Overview of UT/LS Science Issues and New Information from SEAC4RS Steve Wofsy and Jasna Pittman – Harvard UniversityQing Liang – NASA Goddard Space Flight Center, Universities Space Research AssociationPaul Newman – NASA Goddard Space Flight Center

Presented at the SEAC4RS Science Team meetingPasadena, CA, 29 April 2015

Science Questions for this introductory talk:

• What are the transport rates into, within, and out of the stratosphere?

• How can we measure the overall rates of transport? • Is the stratospheric circulation changing in response to

warming climate and increasing greenhouse gases?• How well to we know the global removal rates/lifetimes

of ODSs?

The discussion will consider tracer transport phenomena and key observational constraints on the emergent properties of the stratosphere.

  52    CFC-11      56.2 yr (monthly lifetimes between 49.4 – 63.4)102   CFC-12    100.4 yr (93.6 – 105.9)  93    CFC-113   91.3 yr (84.8 – 97.4)114    N2O        118.5 yr (111.4 – 126.8)  26    CCl4           50.4 yr (42.4 – 54.5) [35 with tropospheric loss]

τWMO Species   τGEOSCCM  variability, range           

Brewer-Dobson Circulation is Projected to Increase with Climate Warming

Implications: Changes in O3, feedback on

climate, shorter lifetimes for

ODSs

JGR, 1994

Mean age of air (average time since an air volume entered the stratosphere) directly measures stratospheric circulation

Definition of age of air, age spectrum, width of age spectrum

age spectrum

mean age

Andrews et al, 2001

N2O Isopleth

Mean Age = Δ CO2 / bCO2

= Δ SF6 / bSF6

Vertically integrated global loss bx time rate of change of X

bx time rate of change of x=CO2 (corrected for CH4 ) or change of SF6 (corrected for mesospheric loss )

Mean Age Tracer-tracer and flux ratios

Rate of change for tracer (e.g. N2O) affects slope and mean lifetime

In a 1-d atmosphere, with gases emitted at the surface, the slope of a tracer-tracer plot at Z is proportional to the vertically integrated loss rate above Z + the rate-of-change of the concentration at the surface.

We have (almost) inert tracers that increase linearly with time direct measure of mean age

De-seasonalized Tracer Trends

The stratosphere approximates the 1-dimensional atmosphere:quasi-horizontal transport rate >> chemistry, vertical transport rate

How good is this approximation?CO2 vs Age of Air: Age of Air vs height

Data from GEOSCCMcompact simple relationship above about 20 km

Models and Data:

Changes over time of mean age and lifetimes for gases in the stratosphere

Vertical Profiles of Mean Age from CO2

Engel et al., 2009

-2 0 2 4 6 8 CO2-derived mean age

GRL, 2006 Models:Large changes, noisy signal.

3.35 2.85

Austin and Li, 2006

Age of Air Global lifetime of ODSs and tracers

100 150 200 250 300 356 358 360 362 364 366 368 370 N2O (ppb) CO2 (ppm)

0

5

0

100

150

20

0

250

0

5

0

100

150

20

0

250

CFCl

3

CFCl

3

Observations of the Middle Stratosphere (OMS)1998 05 18 Ft. Sumner (34 N)

GEOSCCM Obs (corr CH4)

GEOSCCM Obs

Z* (k

m)

Eric Ray Andreas Engel

Mea

n Ag

e of

Air

0

2

4

6

8

Observed mean ages in the middle stratosphere are older by 15-25% than for the GEOSCCM model – what is implied about ODS lifetimes?

No trend can be discerned due to severe under-sampling, ~ 0 today

32 +/-2 km

Mean Ag

e     4.5     5.0       5.5      6.0

1985          1995            2005Year

Tropical average local photolytic lifetimes, 20N—20S. Colored dashed lines represent the cutoff levels for each of the trace gases in the tropics. Source: Moore et al., BAMS 2014.

Photolysis rates increase exponentially vs. altitude large change in loss rate w/ small change in vertical extent

(Local Loss τ )

16.1             24.5           32.2            40.7          48

.3 

      

Z* (k

m)

Small shifts in tracer distributions in response to 10% increase in overturning circulation

Fractional change in tracer vs. alt

    Change in mean age

Fractional change in tracer vs. N2O

CFC-

1235

0

4

00

4

50

500

390 392 394 396CO2

SEAC4RS: Seasonal signal, attenuated full age spectrum

Model Age Spectrum is generally excellent!

Age

of A

ir (d

ays;

GEO

SSCM

)0

2

00

400

60

0 8

00 1

000

120

0

0 500 1000 1500 2000 2500 3000 0 20 40 60 80 100 120 Ethane Ethyne

Ethane vs. GEOSCCM mean age Ethyne

EthyneCH3Cl

CH3Cl

CFC-12 CO2

SEAC4RS (all): Tracers of different character vs Potential T

Source: Anderson, Margitan, Stedman, Science, 1977

Summary and conclusions

• CO2 and SF6 observations provide accurate mean ages for the middle stratosphere and lifetimes for species removed in the stratosphere. 

• CO2 is better at high altitudes and latitudes (no mesospheric losses; need to measure CH4 also); 

• SF6 in lower stratosphere (small seasonal variation).

• CO2 provides age spectra for the UT/LS.• GEOSCCM tracer fields appear to agree well with many tracers, but too young in middle stratosphere 

• We need a strategy for observing the ongoing evolution of stratospheric climate and composition.

SF6

CCl4 CFCl3

HIPPO cross sections, mid-Pacific, Nov 2009

Also Jan 2009, Mar-April 2010, Jun-Jul-Aug-Sep 2011

slope=.601

Annual Emission Fluxes derived from HIPPO lower tropospheric gradientsSF6 10 Gg/yr 2010 (adopted from Rigby, based on interannual trend)CCl4 62 Gg/yr (ratio 6.0 mole/mole vs. SF6 ; no sink asymmetry adj)CFCl3 86 Gg/yr (ratio 9.2 mole/mole vs. SF6 : the elusive residual source)

Note: SF6 latent heat 12 kJ/mole > CFC-11; greater ΔPvap in summer?

Similar approach to get emissions of species emitted in the NH & inert in the troposphere

TTL transport rates, Atmospheric Lifetimes, and Global Stratospheric Removal Rates for Greenhouse Gases and Ozone Destroying Substances from CO2-tracer relationships

Steven C. Wofsy, Jasna V. Pittman, Bruce C. DaubeHarvard University

Paul A. Newman and Qing LiangGoddard Space Flight Center

Elliot AtlasUniversity of Miami

Arlyn E. AndrewsNOAA ESRL, Global Monitoring Division

Michael Volk et al.

SH high lat

       N2O    CFC11 CFC12 H1211 lvl0         356.9   358.5  357.2   359.0 co2..ch4     1.5       0.7     1.3    0.6

#age.of.air.lvl0#     N2O    CFC11    CFC12    H1211 #6.277696 4.698718 5.960831 4.310113 ##  --- below, not corrected for CH4 (big error !)     #N2O    CFC11    CFC12    H1211 #5.290422 4.248600 5.095080 3.923031 

Z* 7*ln( 100

0/P)

0       10

        20       

30

Z* 7*ln( 100

0/P)

0       10

        20       

30