© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Tropospheric Water Vapour and the Hydrological Cycle Richard Allan University of Reading, UK

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Tony Slingo

© University of Reading 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

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate A Satellite perspective

© University of Reading 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

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Climate sensitivity and water vapour feedback = ─

© University of Reading 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)

© University of Reading 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 = ─

© University of Reading 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 ~ = -1.8 Wm -2 K -1

© University of Reading 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?

© University of Reading 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

© University of Reading 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: Observations Model Constant RH model Constant water vapour model

© University of Reading 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

© University of Reading 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?

© University of Reading 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)

© University of Reading 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

© University of Reading 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

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Evaporation cannot explain moistening John and Soden (2006) GRL; Luo and Rossow (2004) g m -3

© University of Reading 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

© University of Reading 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

© University of Reading 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

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate How should precipitation respond to climate change? Allen and Ingram (2002) Nature

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

© University of Reading 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

© University of Reading 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 BAMS)

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Intensification of heaviest rainfall with warming Allan and Soden (2008) Science

© University of Reading 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

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate IPCC 2007 WGI Mean projected precipitation changes

© University of Reading 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)

© University of Reading 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

© University of Reading 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?

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Extra Slides

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Can we use reanalyses? Global coverage All levels of the troposphere Changes in observing system: spurious longer-term variability

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Surface longwave radiation and the hydrological cycle Hartmann & Michelsen (1993) J Climate Water vapour (mm)  Surface LW (Wm -2 )

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Schematic showing effect of changes in water vapour on surface, top of atmosphere and atmospheric radiation budgets Longwave radiative cooling  Surface Top of atmosphere Atmosphere Troposphere Water vapour feedback  Hydrological  cycle

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Changes in precipitation: “the rich get richer”? precip trends 0-30 o N Rainy season: wetter Dry season: drier Chou et al. (2007) GRL

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Are mean precipitation and evaporation changing following Clausius Clapeyron (7%/K), larger than the model estimates Yu and Weller (2007) BAMS (Wentz et al. 2007, Science)

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Conclusions Free troposphere humidity crucial for water vapour and cloud feedback Low-level water vapour crucial for hydrological cycle Is cloud feedback a small indirect forcing? Are observed changes in the hydrological cycle real and if so do they cast doubt on model forcings/physics?

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Calculating Feedback: kernels Shell et al. (2007) J Clim; Soden et al. (2008) J. Clim Clear-sky All-sky

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Model reproduces water vapour feedback response to Pinatubo eruption Soden et al. (2002) Science; Forster and Collins (2004) Clim Dyn

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Timescales of feedbacks Water Vapour (mm) 6.7 μm T (K) Surface T (K) Harries and Futyan (2006) GRL

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Radiative Impact of Cloud Most of the water in the atmosphere is invisible vapour –Clouds are like the tip of the iceberg Clouds are water vapour with attitude Strong interaction with longwave and shortwave radiation (emission, absorption, scattering)

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Interanual Variability: Response of water vapour & clear-sky LW radiation at the surface and TOA in models, reanalyses & observations Water vapour Surface clear LW Clear-sky OLR

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Quantifying Feedbacks Climate Sensitivity parameter + Black body feedback x denotes feedback variable, e.g. cloud, water vapour, ice-albedo, etc ≈ Black body feedback ~ -3.8 Wm -2 K -1 assuming T=255 K (using GCMs ~ -3.2 Wm -2 K -1 )

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate 2xCO 2 response + water vapour feedback = ─ 3.7 = ─ ( ) ∆T; ∆T ~ 2 K So water vapour feedback approximately doubles no feedback temperature response to doubling of CO2 Including feedbacks from temperature lapse rate (negative), ice albedo (positive) and clouds (positive), models produce a best estimate ∆T ~ 3 K

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Observations and cloud feedback

© University of Reading Chapman Conference on Atmospheric Water Vapor and Its Role in Climate Changes in cloud from ISCCP Decadal changes in ISCCP cloud relate to viewing artifacts due to changes in geo- stationary satellite coverage Cloud thickness/cloud fraction compensation e.g. Norris (2005) JGR; Evan et al. (2007) GRL