global modelling of utls ozone david stevenson + many others [email protected] institute...
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Global Modelling of UTLS Ozone
David Stevenson + many others
[email protected]/homes/dstevens
Institute of Atmospheric and Environmental ScienceThe University of Edinburgh
Royal Met. Soc. 18th October 2006, London Zoo
Very few observations of long-term trends in tropospheric ozone…
Year
J F M A M J J A S O N D
1850 1900 1950 2000Montsouris (Volz and Kley, 1987)
Arkona (Feister and Warmbt, 1987)
Time (month)
30
20
10
Ozo
ne
[p
pb
]
Ozo
ne
[p
pb
]
30
25
20
15
10
5
0
Year
J F M A M J J A S O N D
1850 1900 1950 2000Montsouris (Volz and Kley, 1987)
Arkona (Feister and Warmbt, 1987)
Time (month)
30
20
10
Ozo
ne
[p
pb
]
Ozo
ne
[p
pb
]
30
25
20
15
10
5
0
Surface ozone at Arosa, Switzerland
Staehelin et al., 2001
NH mid-lats, mid-troposphere
Logan et al., 1999; O3 sonde data
Even shorter time period of observationsfrom the free atmosphere…
Largeinterannualvariability
Regionallydifferent trends;regionallydifferent AQmeasures
Models of tropospheric ozone
Limited observational evidence suggests that O3T has increased substantially since pre-industrial times
No ice-core record of O3 (too reactive)
Recent (last 30 years) trends show regional differences and are obscured by large interannual variations
We are dependent on models to produce a global picture of O3T change (past and future)
Best we can do is produce models that closely match the limited set of observations of O3 and its precursors, and hope they can reliably simulate the past/future
But it is difficult to know the true ‘ozone sensitivity’ – i.e. O3/emissions or O3/climate
However, we can assess the consistency (or otherwise) between models – i.e. intercomparisons
Trop. O3 radiative forcing
1860-1990 O3 (Stevenson et al., 1998)
Zonal mean O3 change
Tropospheric O3 radiative forcing
• Simulate pre-industrial and present-day O3T,use the change to calculate a radiative forcing
• A large part of this is due to changes in UT O3
15-40°N: Cold, high tropopause, hot surface, clear skies
About ¼ ofCO2 forcing
Warming from increasesin GHGs +3 W m-2
A commonly held view?
• “Nobody believes a modelling paper except the author; everybody believes an observational paper – except the author”
• One solution…
ACCENT model intercomparison for IPCC-AR4
• 26 different models perform same experiments– 16 Europe:
• 4 UK (Edinburgh, Cambridge x2, Met. Office)• 4 Germany (Hamburg x2, Mainz x2)• 2 France (Paris x2)• 2 Italy (Ispra, L’Aquila)• 1 Switzerland (Lausanne)• 1 Norway (Oslo)• 1 Netherlands (KNMI)• 1 Belgium (Brussels)
– 7 US– 3 Japan
• Large ensemble reduces uncertainties, and allows them to be quantified
Intercomparison simulations
• Year 2000 – using EDGAR3.2 emissions– Fix biomass burning & natural emissions
• 3 Emissions scenarios for 2030– ‘Likely’: IIASA CLE (‘Current Legislation’)– ‘High’: IPCC SRES A2– ‘Low’: IIASA MFR (‘Maximum technically
Feasible Reductions’)
• Also assess climate feedbacks – expected surface warming of ~0.7K by 2030
Comparison of ensemble mean model with O3 sonde measurements
J F M A M J J A S O N D
Observed ±1SD
Model ±1SD
90-30°S 30°S-Eq 30°N-Eq 90-30°N
UT250 hPa
MT500hPa
LT750hPa
Individualmodels in
grey
E. Asian NOx emissions too low; Biomass burning emissions too high
GOME NO2 Tropospheric Column 2000
Mean of 3 retrieval methods Std. Dev. of 3 retrieval methods
Mean of 17 models Std. Dev. of 17 models
Models’ CO underestimates observations in Northern Hemisphere- Asian CO emissions too low
Where is modelled O3T most uncertain?Z
on
al m
ean
yea
r 20
00 O
3T
Year 2000Ensemble meanof 26 models
AnnualZonalMean
Annual TroposphericColumn
Year 2000Inter-modelstandard deviation (%)
AnnualZonalMean
Annual TroposphericColumn
Models show large variationsin the crucial tropical UT region
Annual Zonal MeanΔO3 / ppbv
Annual Tropo-spheric ColumnΔO3 / DU
‘Likely’IIASA CLE
SRES B2 economy +Current AQ Legislation
‘Optimistic’IIASA MFR
SRES B2 economy +Maximum Feasible
Reductions
‘Pessimistic’IPCC SRES A2
High economic growth +Little AQ legislation
Change in tropospheric O3 2000-2030 under 3 scenarios
Main candidates for inter-model differences in tropical UT O3
• Convection–Vertical mixing of both O3 and its precursors–Lightning NOx production– In-cloud chemistry, washout–Distribution of water vapour
• Different treatments of emissions– Injection height of biomass burning–Biogenic VOCs and degradation chemistry–Lightning NOx (magnitude/profile)
• Stratosphere-troposphere exchange• All of above also sensitive to climate change…
Effect of switching on convection in 2 models
STOCHEM-HadAM3(Doherty et al., 2005)
MATCH-MPIC(Lawrence et al., 2003)
Convection increases ozone everywhere
Convection increases ozone in tropical MTDecreases elsewhere
We don’t know what convectiondoes to UT O3 !
Convective mass fluxes differ markedly
STOCHEM-HadAM3;Too strong/high?
ERA-40The truth?
MATCH-MPICToo weak/low?
Or are differencesin the chemicalschemes the causeof the differences?
Impact of Climate Change on Ozone by 2030(ensemble of 10 models)
MeanMean - 1SD Mean + 1SD
Negative watervapour feedback
Positive stratospheric
influx feedback
Positive and negative feedbacks – no clear consensus
Climate impact of aircraft NOx emissions
ΔNOx
NB negative scale expanded
ΔO3
ΔOH
ΔCH4
NB negative scale expanded
Decay with e-folding timescale of 11.1 years
Short-term warming from ozone
Long-term cooling from methane
Plus minor ozone long-term cooling
UT crucial for correct quantification of aircraft NOx impacts…
Summary• Models are essential to simulate past/future ozone (lack
of observations)
• Comparison of models and observations suggest similar levels of uncertainty in both
• Uncertainties in modelled O3 are large in the UT – translates directly into climate forcing
• Convection is poorly understood and a major source of uncertainty – not even clear if convection increases or decreases UT O3
• Likely effects of climate change (water vapour increases, STE changes) on O3 even less well constrained
• Conclusion: plenty to do…
[email protected]/homes/dstevens