Tropopause Folding and Stratosphere-Troposphere Exchange (STE)

1
Tropopause FoldingandStratosphere-Troposphere
Exchange (STE)
http//www.gsfc.nasa.gov/gsfc/earth/pictures/2003/
1117aura/frontF.mpg

• AOSC 637 Presentation
• David Kuhl

2
Overview

• Background
• Climatological tropopause
• General circulation of Stratosphere
• Mechanisms for tropopause folding
• Other STE mechanisms
• Seasonality in STE
• Conclusions

3
Main References

• Holton, J.R. et al. 1995 Stratosphere-troposphere
  exchange. Rev. Geophys. Vol. 33, pp 403-439.
• United States Environmental Protection Agency
  (EPA) (2006), Air Quality Criteria for Ozone
  and Related Photochemical Oxidants, Vol.1.
• World Meteorological Organization (WMO),
  Atmospheric ozone 1985, WMO 16, Geneva,
  Switzerland, 1986.

4
Background Earths Atmosphere

• Troposphere Mixed Layer near the surface
• Neg. Temp. Gradient
• Pos. Lapse Rate (unstable)
• Low in ozone O(0.1 ppm)

Troposphere
5
Background Earths Atmosphere

• Stratosphere Stratified Layer above the
  Troposphere
• Pos. Temp. Gradient
• Neg. Lapse Rate (stable)
• High in ozone O(10 ppm)

Stratosphere
Troposphere
6
Background Earths Atmosphere

• Tropopause Layer between Troposphere and
  Stratosphere
• Temp. Gradient lt
• 2 K/km

Stratosphere
Tropopause
Troposphere
7
Background Earths Atmosphere
Mesosphere
Stratosphere
Tropopause
Troposphere
8
Background Earths Atmosphere
Thermosphere
Mesosphere
Stratosphere
Tropopause
Troposphere
9
Stratospheric Air Tropospheric Air

• Stratospheric Air
• High Ozone which is good for protecting life from
  harmful radiation from the sun
• At times it was high in radiation (In the 1950s
  and 1960s from nuculer bomb testing)
• High in potential vorticity (values greater than
  1)
• Tropospheric Air
• Low Ozone which is good since ozone is not good
  for plants or animals
• Low in radiation
• Low in potential vorticity (values less than 1)
• The thermal gradients keep the two air masses
  from mixing most of the time

10
Climatalogical Tropopause

• 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

Climatological Mean Tropopause Structure
Tropopause
Pole
Equator
Figure 3 Holton et. al 1995
11
Climatalogical Tropopause

• Fluid parcels tend to follow lines of constant
  potential temperature
• Lines of constant potential temperature are
  isentropes
• Transport occurs across isentropes is caused
  diabatic heating and turbulent mixing.
• In General the atmosphere tends to flow along
  isentropes

Tropopause
Isentrope
Figure 3 Holton et. al 1995
12
Tropical Transport

• In the tropics we see diabatic or moist adiabatic
  heating, fueled by water vapor, producing rapid
  vertical transport across the insentropes in
  convective cells.
• Sometimes this transport even reaches past the
  troposphere and into the stratosphere
• This is the main input and mechanism for
  transport into the stratosphere from the
  troposphere.

Tropopause
Isentrope
Figure 3 Holton et. al 1995
13
Midlatitude/Polar Transport

• In the midlatitudes and polar regions (shown
  through in-situ measurements) downward transport
  of stratospheric air into the troposphere occurs
  along the sloping lines of constant potential
  temperature
• In this way the transport is adiabatic and
  requires no heating to drive it.

Tropopause
Isentrope
Figure 3 Holton et. al 1995
14
Climatalogical Tropopause

• Upper Stratosphere
• Area above highest isentrope over the tropics
• Lower Stratosphere
• Area between Upper Stratosphere and tropopause
• Mixing occurs between troposphere and
  stratosphere in this lower stratospheric area

Upper Stratosphere
Low Stratosphere
Troposphere
Figure 3 Holton et. al 1995
15
Motivation

• So 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

16
Motivation

• Chemical species with sources in the troposphere
  and sinks high in the stratosphere, such as
• Methane
• Nitrous oxide
• Chlorofluoro carbons
• Transport maybe viewed as part of global scale
  circulation
• Chemical species with sources in the high
  stratosphere and sinks in the troposphere are
  similar so that transport maybe viewed as part of
  global scale circulation
• However for Chemical species with sources or
  sinks in this lower-stratospheric/upper-tropospher
  ic area
• Aircraft emission
• Heterogeneous chemistry responsible for ozone
  depletion
• Tropospheric nonurban photochemical ozone
  production
• It very important to understand the complete
  dynamics of the transport between the air masses.

