The tropical tropopause layer - NASA...The tropical tropopause layer A. E. Dessler, C. M. Alcala Earth System Science Interdisciplinary Center University of Maryland S. C. Sherwood

The tropical tropopause layer

A. E. Dessler, C. M. AlcalaEarth System Science Interdisciplinary Center

University of Maryland

S. C. SherwoodDept. of Geology and Geophysics

Yale University

Brewer, A.W., Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the

stratosphere, Q. J. R. Meteorol. Soc., 1949.

Very low water vapor

16 km

TropopauseDehydration

8 km

Pole Equator Pole

Theories of TTL processes

• Slow ascent

• Convective dehydration

Atmospheric Structure

300280260240220200

Temperature (K)

50

40

30

20

10

0

Altitude (km)

Tropopause

Troposphere

Stratosphere

convection

Slow Ascent

TropicalTropopauseLayer

The stratospheric “drain”

Sherwood, S.C., A stratospheric "drain" over the maritime continent, Geophys. Res.Lett., 2000.

Constraints

Positive or negative?Water-ozone correlation

HDO: -679±20 per mil,H2Q: -128±31 per mil[Johnson et al., 2001]

Isotopic abundance of HDO and H2Q

3.85 ppmv ( ±5%) in the mid 1990s[Dessler and Kim, 1999, SPARC Water Vapor Assessment, 2000]

Water vapor entering the stratosphere

Randel et al., JGR, 2001

January

Randel et al., JGR, 2001

July

Chemistry

• Mix of troposphere and stratosphere• O3 is produced by both photolysis and

HOx+NOx• CO is destroyed by oxidation

– CO & O3: photochemical clock (e.g., Weinstock et al., 2001)

• CH4 and N2O are essentially inert

Theories of Dehydration

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Tropopause

14 km, 355 K

18 km, 420 KStratosphere

Troposphere

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Tropopause

14 km, 355 K


Troposphere

Cold Pool

ConvectiveDehydration

“Cold Trap”Dehydration

Isotope fundamentals

• R=[HDO]/[H2O]• delta D = 1000(R/Ro-1)• Only changes in HDO relative to H2O

change these values• Condensation preferentially reduces HDO

Isotopic Constraint

50 100 150 200 250delta O18

200

400

600

800

1000delta D

Johnson et al., Isotopic composition of stratospheric water vapor: Implications for transport, J. Geophys. Res., 2001.

Keith, D.W., Stratosphere-Troposphere exchange: Inferences from the isotopic composition of water vapor, J. Geophys. Res., 2000.

Moyer et al., ATMOS stratosphericdeuterated water and implications for troposphere-stratosphere transport, Geophys. Res.Lett., 1996.

150 hPa

200 hPa

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14 km, 355 K


Troposphere

Cold Pool

Theories of Dehydration

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Tropopause

14 km, 355 K


Troposphere

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Tropopause

14 km, 355 K


Troposphere

Cold Pool

ConvectiveDehydration

“Cold Trap”Dehydration

Temperature profiles

300280260240220200Temp (K)

8

9

100

2

3

4

5

6

7

8

9

1000

P (hPa)

-8 -4 0Adiabat-Env (K)

8

9

100

2

3

4

5

6

7

8

9

1000

P (hPa)

12 4 6 8

102 4 6 8

100

Saturation WV (ppmv)

Feb. 1995 sonde data, avg. between 10N and 10SPseudo-adiabatic profile lifted from 850 hPa

Conservation Equation in the TTL∂ χ[ ]∂t

= Pχ − Lχ χ[ ]− Ý θ ∂ χ[ ]∂θ

−χ[ ]− χ[ ]PBL

τ

Transport due to slow ascent from mean stratcirculation

rapid vertical transport viaconvection of planetary boundary layer (PBL) air into the TTL with a transport time constant of τ.

Note: θ is potential temperature and Ýθ is the vertical velocity inθ coordinates

Calculated mixing time, τ

80400τ (days)

380

375

370

365

360

355

Potential Temperature (K)

16

15

14

Approximate Altitude (km)

O3 CO

τ is the turnover time for the TTL due to convection.

The increase in τ with altitude is consistent with our view of the thermodynamics of the PBL.

Importantly, our results show that convection does influence thechemistry around the tropical tropopause

Mass crossing the tropopause

1.00.80.60.40.20.0

Fraction crossing the 380-K surface

380

375

370

365

360

355

Potential Temperature (K)

16

15

14

Approximate Altitude (km)

O3 CO

Tropopause

This plot shows the fraction of mass that’s crossing the 380-K surface that detrained above each θ surface

As expected, virtually 100% of the mass detrained above 355 K.Surprisingly, 45-60% of the mass crossing the 380-K surfacedetrained above 370 K.

A solution to the isotopic conundrum

• Johnson et al. [2001]• Imagine that air going into the stratosphere

comes through two pipes. 170 hPa, 210 K 80 hPa, 170 K

80 ppmv-690,-110

0.1 ppmv-980,-3005% 95%

4 ppmv-696,-114

“most of the moisture comes from convective outflow near 11 km, most of the air in the upper troposphere consists of dehydrated air from convective systems with cloud top temperatures below that of the mean tropical tropopause.”

Conclusions• Dehydration and strat-trop exchange are different

processes• Air is transported to the TTL via convection; transported

out of the TTL via Brewer-Dobson circulation• Tracer data show

– Air detrains throughout the TTL– Positive and negative correlations with H2O

• Isotopic data show that air cannot be dehydrated along ONE path

This work was supported by a NASA New Investigator Program in Earth Science grant, an EOS IDS grant, and several ACMAP grants, all to the Univ. of Maryland. Useful conversations with many, many people are also acknowledged.

The tropical tropopause layer - NASA...The tropical tropopause layer A. E. Dessler, C. M. Alcala Earth System Science Interdisciplinary Center University of Maryland S. C. Sherwood

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