Stratospheretroposphere Exchange

The exchange of mass between the stratosphere and troposphere is important to the chemistry of both regions as it brings chemical species with sources in the troposphere (such as CFCs) into the stratosphere, while species with stratospheric origin (such as ozone) can be brought into the troposphere. Thus, the transport can be important for driving the chemistry in both regions. Analogous to the boundary layer being isolated from the free troposphere because of the presence of a substantial inversion, the troposphere is isolated from the stratosphere by the high static stability of the stratosphere. Similarly, just as the boundary layer is turbulent and well mixed compared to the free troposphere, the troposphere is relatively well mixed vertically and horizontally compared to the stratosphere. The mixing time in the troposphere (on the order of months within each hemisphere; on the order of a year between the hemispheres) is much shorter than the time required to exchange the mass of the entire troposphere with the stratosphere (on the order of 18 years, although due to the difference in mass the entire stratosphere mixes with the troposphere every 2 years).

The simplest way to visualize a model for stratosphere-troposphere exchange is to consider bulk exchange between the two domains accomplished by uniform rising motion across the tropical tropopause, poleward drift in the stratosphere, and by continuity of mass, a return flow into the troposphere at middle and high latitudes. Such a circulation was first proposed in the 1940s by Brewer to explain the observed low water vapor mixing ratios in the stratosphere. The only place near the tropopause where the temperature is low enough to accompany such low values of relative humidity is in the tropics, where the tropopause is high and cold. Dobson pointed out that poleward and downward advection of this type of mean circulation was consistent with the observed high concentration of ozone in the lower polar stratosphere, far from the region of photochemical production. Although the Brewer-Dobson model does not provide a complete description of the exchange process, it is believed to be essentially correct; see Holton et al. (1995).

The Brewer-Dobson circulation cell is now known to be predominantly wave driven. The morphology of stratospheric wave forcing indicates that upward movement of air into the stratosphere occurs in the tropics and downward movement of air into the troposphere occurs preferentially in winter in middle and high latitudes. Net cooling is required to transport air from the stratosphere into the troposphere

9 STRATOSPHERE-TROPOSPHERE EXCHANGE 25

whereas net diabatic heating is required to transport air from the troposphere into the stratosphere. Extensive measurements during the STEP (Stratosphere-Troposphere Exchange Project) in the 1980s showed that specific vigorous convective events were primarily responsible for transporting tropical air into the stratosphere. Troposphcric air can be either mixed directly into the stratosphere when the cumulo-nimbus towers overshoot, mixed across the tropopause by turbulent motion, or moved upward due to radiative heating of cloud tops. The dehydration occurs because some or all of the condensed ice particles are returned to the troposphere by sedimentation while the dry air remains in the stratosphere. Soluble chemical species will be found in the ice particles rather than in the dry air surrounding them, so there may be a greater resistance to cross-tropopause transport of soluble compounds, A schematic diagram illustrating the general concept of the circulation between the troposphere and stratosphere is shown in Figure 9.

Mass flow from the stratosphere to the troposphere tends to be concentrated in dynamical events known as tropopause folds, in which the tropopause on the poleward side of the jet stream is distorted during the development of iarge-scale weather

Large-scale ascent

1000

Wave-driven

extratropical -V-

Large-scale subsidence

Two-way exchange blocking anticyclones cut-off cyclones tropopause (olds

Some cumulonimbus clouds penetrate stratosphere

Pole

Equator

Latitude

Figure 9 Schematic diagram showing the large-scale dynamical aspects of stratosphere-troposphere exchange. The wiggly double-headed arrows denote meridional transport by large-scale eddy processes. The broad arrows show transport by the global-scale circulation, which is the primary exchange mechanism that moves air across ¡sen tropic surfaces. (Reprinted with permission from Holton et al., 1995.)