17
History

• STE is not the only way to create tropospheric
  ozone!
• 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)
• In 1976 Chatfield and Harrison questioned the
  1973 Chameides and Walker photochemical
  hypothesis
• 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)

18
Global Budgets of Trop. Ozone
IPCC 4th Assessment
Strat-Trop Exchange Chemical Production
770 /- 400 Tg/yr 3420 /- 770 Tg/yr

• Strat-Trop Exchange accounts for 18 of Ozone in
  the troposphere (with a range of 8-44 -- large
  amount of error!)
• Although photochemistry in the lower troposphere
  is the major source of tropospheric ozone, the
  stratosphere-troposphere transport of ozone is
  important to the overall climatology, budget and
  log-term trends of tropospheric ozone. Hocking
  2007

19
Tropopause Folding

• From experimental and computational modeling
  research it has been shown that tropopause
  folding accounts for a major extent of the
  tropospheric ozone (EPA 2006)
• In the 1985 WMO report it states that tropopause
  folding could account for as much as 20 of the
  tropospheric ozone (though this is an old number
  and people are still trying to get a hold of the
  magnitude)

20
Tropopause Folding

• First Theorized in the 1950s (Reed 1955) and
  later proven using many different methods looking
  at tracers such (Danielsen 1968)
• Radiation
• Radiation injected into the stratosphere prior to
  the 1958 moratorium on nuclear testing
• Ozone
• Produced in the stratosphere due to solar
  radiation
• Potential Vorticity
• Conserved quantity with no diabatic heating or
  turbulent mixing
• High values in the stratosphere and low values in
  the troposphere

21
Potential Vorticity
p. 96 Holton 2004

• Relationship between the relative vorticity,
  Coriolis parameter, gravity, gradient of
  potential temperature in pressure coordinates
• Transport only occurs along lines of constant
  potential vorticity unless you have diabatic
  heating or turbulent mixing (p. 108 Holton 2004).
  
• The conservation holds true for weather
  disturbances such as jets and fronts (p. 110
  Holton 2004) where tropopause folding occurs
• Thus potential vorticity is a good tracer for
  stratospheric air masses and tropopause folding
  events

22
Tropopause Folding

• Tropopause folding occurs in areas with large
  vertical shear and strong meridional thermal
  gradients (p.144 Holton 2004)

http//www.srh.noaa.gov/jetstream/global/jet.htm
Pole
Equator
23
Tropopause Folding

• Tropopause folding occurs in areas with large
  vertical shear and strong meridional thermal
  gradients (p.144 Holton 2004)

http//www.srh.noaa.gov/jetstream/global/jet.htm
Large Vertical Shear
Strong meridional Thermal gradient
24
Tropopause Folding

• Tropopause folding occurs in areas with large
  vertical shear and strong meridional thermal
  gradients (p.144 Holton 2004)

http//www.srh.noaa.gov/jetstream/global/jet.htm
Large Vertical Shear Polar Jet core 140mph Up
to 275mph
Strong meridional Thermal gradient
Cold Polar Air
Warm Tropical Air
25
Tropopause Folding
Polar Jet
Holton 2004
Zonal Wind (m/s)

• A common situation with tropopause folding is
  shown in the figure from January 14, 1999 00 UTC
  80W logitude
• The above figure clearly shows a strong polar jet
  core above a cold front at the surface

Pot. Temp.
Cold Front
26
Tropopause Folding
Polar Jet
Holton 2004
Zonal Wind (m/s)

• A common situation with tropopause folding is
  shown in the figure from January 14, 1999 00 UTC
  80W logitude
• The lower figure shows potential vorticity
  contours dipping deep into the troposphere from
  the stratosphere

