Transcript Tropopause Folding and Stratosphere

Tropopause Folding
and
Stratosphere-Troposphere Exchange (STE)
http://www.gsfc.nasa.gov/gsfc/earth/pictures/2003/1117aura/frontF.mpg
AOSC 637 Presentation
David Kuhl
Overview
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Background
Climatological tropopause
General circulation of Stratosphere
Mechanisms for tropopause folding
Other STE mechanisms
Seasonality in STE
Conclusions
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Main References
• Holton, J.R. et al. 1995 “Stratospheretroposphere 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.
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Background: Earth’s Atmosphere
Troposphere: Mixed
Layer near the
surface
– Neg. Temp. Gradient
– Pos. Lapse Rate
(unstable)
– Low in ozone
O(0.1 ppm)
Troposphere
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Background: Earth’s Atmosphere
• Stratosphere:
Stratified Layer
above the
Troposphere
Stratosphere
– Pos. Temp. Gradient
– Neg. Lapse Rate
(stable)
– High in ozone
O(10 ppm)
Troposphere
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Background: Earth’s Atmosphere
• Tropopause: Layer
between
Troposphere and
Stratosphere
– Temp. Gradient <
2 K/km
Stratosphere
Tropopause
Troposphere
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Background: Earth’s Atmosphere
Mesosphere
Stratosphere
Tropopause
Troposphere
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Background: Earth’s Atmosphere
Thermosphere
Mesosphere
Stratosphere
Tropopause
Troposphere
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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 1950’s and
1960’s 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
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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
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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
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  T  p0 p 

Tropopause Folding
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
R /cp
 const
Tropopause
Isentrope
Figure 3 Holton et. al 1995
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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.
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Tropopause Folding
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Tropopause
Isentrope
Figure 3 Holton et. al 1995
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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.
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Tropopause
Isentrope
Figure 3 Holton et. al 1995
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Climatalogical Tropopause
• Upper Stratosphere
– Area above highest
isentrope over the tropics
• Lower Stratosphere
– Area between Upper
Stratosphere and
tropopause
Upper Stratosphere
Low Stratosphere
• Mixing occurs between
troposphere and
stratosphere in this
lower stratospheric area
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Troposphere
Tropopause Folding
Stratosphere-Troposphere Exchange
Figure 3 Holton et. al 1995
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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
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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-tropospheric 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.
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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 photochemistry (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)
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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 logterm trends of tropospheric ozone.” Hocking 2007
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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)
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Tropopause Folding
• First Theorized in the 1950’s (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
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Potential Vorticity

 
  Const
P    f   g
p 

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
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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
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Equator
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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
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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]
Cold Polar
Air
Strong meridional
Thermal gradient
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Warm Tropical
Air
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Tropopause Folding
Polar Jet
Zonal Wind (m/s)
Holton 2004
• 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
Cold Front
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Pot.
Temp.
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Tropopause Folding
Polar Jet
Zonal Wind (m/s)
Holton 2004
• 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
PV
Polar Air
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Pot.
Temp.
Trop. Air
Cold Front 26
Tropopause Folding
Polar Jet
Zonal Wind (m/s)
Holton 2004
• 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)
PV
Polar Air
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Pot.
Temp.
Trop. Air
Cold Front 27
Tropopause Folding
Polar Jet
Zonal Wind (m/s)
Holton 2004
• 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
PV
Polar Air
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Pot.
Temp.
Trop. Air
Cold Front 28
Classic Cyclogenesis
1
2
3
4
Strong
Polar Jet
Large meridional
Thermal gradients
http://rst.gsfc.nasa.gov/Sect14/Sect14_1d.html
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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
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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
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Cyclone
Polar Jet
Tropopause Folding
Stratosphere-Troposphere Exchange
Cold Front
Polar Jet
Core
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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
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Cyclone
Polar Jet
Dry Clear Sky
S and SW
Tropopause Folding
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Wet Cloudy Sky
N and E
Cold Front
Polar Jet
Core
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Classic Picture (Danielsen 1968)
South
North
Stratosphere
Tropopause
Troposphere
Danielsen 1968
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Classic Picture (Danielsen 1968)
South
North
Jet
Stratosphere
Tropopause
Troposphere
Danielsen 1968
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Classic Picture (Danielsen 1968)
North
South
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Troposphere
Danielsen 1968
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Classic Picture (Danielsen 1968)
North
South
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Troposphere
Cold Air
Warm Air
Mixing of
Strat and
Trop Air
Danielsen 1968
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Classic Picture (Danielsen 1968)
North
South
Stratospheric Air
Jet
Stratosphere
Troposphereric Air
Tropopause
Troposphere
Warm Air
Cold Air
Wet Cloudy Sky
N and E
Mixing of
Strat and
Trop Air
Dry Clear Sky
S and SW
Danielsen 1968
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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
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South
Tropopause Fold
http://www.gsfc.nasa.gov/gsfc/earth/pictures/2003/1117aura/frontF.mpg
North
Cyclone
Clear Dry air
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Wet Cloudy air
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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
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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
It’s very hard to measure and quantify the
contributions from each of these
mechanisims
• Cutoff Cyclones
• Streamers
• Clear air turbulence
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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
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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
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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
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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 don’t see)
• Only in the lowest kilometer or so of the
stratosphere is there evidence of a two-way
exchange.
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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
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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
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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.
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References
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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!