Tropospherer Stratosphere Exchange Ozone

90 120 ISO Ozone (ppbv)

90 120 ISO Ozone (ppbv)

—t—i—i—t—j—i i t < ! i t > i ' i i i i —i—

137.97 136,63 136 18 139 25 E Ion

Figure 10 (see color insert) Three-panel figure showing evidence of ozone input from the stratosphere into the troposphere in both hemispheres. The top panel shows the flight path (heavy line) of a DC-8 airplane on October 3, 1992, from South America to Africa that intersected a trough protruding from higher latitudes. Points A and B on that flight path show high concentrations of ozone being transported to altitudes below 6 km in the middle panel; the data depicted in this panel were obtained from a differential absorption lidar system that measured ozone below the 11-km flight level of the DC-8. The lowest panel shows a similar feature for a flight on March 11,1994, in the Northern 1 Iemisphere. As the airplane flies from north to south in this panel, note the higher tropopause height south of the fold. See ftp site for color image.

9 STRATOSPHERE-TROPOSPHERE EXCHANGE 27

systems. Large amounts of stratospheric air extend into the troposphere and much of that air becomes trapped in, and eventually mixed with, the troposphere. An example of stratospheric air coming into the troposphere during a tropopause fold is shown in Figure 10. This figure illustrates the intrusion of stratospheric tracers into the troposphere using a differential absorption laser radar (lidar) instrument that measures ozone below and above it as it flies in an airplane at a cruising altitude of 11 km. The top panel shows the flight path of the airplane (heavy line) and the geopotential height distribution at 200 hPa. This flight path between South America and Africa was part of a field mission in October 1992. Points A and B refer to the location of the two "tongues" of stratospheric air that have descended into the troposphere as the flight path intersected a trough from southern middle latitudes. The middle panel of Figure 10 shows the descent of ozone from the stratosphere (brown areas, >100ppbv) in conjunction with the tropopause fold (see ftp site for color image). At these points, stratospheric air, as marked by the high concentrations of ozone, has descended to altitudes as low as 6 km. As the flight continues to the east, and as measurements from the upward-looking lidar system became available, the tropopause is located at ~ 15 km. The higher concentrations of ozone in the middle and upper troposphere (denoted by the orange colors) were formed in situ from widespread biomass burning taking place at this time of the year. The bottom panel is from a flight in March 1994 and perhaps better illustrates the distribution of ozone during a folding event. Note how much higher the tropopause is south of the fold (later in the flight), than at higher latitudes in the beginning of the flight (~8 km at the beginning of the flight), consistent with the schematic shown in Figure 9.

Figure 11 Annual mean distribution of global tropopause folding activity obtained from meteorological analysis over a 10-year period, 1984-1993; the size of the dots denotes the activity corresponding to bringing air from the stratosphere into the troposphere. (Reprinted with permission from Beekman et al., 1997.)

Figure 11 Annual mean distribution of global tropopause folding activity obtained from meteorological analysis over a 10-year period, 1984-1993; the size of the dots denotes the activity corresponding to bringing air from the stratosphere into the troposphere. (Reprinted with permission from Beekman et al., 1997.)

Figure 11 shows the climatological location of stratospheric intrusions weighted by the intensity of the tropopause event to derive a depiction of how much stratospheric air enters the troposphere. The data have been obtained from European Center for Medium-Range Weather Forecasting (ECMWF) data using an identification scheme relating potential vorticity to the exchange of air between the stratosphere and troposphere. This analysis, published in 1997, agrees with previous studies suggesting that considerably more exchange takes place in the Northern Hemisphere relative to the Southern Hemisphere and that the flux in the NH is 6 x 1010 molecules 03/cm2 s. This value is in agreement with a number of previous studies since the 1970s that have estimated a cross-tropopause flux using both general circulation models and observations calculating amounts of between 4 and 8 x 1010 molecules 03/cm2 s for the NH. With respect to the global tropospheric ozone budget, this "natural" flux of ozone transported would account for only a relatively small fraction of the ozone now commonly measured near Earth's surface, implying that much of the ozone present in the lower atmosphere would not be there without anthropogenic input. The chapter on tropospheric ozone will discuss the tropospheric ozone budget in more detail.

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