Tropospheric Water Vapour and the Hydrological Cycle Richard

Tropospheric Water Vapour and the Hydrological Cycle Richard Allan University of Reading, UK Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Tony Slingo 1950 -2008 Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

How does water vapour impact climate change? – Amount of warming – Changes in water cycle Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Spectral cooling rate (H 2 O, CO 2, O 3) Clough & Iacono (1995) JGR K d-1 (cm-1)-1 MLS Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Climate sensitivity and water vapour feedback =─ Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Climate sensitivity and water vapour feedback =─ Kernals: Soden et al. (2008) 0. 2 Wm-2%-1 Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Climate sensitivity and water vapour feedback =─ 0. 2 Wm-2%-1 Chapman Conference on Atmospheric Water Vapor and Its Role in Climate 7%K-1 © University of Reading 2008 r. p. allan@reading. ac. uk

Climate sensitivity and water vapour feedback =─ 0. 2 Wm-2%-1 7%K-1 λBB~ -4σT 3 ~ -3. 2 Wm-2 K-1 λ +λ ~1. 4 - 3. 2 WV BB -2 K-1 Chapman Conference on Atmospheric Water Vapor and Its -2 -1 = -1. 8 Wm λWV~(0. 2)(7)=1. 4 Wm Kof Reading 2008 Role in Climate © University r. p. allan@reading. ac. uk

How does moisture respond to warming? • Low level moisture over the ocean seems to behave… • Does moisture really vary with temperature at ~ 7%/K? • Do models capture the essential relationships? - Land, upper troposphere? - Reanalyses, surface measurements or satellite data? Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Does moisture rise at 7%/K over land? Specific humidity trend correlation (left) and time series (right) Land Ocean Willett et al. (2007) Nature; Willet et al. (2008) J Clim But some contradictory results (e. g. , Wang et al. (2008) GRL) Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Is moisture at higher levels constrained by Clausius Clapeyron? Trend in water vapour radiance channels: 1983 -2004 Moistening Observations Model Constant RH model Constant water vapour model Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Soden et al. (2005) Science © University of Reading 2008 r. p. allan@reading. ac. uk

Is the mean state important? Pierce et al. (2006) GRL • Models appear to overestimate water vapour – Pierce et al. (2006) GRL; John and Soden (2006) GRL – But not for microwave data? [Brogniez and Pierrehumbert (2007) GRL] • This does not appear to affect feedback strength – Held and Soden (2006), John and Soden (2006) • What about the hydrological cycle? – Inaccurate mean state? Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

What time-scales do different processes operate on? Soden et al. (2002) Science; Forster/Collins (2004) Clim Dyn; Harries/Futyan (2006) GRL Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Bates and Jackson (2001) GRL Trends in UTH (above) Sensitivity of OLR to UTH (right) Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Reduction in UTH with warming Minschwaner et al. (2006) J Clim Lindzen (1990) BAMS Mitchell et al. (1987) QJRMS Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Moistening processes: diurnal cycle (SEVIRI) Convergence Divergence Evaporation Condensation Moistening Sohn et al. (2008)JGR Drying Evaporation Divergence UTH tendency See also Soden et al. (2004) GRL Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Evaporation cannot explain moistening g m-3 350 250 180 120 90 63 45 30 John and Soden (2006) GRL; Luo and Rossow (2004) Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Cloud feedback: a more complex problem • Depends on: – – Type of cloud Height of cloud Time of day/year Surface characteristics Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Non-trivial relationship between cloud and temperature Response of cloud to warming is highly uncertain © University of Reading 2008 r. p. allan@reading. ac. uk

Spread in cloud feedback in models appears to relate to tropical low altitude clouds Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 colleagues IPCC (2007), after Sandrine Bony and r. p. allan@reading. ac. uk

Is cloud feedback an indirect forcing? • • Clouds respond to – direct forcing from CO 2 – Climate response to ∆SST • • Does cloud feedback uncertainty stem from direct response rather than climate feedback response? Andrews and Forster (2008) GRL (above); Gregory and Webb (2008) J Clim Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

How should precipitation respond to climate change? Allen and Ingram (2002) Nature Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Models simulate robust response of clear-sky radiation to warming (~2 Wm-2 K-1) and a resulting increase in precipitation to balance (~2%K-1) e. g. , Allen & Ingram, 2002; Lambert & Webb (2008) GRL Surface Temperature (K) Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

• But moisture observed & predicted to increase at greater rate ~7%K-1 • Thus convective rainfall expected to increase at a faster rate than mean precipitation (e. g. Trenberth et al. 2003 BAMS) 1979 -2002 Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Intensification of heaviest rainfall with warming Allan and Soden (2008) Science Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Precipitation Contrasting precipitation response expected ) K / % 7 ~ ( e r u t is o m s w o o l t l d e o k f in l n n o i i t ta a i p ) r i K c / e r y % P 3 ~ ( e av ean c M n e la a H b n o radiati Light Precip itation (-? %/K ) Temperature Chapman Conference on Atmospheric Water Vapor and Its Role in Climate e. g. see Held and Soden (2006) J. Clim © University of Reading 2008 r. p. allan@reading. ac. uk

Mean projected precipitation changes IPCC 2007 WGI Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Precipitation change (mm/day) Contrasting precipitation response in ascending and descending portions of the tropical circulation ascent descent GPCP Models Allan and Soden (2007) GRL Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk

Could changes in aerosol be driving recent changes in the hydrological cycle? Chapman Conference on Atmospheric Water Vapor and Its et al. (2002) ©Science; Wong Role in. Wielicki Climate University of Reading 2008 et al. (2006) J. Clim; Loeb et al. (2007) J. Clim r. p. allan@reading. ac. uk

Unanswered questions • • • How does UTH really respond to warming? Do we understand the upper tropospheric moistening processes? Is moisture really constrained by Clausius Clapeyron over land? What time-scales do feedbacks operate on? Apparent discrepancy between observed and simulated changes in precipitation – Is the satellite data at fault? – Are aerosol changes short-circuiting the hydrological cycle? – Could model physics/resolution be inadequate? • Could subtle changes in the boundary layer be coupled with decadal swings in the hydrological cycle? • How do clouds respond to forcing and feedback including changes in water vapour? Chapman Conference on Atmospheric Water Vapor and Its Role in Climate © University of Reading 2008 r. p. allan@reading. ac. uk