Pot. Temp.
PV
Polar Air
Trop. Air
Cold Front
27
Tropopause Folding
Polar Jet
Holton 2004
Zonal Wind (m/s)

• In stituations such as this with a very strong
  jet core and a large thermal gradient at the
  surface the system may be unstable
• So that small perturbations induced into the jet
  (or disturbances) amplify.
• This is called Baroclinic instability
• The instability depends on the meridional
  temperature gradient (particualarly at the
  surface)

Pot. Temp.
PV
Polar Air
Trop. Air
Cold Front
28
Tropopause Folding
Polar Jet
Holton 2004
Zonal Wind (m/s)

• For those of your familier with atmospheric
  dynamics you may recognize this situation as a
  perfect precursor for cyclogenisis
• Thus tropopause folding events usually occur
  along with cyclogenisis

Pot. Temp.
PV
Polar Air
Trop. Air
Cold Front
29
Classic Cyclogenesis
1
2
Strong Polar Jet
Large meridional Thermal gradients
3
4
http//rst.gsfc.nasa.gov/Sect14/Sect14_1d.html
30
Tropause Folding

• A case study from Feb. 23, 1994 12UTC
• This is a common situation for tropopause folding
  with a Low pressure system ahead of the fold

31
Tropause Folding
Cyclone

• A case study from Feb. 23, 1994 12UTC
• This is a common situation for tropopause folding
  with a Low pressure system ahead of the fold

Polar Jet
Cold Front
Polar Jet Core
32
Tropause Folding
Cyclone

• A case study from Feb. 23, 1994 12UTC
• This is a common situation for tropopause folding
  with a Low pressure system ahead of the fold

Wet Cloudy Sky N and E
Polar Jet
Dry Clear Sky S and SW
Cold Front
Polar Jet Core
33
Classic Picture (Danielsen 1968)
South
North
Stratosphere
Tropopause
Troposphere
Danielsen 1968
34
Classic Picture (Danielsen 1968)
South
North
Jet
Stratosphere
Tropopause
Troposphere
Danielsen 1968
35
Classic Picture (Danielsen 1968)
South
North
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Troposphere
Danielsen 1968
36
Classic Picture (Danielsen 1968)
South
North
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Mixing of Strat and Trop Air
Troposphere
Warm Air
Cold Air
Danielsen 1968
37
Classic Picture (Danielsen 1968)
South
North
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Mixing of Strat and Trop Air
Troposphere
Warm Air
Cold Air
Wet Cloudy Sky N and E
Dry Clear Sky S and SW
Danielsen 1968
38
Tropopause Folds

• The result is an irreversible transfer of
  stratospheric air from the polar reservoir to
  lower latitudes and to lower altitudes
• Shapiro 1980 estimated observationally that 50
  of the mass within a fold is exchanged with
  tropospheric air during downward penetration.
• Significant intrusions of stratospheric air occur
  in ribbons 200 to 100 km in length, 100 to 300
  km wide and about 1 to 4 km thick (EPA 2006).
• These events occur throughout the year and their
  location follows the seasonal displacement of the
  polar jet stream

39
Tropopause Fold
North
South
http//www.gsfc.nasa.gov/gsfc/earth/pictures/2003/
1117aura/frontF.mpg
Cyclone
Wet Cloudy air
Clear Dry air
40
Tropopause Fold Model

• In the model the intrusion crept way down in the
  troposphere. Intrusions which reach the surface
  are rare. Much more common are intrusions which
  penetrate only to the middle and upper
  troposphere (EPA 2006).
• Though it should be said that even middle and
  upper tropospheric ozone is transported to the
  surface much quicker than stratospheric air due
  to various exchange mechanisms that mix
  tropospheric air

41
Other STE Mechanisms

• In the areas of tropopause folding there are
  other STE mechanism which have been identified.
• This is understandable since it is an area with
  large cyclones and a fast jetstream
• Its very hard to measure and quantify the
  contributions from each of these mechanisims
• Cutoff Cyclones
• Streamers
• Clear air turbulence

42
Cut-off Cyclones

• 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

43
Streamers

• Streamers are stratospheric Intrusions sheared
  into long filamentary structures that often roll
  into vortices and mix with with subtropical
  tropospheric air
• Stretching of stratospheric intrusions to ever
  finer scales leads to irreversible transport,
  often speeded up by turbulence resulting from
  shear instabilities

44
Clear Air Turbulence

• CAT occurs in the vicinity of jet streams
  (resulting from vertical wind shear instabilities
  within tropopause folds) and in the region of
  decreasing winds in the stratosphere above the
  jet core (Shapiro 1980)

Jet
CAT
45
Trop. to Strat. exchange?

