Tropopause Folding

and

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 Folding

Stratosphere-Troposphere Exchange

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Overview

• Background

• Climatological tropopause

• General circulation of Stratosphere

• Mechanisms for tropopause folding

• Other STE mechanisms

• Seasonality in STE

• Conclusions
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Stratosphere-Troposphere Exchange

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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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Stratosphere-Troposphere Exchange

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Background: Earth’s Atmosphere

Troposphere

Troposphere: Mixed 

Layer near the 

surface 

– Neg. Temp. Gradient 

– Pos. Lapse Rate 

(unstable) 

– Low in ozone 

O(0.1 ppm)
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Stratosphere-Troposphere Exchange

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Background: Earth’s Atmosphere

Stratosphere

Troposphere

• Stratosphere: 

Stratified Layer 

above the 

Troposphere

– Pos. Temp. Gradient 

– Neg. Lapse Rate 

(stable)

– High in ozone 

O(10 ppm)
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Stratosphere-Troposphere Exchange

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Background: Earth’s Atmosphere

Stratosphere

Tropopause

Troposphere

• Tropopause: Layer 

between 

Troposphere and 

Stratosphere

– Temp. Gradient < 

2 K/km
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Stratosphere-Troposphere Exchange

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Background: Earth’s Atmosphere

Stratosphere

Tropopause

Troposphere

Mesosphere
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Stratosphere-Troposphere Exchange

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Background: Earth’s Atmosphere

Stratosphere

Tropopause

Troposphere

Mesosphere

Thermosphere
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Stratosphere-Troposphere Exchange

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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

const

p

p

T

p R c

 













/

 0

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.

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.

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

• Mixing occurs between 

troposphere and 

stratosphere in this 

lower stratospheric area

Upper Stratosphere

Low Stratosphere

Troposphere

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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Stratosphere-Troposphere Exchange

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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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Stratosphere-Troposphere Exchange

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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

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 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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Stratosphere-Troposphere Exchange

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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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Stratosphere-Troposphere Exchange

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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

  Const

p

P f g 



















  





p. 96 Holton 2004
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Stratosphere-Troposphere Exchange

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Tropopause Folding

• Tropopause folding occurs in areas with 

large vertical shear and strong 

meridional thermal gradients (p.144 

Holton 2004)

Pole Equator

http://www.srh.noaa.gov/jetstream/global/jet.htm
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Stratosphere-Troposphere Exchange

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Tropopause Folding

• Tropopause folding occurs in areas with 

large vertical shear and strong 

meridional thermal gradients (p.144 

Holton 2004)

Large Vertical Shear

Strong meridional

Thermal gradient

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

• Tropopause folding occurs in areas with 

large vertical shear and strong 

meridional thermal gradients (p.144 

Holton 2004)

Large Vertical Shear

[Polar Jet core ~140mph

Up to 275mph]

Strong meridional

Thermal gradient Cold Polar

Air

Warm Tropical

Air

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

Holton 2004 Polar Jet

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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Stratosphere-Troposphere Exchange

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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

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

Cold Front

Cyclone

Polar Jet

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

Cold Front

Cyclone

Dry Clear Sky

S and SW

Wet Cloudy Sky

Polar Jet N and E

Polar Jet

Core
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Classic Picture (Danielsen 1968)

Danielsen 1968

North South

Stratosphere

Troposphere

Tropopause
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Classic Picture (Danielsen 1968)

Danielsen 1968

North South

Stratosphere

Troposphere

Tropopause

Jet
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Classic Picture (Danielsen 1968)

Danielsen 1968

North South

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air
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Classic Picture (Danielsen 1968)

Danielsen 1968

North South

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air

Cold Air Warm Air

Mixing of

Strat and 

Trop Air
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Classic Picture (Danielsen 1968)

Danielsen 1968

North South

Stratosphere

Troposphere

Tropopause

Stratospheric Air

Jet

Troposphereric Air

Cold Air Warm Air

Dry Clear Sky

S and SW

Wet Cloudy Sky

N and E

Mixing of

Strat 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 Fold

South 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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Stratosphere-Troposphere Exchange

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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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Stratosphere-Troposphere Exchange

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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!