Tropopause Folding and Stratosphere-Troposphere Exchange (STE) AOSC 637 Presentation David Kuhl

Tropopause Foldingand

Stratosphere-Troposphere Exchange (STE)

AOSC 637 Presentation

David Kuhl

http://www.gsfc.nasa.gov/gsfc/earth/pictures/2003/1117aura/frontF.mpg

December 6, 2007 Tropopause FoldingStratosphere-Troposphere Exchange

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: Earth’s Atmosphere

TroposphereTroposphere

Troposphere: Mixed Layer near the surface – Neg. Temp. Gradient – Pos. Lapse Rate

(unstable) – Low in ozone

O(0.1 ppm)


5


StratosphereStratosphere

TroposphereTroposphere

• Stratosphere: Stratified Layer above the Troposphere – Pos. Temp. Gradient – Neg. Lapse Rate

(stable)– High in ozone

O(10 ppm)


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TropopauseTropopauseTroposphereTroposphere

• Tropopause: Layer between Troposphere and Stratosphere– Temp. Gradient < 2 K/km


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MesosphereMesosphere


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MesosphereMesosphere

ThermosphereThermosphere


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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

Figure 3 Holton et. al 1995

Pole Equator


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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

Tropopause

Isentrope

constppT

pcR

/

0



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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.

Tropopause

Isentrope



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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.

Tropopause

Isentrope



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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



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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 carbonsTransport 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 productionIt 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 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)


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Global Budgets of Trop. Ozone

Strat-Trop Exchange

Chemical Production

770 +/- 400 Tg/yr

3420 +/- 770 Tg/yr

IPCC 4th Assessment

• 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


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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

• 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

Constp

gfP

p. 96 Holton 2004


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Tropopause Folding

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

EquatorPole

http://www.srh.noaa.gov/jetstream/global/jet.htm


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Tropopause Folding


Large Vertical Shear

Strong meridionalThermal gradient



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Tropopause Folding


Large Vertical Shear[Polar Jet core ~140mph

Up to 275mph]

Strong meridionalThermal gradient

Cold PolarAir

Warm TropicalAir



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Tropopause Folding

• 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

Polar JetHolton 2004

Zonal Wind (m/s)


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Tropopause Folding

• 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

Cold Front

Polar Jet

Polar Air Trop. Air

Holton 2004

Zonal Wind (m/s)


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Tropopause Folding• 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.

Zonal Wind (m/s)

PV

Cold Front

Polar Jet

Polar Air Trop. Air

Holton 2004


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Tropopause Folding

• 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

Cold Front

Polar Jet

Polar Air Trop. Air

Holton 2004

Zonal Wind (m/s)


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Classic Cyclogenesis

1 2

3 4

StrongPolar Jet

Large meridionalThermal 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



Cold Front

Cyclone

Polar Jet

Polar JetCore


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Tropause Folding



Cold Front

Cyclone

Dry Clear SkyDry Clear SkyS and SWS and SW

Wet Cloudy SkyWet Cloudy SkyN and EN and EPolar Jet

Polar JetCore


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Classic Picture (Danielsen 1968)

Danielsen 1968

SouthNorth

Stratosphere

Troposphere

Tropopause


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Danielsen 1968

SouthNorth

Stratosphere

Troposphere

Tropopause

Jet


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Danielsen 1968

SouthNorth

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air


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Danielsen 1968

SouthNorth

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air

Cold AirWarm Air

Mixing ofStrat and Trop Air


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Danielsen 1968

SouthNorth

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air

Cold AirWarm Air

Dry Clear SkyS and SW

Wet Cloudy SkyN and E

Mixing ofStrat and Trop Air


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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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Tropopause FoldSouth North

Cyclone

Clear Dry air Wet Cloudy air

http://www.gsfc.nasa.gov/gsfc/earth/pictures/2003/1117aura/frontF.mpg


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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)

CAT

Jet


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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• 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.

Thank you!

Documents

Tropopause Folding and Stratosphere-Troposphere Exchange (STE) AOSC 637 Presentation David Kuhl