• We know that ozone comes down but how do we know
  that tropopause folding does this not cause
  mixing up into the stratosphere?
• We basically know how much ozone is transported
  down, and if a similar amount of water vapor was
  transported up at the same time there would be
  much higher quantities of water vapor in the
  stratosphere (which we certainly dont see)
• Only in the lowest kilometer or so of the
  stratosphere is there evidence of a two-way
  exchange.

46
Seasonal Cycle (EPA 2006)

• The seasonal cycle of STE ozone is related to the
  large scale pattern of tracer transport in the
  stratosphere (not the peak in tropospheric
  cyclone activity).
• During winter in the Northern Hemisphere, there
  is a maximum in the poleward, downward transport
  of mass, which moves ozone from the the tropical
  upper stratosphere to the lower stratosphere of
  the polar and midlatitdes.
• This global scale pattern is controlled by the
  upward propagation of large-scale and small-scale
  waves generated in the troposphere.
• As the energy from these disturbances dissipates,
  it drives this stratosphere circulation.
• As a result of this process, there is a
  springtime maximum in the total column abundance
  of ozone over the poles

47
Seasonal Cycle (EPA 2006)

• The concentration of ozone (and other trace
  gases) build up in the lower stratosphere until
  their downward fluxes into the lower stratosphere
  are matched by increased fluxes into the
  troposphere
• Thus, there would be a springtime maximum in the
  flux of ozone into the troposphere even if the
  flux of stratospheric air through the tropopause
  by tropopause folding remained constant
  throughout the year (Holton 1995)
• Indeed, cyclonic activity in the upper tropophere
  is active throughout the entire year in
  transporting air from the lower stratosphere into
  the troposphere

48
Conclusion

• I hope this gives an idea of the general size and
  scale of tropopause folding events and how they
  fit into the broader general circulation of the
  atmosphere between the stratosphere and
  troposphere
• Even though we have two seemingly separate layers
  (Troposphere and Stratosphere), there is
  interaction and how and when interaction occurs
  is an important piece of the puzzle for
  understanding the chemistry of the earths
  atmosphere.

49
References

• Danielsen, E.F. 1968 Stratospheric-tropospheric
  exchange based upon radioactivity, ozone, and
  potential vorticity, J. Atmos. Sci., Vol. 25,
  pp. 502-518.
• Hocking, W.K. et al. 2007 Detection of
  stratospheric ozone intrusions by windprofiler
  radars, Nature, Vol. 250, Nov. 8, pp. 281-284.
• Holton, J.R. et al. 1995 Stratosphere-troposphere
  exchange. Rev. Geophys. Vol. 33, pp 403-439.
• Holton, J.R. 2004 An Introduction to Dynamic
  Meteorology, 4th Edition, Elsevier Academic
  Press.
• Intergovernmental Panel on Climate Change (IPCC).
  (2006) Working. Group I Report The Physical
  Science Basis Cambridge, United Kingdom
  Cambridge University Press
• Reed, R.J. 1955 A study of a characteristic
  type of upper-level frontogenesis. J. Meteor.
  Vol 12, pp. 226-237.
• Shapiro, M.A., 1980 Turbulent mixing within
  tropopause folds as a mechanism for the exchange
  of chemical constituents between the stratosphere
  and the troposphere, J. Atmos. Sci., Vol. 37,
  pp. 994-1004.
• United States Environmental Protection Agency
  (EPA) (2006), Air Quality Criteria for Ozone
  and Related Photochemical Oxidants, Vol.1.
• World Meteorological Organization (WMO),
  Atmospheric ozone 1985, WMO 16, Geneva,
  Switzerland, 1986.

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Thank you!