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© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Tropospheric Water Vapour and.

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Presentation on theme: "© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Tropospheric Water Vapour and."— Presentation transcript:

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

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

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

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

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

6 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Climate sensitivity and water vapour feedback 0.2 Wm -2 % -1 = Kernals: Soden et al. (2008)

7 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Climate sensitivity and water vapour feedback 0.2 Wm -2 % -1 7%K -1 =

8 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Climate sensitivity and water vapour feedback 0.2 Wm -2 % -1 7%K -1 λ BB ~ -4σT 3 ~ -3.2 Wm -2 K -1 λ WV ~(0.2)(7)=1.4 Wm -2 K -1 = λ WV +λ BB ~1.4 - 3.2 = -1.8 Wm -2 K -1

9 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate 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? How does moisture respond to warming?

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

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

12 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Is the mean state important? 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? Pierce et al. (2006) GRL

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

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

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

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

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

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

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

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

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

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

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

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

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

26 © University of Reading 2008r.p.allan@reading.ac.uk Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Contrasting precipitation response expected Precipitation Heavy rain follows moisture (~7%/K) Mean Precipitation linked to radiation balance (~3%/K) Light Precipitation (-?%/K) Temperature e.g. see Held and Soden (2006) J. Clim

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

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

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

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

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