1. How ozone affects global precipitation
Department of Meteorology
How ozone affects global precipitation
Bill Collins
Laura Baker, Richard Allen, Keith Shine, Claire MacIntosh, Zahra Mousavi

2. Outline Conservation of energy Atmospheric forcing
Radiative transfer model
Atmosphere-only climate model
Transient responses
Conclusions

4. Introduction Ozone is a radiatively-active gas. Absorbs in short-wave
Absorbs and emits in long-wave
Locally heats/cools the atmosphere
Leads to circulation changes, eg. Antarctic
Regionally multiple complex precipitation changes
Globally atmospheric absorption balances decrease in latent heat.

5. Conservation of energy
Δ FTOA
kaΔTsurf
Δ Fatm
LΔP
Δ Fsurf
Δ SHslow (= kSHΔTsurf)
Δ SHfast
Flux in = Flux out
LΔP+Δ FTOA+Δ SHslow+Δ SHfast = kaΔTsurf+Δ Fsurf
LΔP = (ka-kSH)ΔTsurf-(Δ FTOA-Δ Fsurf)- Δ SHfast
LΔP = kΔTsurf Δ Fatm Δ SHfast
Slow Fast

6. Δfatm for ozone
Atmospheric forcing (ΔFatm ) for increase in ozone (Δε, Δτ +ve)
-ve in lower troposphere
TA>0.84 Ts ; RUV small
+ve in upper troposphere
TA<0.84 Ts ; RUV small
Very +ve in stratosphere
TA<<0.84 Ts ; RUV large
ΔFatm=Δεσ(TS4 - 2TA4)+SW
-ve if TA>0.84 Ts ; RUV small
ΔτRUV(z)
2ΔεσTa4
Ta(z)
ΔεσTs4 Ts

7. Radiative transfer model
Increase ozone by 20% one layer at a time in Edwards-Slingo RTM
Allowing the stratospheric temperatures to adjust
Below 700 hPa
ΔFatm –ve
700 hPa to 300 hPa
0<ΔFatm< ΔFTOA
Above 300 hPa
ΔFatm> ΔFTOA
ΔFsurf –ve
ΔFatm
ΔFatm
ΔFatm/ ΔFTOA
ΔFTOA

8. Climate model
HadGEM3 climate model with fixed sea surface temperatures
Ozone specified as climatology (no chemistry)
3 year simulations
Idealised
+100% in lower trop and upper trop,
-20% in strat (scaled up to +100% in table)
Historical
changes (from ACCMIP)

9. Results Ozone in LT/UT increases/decreases fast precipitation response
Sensible heat changes offset about 20% of the atmospheric forcing
Overall tropospheric ozone decreases fast precipitation response (UT dominates)
Stratospheric ozone depletion increases fast precipitation response
Overall historical ozone changes balance
Experiment
ΔFTOA
(W m-2)
ΔFatm
ΔSHfast
ΔFatm+ ΔSHfast
LΔP (W m-2)
(mm day-1)
ΔFatm/ ΔFTOA
100% increase
LT (>700 hPa)
0.28
-0.12
0.02
-0.10
0.10
-0.42
UT (<700 hPa)
0.83
0.58
-0.13
0.46
-0.45
0.70
Strat
1.35
2.30
-0.50
1.8
-1.8
1.70
change
TROP
0.36
0.13
-0.03
( )
0.36 
STRAT
-0.096
(0.0034)
1.27 
FULL
0.26
0.006 
-0.009
-0.003
0.005
(0.0002)
Global average precipitation is 2.7 mm day-1, so mm day-1 is 0.12%

10. Perfect agreement between theoretical calculation and GCM response
(Just shows GCM conserves energy)

11. Slow contribution LΔP = kΔTsurf - Δ Fatm - Δ SHfast ; k=2.2 Wm-2K-1
Transient ΔTsurf from ΔFTOA
Using analytical climate model with shallow and deep ocean
ΔFTOA from IPCC AR5
Tabulated f = ΔFatm /ΔFTOA
Fast and slow responses have opposite sign
For trop. ozone and CO2 , slow (ΔT) response larger than fast
For CO2 fast response offsets more of the slow response (f=0.8)
Trop. Ozone has had proportionally larger effect on historical precipitation (for 20% of the forcing)

12. Global precipitation potential
LΔP = kΔTsurf Δ Fatm - Δ SHfast
Can generalise this for unit pulse emission,
analogous to absolute (A) GWP and GTP
L×AGPP(t) = k×AGTP(t) – f×AGWP(t) ; where f= (ΔFatm+ΔSHfast)/ ΔFTOA
AGPP(t) in kg m-2kg-1

13. Comparison CMIP5 ΔP=0.009±0.054 mm day-1 1850-2000
ΔP=0.178±0.096 mm day (RCP8.5)
Observations:
Variability in models and observations to great to test theory
Allan et al Surv. Geophys.

14. Precipitation patterns
Zonal patterns of ΔP don‘t follow forcing pattern
Would need to diagnose meridional energy transport to do zonal calculations

15. Conclusions
Ozone affects global precipitation through the atmospheric energy balance
Direct (fast) response to atmospheric absorption
Negative except for lower tropospheric ozone
Most negative for stratospheric ozone
Indirect (slow) response via surface warming
Positive
Total effect is more positive (per unit forcing) for ozone than CO2
There is no simple relationship between zonal patterns of forcing and precipitation change.