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

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

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.

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

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

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

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 1850-2000 changes (from ACCMIP)

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 1850-2000 change TROP 0.36   0.13 -0.03 (-0.0034) 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 0.0034 mm day-1 is 0.12%

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

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)

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

Comparison CMIP5 ΔP=0.009±0.054 mm day-1 1850-2000 ΔP=0.178±0.096 mm day-1 2000-2100 (RCP8.5) Observations: Variability in models and observations to great to test theory Allan et al. 2014 Surv. Geophys.

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

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.