structures that are drawn poleward in relation with anticyclonic circulation in the upper troposphere (Chen, 1995). 

Equations (3.67) to (3.69) show that, in a hypothetical case where the effects of wave transience and dissipation are ignored (Gu and G = 0) in both the thermodynamic and momentum equations and steady state is assumed, the atmosphere would eventually reach a radiative equilibrium, corresponding to a temperature distribution similar to that displayed in Figure 3.31. In this case, the residual circulation (v, w ) would vanish and the temperatures would be quite different from those observed, particularly at the tropical tropopause and in the mesosphere. At the same time, zonal winds would be considerably faster than are observed with a particularly strong polar vortex during winter. 

At lower altitudes, except at the tropical tropopause and in the winter hemisphere, the stratosphere may be considered to be approximately in radiative equilibrium, while the mesosphere is very far from these conditions. The most dramatic manifestation of the importance of meridional motions for the temperature structure of the middle atmosphere occurs at the summer polar mesopause, where the temperature (about 130 K) is the lowest observed anywhere on Earth, even though the region receives substantially more solar radiation than does the winter mesopause (Box 3.2). 

Jensen, E. J., et al., High humidities and subvisible cirrus near the tropical tropopause, Geophys Res Lett 25, 151, 1998. 

Schauffler, S.M., L.E. Heidt, W.H. Pollack, T.M. Gilpin, J.F. Vedder, S. Solomon, R.A. Lueb, and E.L. Atlas, Measurements of halogenated organic compounds near the tropical tropopause. Geophys Res Lett 20, 2567, 1993. 

There are indeed large ozone losses in the vortex, but all of the large-scale flow is from the tropics northward and downward. Also, because we know the seasonal phase of C02 and water over the tropical tropopause, we know that there is no communication backward from the polar regions to midlatitudes in these key months of ozone loss. Therefore, it isn t simply ozone-depleted arctic air moving back into midlatitudes. 

Secondary Tropical Tropopause (STT) Maximum Convective Outflow... 

The halogen gases enter the stratosphere primarily across the tropical tropopause with air pumped into the stratosphere by deep convection. Air motions in the stratosphere then... 

The tropical tropopause, essentially defined by the 380 K potential temperature surface (see Chapter 21), is located at about 16-17 km. Tropical convection occurs up to an altitude of about 11-12 km. Between these two levels lies a transition region between the tropopause and the stratosphere, call the tropica tropopause layer (TTL). Together with the lowermost stratosphere, the region is referred to as the upper troposphere/lower... 

Simultaneous in situ measurements of CO2 and water vapor in the lower stratosphere in November 1992 and May 1993, for example, were analyzed to infer a mean transport time of 4 to 6 months from the tropical tropopause (—16 km) to — 18.5 to 19 km at midlatitudes (Boering et al., 1995). It takes many years for a species to reach the upper levels of the stratosphere from the time it crosses the tropopause. 

Tropospheric air enters the stratosphere predominantly at the tropical tropopause (see Chapter 1). Once in the stratosphere, it is dispersed upward and toward the poles. Stratospheric air is lofted most efficiently in the tropics, and exchange of air between the... 

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