This slide will present STE which happens in both tropical and extratropical.

1. 1DEPARTMENT OF EARTH AND SCIENCES
Bandung Institute of Techonolgy
Department of Earth and Sciences
Stratosphere Troposphere Exchange
THOEURN Thean 22417701
Year: 2018

2. Introduction
2
Stratosphere–troposphere exchange (STE) is a part of the general
circulation of the atmosphere that transports air and atmospheric
constituents across the tropopause.
Stratosphere and troposphere are the two adjoining regions of different
characteristics which are coupled radiatively, chemically, and
dynamically.

3. History
3
•Previous to 1973 it was thought that tropospheric ozone was produced
by only dynamic processes transporting from high levels in the
stratosphere into the troposphere
•Then in 1973 Chameides and Walker produced the photochemical
theory for tropospheric ozone where they believed that most tropospheric
ozone came from photo-chemistry (primarily from methane oxidation).
•According to WMO 1995, anthropogenic transported form troposphere
to stratosphere initiate much of chemistry that cause ozone depletion.
•Now general consensus is that “The abundance and distribution of
ozone in the atmosphere is determined by complex interactions between
meteorology and chemistry.” (p. AX2-60 2006 EPA)

4. Motivation
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why do we care when and how the stratospheric air mass
mixes with the tropospheric air mass?
 When mixing occurs it
– depletes the stratosphere of helpful chemical constituents
– increases the levels of harmful chemicals in the troposphere
 Mixing regions are areas of interest for atmospheric
chemistry because combining parcels of air with differing
compositions and lifetimes provides potential for reactions

5. Tropopause
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Isentropic
tropopause
Lowermost stratosphere
The tropic Tropopause corresponds to isentropic whose potential Temperaute
=380K in the annual mean.
The Extratropical Tropopause corresponds to a constant surface potential
vorticity which is 2PVU.

6. Transport Across the Tropopause
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Overworld is the region where the
isentropic surfaces lie entirely in the
stratosphere. Air parcels in this region
cannot reach the troposphere directly.
However, air parcels from the
stratosphere descend first slowly across
the isentropic surfaces, a process that
must be accompanied by diabatic cooling
and reach the troposphere.
Underworld is the region where the isentropic
surfaces lie entirely in the troposphere. The
tropospheric air parcel reaches the stratosphere
by first rising across the isentropic surfaces.
The middle world, the region that lies
between the tropopause and the 380 K
θ surface, is a convergence zone
between tropospheric and
stratospheric air

7. Transport Across the Tropopause
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Tropospause low at mid-
latitudes and poles
where jet streams and
storm tracks occur.
Tropospause high at the
equator where large
amounts of convection
occurs.

8. Transport Across the Tropopause
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Withdraw air upward
and poleward form
tropical LS and
pushes it poleward
and downward into
Extra Troposphere.
Transport along
isentropic surfaces
occurs in adiabatic
motion, whereas
transport across the
isentropes requires
diabatic processes.
In the region of UT and LS consisting
of isentropic surfaces that intersect
the tropopause, air and chemical
constituents can be irreversibly
transported as adiabatic eddy
motions.

9. Exchange Processes Near the Midlatitudes and Subtropics
Tropopause
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In the midlatitudes, exchange appears to be dominated by
processes associated with tropopause folds and cut-off
lows.

10. Tropopause Folding
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Tropopause folding is a process in which a thin band of stratosphere
air intrude more or less deeply into troposphere along the strongly
tilted isentrope associated with an upper troposphere frontal zone.

11. Cutoff cyclone
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 Some parts of the tongues of stratospheric air may
roll up to form isolated coherent structures containing
high-PV air, generally referred to as “cutoff
cyclones”.
 Exchange in cutoff cyclones can occur by convective
or radiative erosion of the anomalously low
tropopause that is characteristic of cutoff cyclones,
by turbulent mixing near the jet stream associated
with the cutoff system, or as a result of tropopause
folding along the flank of the system.

12. Exchange Processes Near the Tropical Tropopause
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Exchange processes near the tropical tropopause have been a subject of much debate
for a few decades, and it is still ongoing. Possible mechanisms include:
• overshooting convection (as shown in Fig. 8.8) in Micronesia, where the air may
overshoot, but getting it to stay at higher potential temperature may be difficult;
and
• a stratospheric fountain, where the air is lifted into the stratosphere by heating

13. Freeze drying
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Freeze Drying is the process in which air passing through of the
tropical tropopause has its water vapor mixing ratio reduced to the
ice saturation value at or near the tropopause.
Air entering the tropical stratosphere from below, being
dehydrated as it enters, and then being gradually moistened,
 by methane oxidation and
 perhaps by weak mixing from the extratropics,

14. Stratospheric Fountain
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A stratospheric fountain is an area where air enters the
stratosphere from the troposphere.
The fountain occurs over the western tropical Pacific, northern
Australia, Indonesia, and Malaysia in the November–March
period and over the Bay of Bengal and India during the monsoon.

15. Conclusion
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• The overword and lowermost stratosphere need to be
distinguished .
• The wave induced forces drive a kind of global-scale
extratropical “ Fluid dynamical Suction Pump” which
withdraws air upward and poleward from the tropical lower
stratosphere and pushes it poleward and downward into the
extratropic tropopause.
• The smaller scale process must be considered in order to
understand the finer detail exchange.
• Moist convection plays important role in the tropic for extreme
dehydration of air entering the stratosphere.