Three-Dimensional Chemical Transport Model Studies of Arctic Ozone Depletion

Wuhu Feng and Martyn ChipperfieldSchool of the Earth and Environment, University of Leeds

AcknowledgmentsS. Davies, B. Sen, G. Toon, J.F. Blavier, C.R. Webster, C.M. Volk, A. Ulanovsky, F. Ravegnani, P. von der Gathen, H. Jost, E.C. Richard, H. Claude NERC, EU TOPOZ III, QUILT , QUOBI projects

• Model descriptionRecent improvements to SLIMCAT 3D CTM• ResultsComparisons of new/old CTM for Arctic winter 2002/3Improved decadal simulations of Arctic O3 loss (Rex plot) • Conclusions

Aim of the work 1: Quantify and understand the degree of chemical ozone loss

Aim of the work 2: Improve the Chemical Transport Model (e.g. Rex plot)

Measurements: colored squaresOld SLIMCAT: black points

Rex et al. (GRL,2004)

ARCTIC OZONE LOSS

• Off-line chemical transport model [e.g. Chipperfield, JGR, 1999]

• Extends to surface using hybrid - levels (SLIMCAT version). Variable horizontal/vertical resolution.

• Horizontal winds and temperatures from analyses (e.g. UKMO, ECMWF (ERA-40 or operational)).

• Vertical motion from diagnosed heating rates or divergence.

Radiation scheme MIDRAD or CCM scheme

• Tropospheric physics: convection, PBL mixing etc.

• Chemistry: ‘Full’ stratospheric chemistry scheme (41 species, 160 reactions) with heterogeneous chemistry on liquid/solid aerosols/PSCs and an equilibrium denitrification scheme.

NAT-based denitrification scheme included.

www.env.leeds.ac.uk/slimcat

SLIMCAT/TOMCAT 3D CTM

2002/03 Meteorology

PSC extent decreases with height

Very Low Temp. in Dec. 2002(produces early O3 loss)

NEW SLIMCAT VS OLD SLIMCAT comparison with MK4 balloon data

Old SLIMCAT model (with lower boundary at 350K) overestimates N2O above 20 km. New version of SLIMCAT (which extends to the surface) gives better N2O distribution. Different radiation schemes result in different transport (CCM better).

N2O CH4

2) NEW SLIMCAT VS OLD SLIMCAT comparison with M55 aircraft data

New version of SLIMCAT with CCM radiation scheme gives more (better) descent than MIDRAD radiation scheme in the old version of SLIMCAT

N2O

CH4

3) NEW SLIMCAT VS OLD SLIMCAT Comparison with O3 sonde data

Significant improvements in the new version of SLIMCAT (I.e. better representation of O3 in the lower stratosphere – better transport and better chemical loss)

425K

460K

495K

OLD SLIMCAT Run NEW SLIMCAT Run

4) NEW SLIMCAT VS OLD SLIMCAT comparison with POAM data

Singleton et al., ACP(submitted),2004

Significant improvements in the NEW SLIMCAT when compared with POAM satellite data (daily average in the vortex).

450K450K

Observations

SLIMCAT - OLD

1990

2004

SLIMCAT – NEW7.5o x 7.5o

New Model: Multiannual Simulations of Polar O3 Loss

New SLIMCAT reproduces the Rex plot much better

99

98

03

97

93

95

96

00

9496

Jan 15Mar 25

SLIMCATObs.

Run OLD

Profile of O3 loss looks ok generally, even in warm winters.

Model has larger changes near 550K.

Model vortex-average does not get very low values of 2000.

1993

20001999

19981997

19961995

1994

C

C

C

C

C

W

W

400

550

40

New SLIMCAT: Vortex Averaged Profiles

1993-2000 for O3 (left) and O3 (right)

A lot! Key points for polar O3 are probably:

• Updated kinetics (JPL 2002) + faster JCl2O2 (Burkholder et

al extended to 450 nm).• NAT-based denitrification scheme.• Minimum aerosol (H2SO4) loading.

• Better vertical transport (more Cly in lower stratosphere) and no lower boundary near tropopause.

• ECMWF analyses (ERA40 + operational).

• Source gas scenarios: + 100pptv short-lived organic Cl, + shift in long-lived organic loading to shorter lived species.

What else has changed in model between old and new model?

Importance of model resolution

Higher resolution model produces large chemical ozone depletion, which agrees better with observations

1999/2000

425K

460K

495K

2.8o x 2.8o

7.5o x 7.5o

1999/2000

Clyy (ppbv) NOy (ppbv)

More denitrification at 2.8o x 2.8o

ERA 40

Vortex maintains stronger gradients – more isolated

Op

Effect of Resolution: New Model

2.8o x 2.8o 7.5o x 7.5o

99/00

02/03

03/04

ClOx (ClO + 2Cl2O2) (ppbv) Ny Alesund (79oN, 12oE)ERA40!

Effect of Resolution: New Model

Conclusion• Updated New SLIMCAT CTM now gives a good simulation of seasonal O3

column loss (and better January loss rates – not shown here).

• Significant improvement in modelling of cold winters in mid 1990s – more modelled O3 loss.

• Higher resolution (2.8o) does increase O3 loss especially in late winter/spring through maintaining active Cl for longer.

• Importance of radiation scheme in the model:Different radiation schemes used in the model can result in different transport and polar ozone loss. More sophisticated CCM scheme gives a better simulation than other schemes.

• Chemical models/modules (based on tested/validated code) within CCMs can be expected to produce reasonable simulations of chemical polar O3 loss (under conditions so far experienced) – more positive than results of Rex et al (2004)!