1. Current Changes in the Global Water Cycle Richard P. Allan Department of Meteorology, University of Reading Thanks to Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev, Mark Ringer and Tony Slingo http://www.met.reading.ac.uk/~sgs02rpa r.p.allan@reading.ac.uk

2. Introduction Observational records and climate projections provide abundant evidence that freshwater resources are vulnerable and have the potential to be strongly impacted by climate change, with wide-ranging consequences for human societies and ecosystems. IPCC (2008) Climate Change and Water

3. How should the water cycle respond to climate change? Precipitation Change (%) relative to 1961-1990: 2 scenarios, multi model (IPCC, 2001) See discussion in: Allen & Ingram (2002) Nature; Trenberth et al. (2003) BAMS

4. Increased Precipitation More Intense Rainfall More droughts Wet regions get wetter, dry regions get drier? Regional projections?? Precipitation Change (%) Climate model projections (IPCC 2007) Precipitation Intensity Dry Days

5. NCAS-Climate Talk 15 th January 2010 Trenberth et al. (2009) BAMS Physical basis: energy balance

6. NCAS-Climate Talk 15 th January 2010 CCWindT s -T o RH o Muted Evaporation changes in models are explained by small changes in Boundary Layer: 1) declining wind stress 2) reduced surface temperature lapse rate (T s -T o ) 3) increased surface relative humidity (RH o ) Richter and Xie (2008) JGR Evaporation

7. Surface Temperature (K) Lambert & Webb (2008) GRL Latent Heat Release, LΔP (Wm -2 ) Radiative cooling, clear (Wm -2 ) Physical Basis: clear-sky radiative cooling: models simulate robust response of clear-sky radiation to warming (~2 Wm -2 K -1 ) & resulting precipitation increase e.g. see Stephens and Ellis (2008); Lambert and Webb (2008) GRL

8. Physical basis: water vapour 1979-2002 Clausius-Clapeyron –Low-level water vapour (~7%/K) –Intensification of rainfall: Trenberth et al. (2003) BAMS; Pall et al. (2007) Clim Dyn Changes in intense rainfall also constrained by moist adiabat -OGorman and Schneider (2009) PNAS Could extra latent heat release within storms enhance rainfall intensity above Clausius Clapeyron? –e.g. Lenderink and van Meijgaard (2008) Nature Geoscience

9. Physical basis: water vapour Clausius-Clapeyron –Low-level water vapour (~7%/K) –Enhanced moisture transport (F) –Enhanced P-E patterns (below) See Held and Soden (2006) J Clim AR5 scaling

10. Models/observations achieve muted precipitation response by reducing strength of Walker circulation. Vecchi and Soden (2006) Nature P~Mq Circulation response

11. 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.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature

12. Models ΔP [IPCC 2007 WGI] Is there a contrasting precipitation responses in wet and dry regions? Some limited observational evidence, e.g. Zhang et al. (2007) Nature Rainy season: wetter Dry season: drier Chou et al. (2007) GRL Precip trends, 0-30 o N The Rich Get Richer?

13. Current changes in the water cycle As observed by satellite datasets and simulated by models Focus on tropical oceans.

14. Current changes in tropical ocean column water vapour …despite inaccurate mean state, Pierce et al.; John and Soden (both GRL, 2006) - see also Trenberth et al. (2005) Clim. Dyn., Soden et al. (2005) Science John et al. (2009) models Water Vapour (mm)

15. Sensitivity of water vapour and clear-sky radiation to surface temperature Allan (2009) J. Climate ERA40 NCEP ERAINT SSM/I ERA40 NCEP SRB SSM/I

16. NCAS-Climate Talk 15 th January 2010 Radiative cooling, clear (Wm -2 K -1 ) Allan (2006) JGR 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 and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim

17. Trends in clear-sky radiation in coupled models Clear-sky shortwave absorptionSurface net clear-sky longwave Can we derive an observational estimate of surface longwave? Prata (1996) QJRMS

18. Variability in clear-sky radiative cooling John et al. (2009) GRL

19. NCAS-Climate Talk 15 th January 2010 Precip. (%) Allan and Soden (2008) Science Tropical ocean variation in water vapour and precipitation

20. Tropical ocean precipitation dP/dSST: GPCP:10%/K (1988-2008) AMIP:3-11 %/K (1979-2001) dP/dt trend GPCP: 1%/dec (1988-2008) AMIP: 0.4-0.7%/dec (1979-2001) (land+ocean) SSM/I GPCP

21. Contrasting precipitation response in wet and dry regions of the tropical circulation Updated from Allan and Soden (2007) GRL descent ascent ModelsObservations Precipitation change (%) Sensitivity to reanalysis dataset used to define wet/dry regions

22. Is the contrasting wet/dry response robust? Large uncertainty in magnitude of change: satellite datasets and models & time period TRMM GPCP Ascent Region Precipitation (mm/day) John et al. (2009) GRL Robust response: wet regions become wetter at the expense of dry regions. Is this an artefact of the reanalyses?

23. Avoid reanalyses in defining wet/dry regions Sample grid boxes: –30% wettest –70% driest Do wet/dry trends remain?

24. Current trends in wet/dry regions of tropical oceans Wet/dry trends remain –1979-1987 GPCP record may be suspect for dry region –SSM/I dry region record: inhomogeneity 2000/01? GPCP trends 1988-2008 –Wet: 1.8%/decade –Dry: -2.6%/decade –Upper range of model trend magnitudes Models DRY WET

25. Analyse daily rainfall over tropical oceans –SSM/I v6 satellite data, 1988-2008 (F08/11/13) –Climate model data (AMIP experiments) Create rainfall frequency distributions Calculate changes in the frequency of events in each intensity bin Does frequency of most intense rainfall rise with atmospheric warming? Precipitation Extremes Trends in tropical wet region precipitation appear robust. – What about extreme precipitation events? METHOD

26. Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I) Updated from Allan and Soden (2008) Science Reduced frequencyIncreased frequency

27. Increase in intense rainfall with tropical ocean warming (close to Clausius Clapeyron) SSM/I satellite observations at upper range of model range Model intense precipitation dependent upon conservation of moist adiabatic lapse rate but responses are highly sensitive to model-specific changes upward velocities (see OGorman and Schneider, 2009, PNAS; Gastineau & Soden 2009).

28. One of the largest challenges remains improving predictability of regional changes in the water cycle… Changes in circulation systems are crucial to regional changes in water resources and risk yet predictability is poor. How will catchment-scale runoff and crucial local impacts and risk respond to warming? What are the important land-surface and ocean-atmosphere feedbacks which determine the response?

29. Precipitation in the Europe- Atlantic region (summer) Dependence on NAO

30. NCAS-Climate Talk 15 th January 2010 Water vapourTemperature Current changes water cycle variables: Europe-Atlantic region

31. NCAS-Climate Talk 15 th January 2010 EvaporationPrecipitation Current changes water cycle variables: Europe-Atlantic region

32. Outstanding issues Are satellite estimates of precipitation, evaporation and surface flux variation reliable? Are regional changes in the water cycle, down to catchment scale, predictable? How well do models represent land surface feedbacks. Can SMOS mission help? How is the water cycle responding to aerosols? Linking water cycle and cloud feedback issues

33. How does the hydrological cycle respond to different forcings? Andrews et al. (2009) J Climate Partitioning of energy between atmosphere and surface is crucial to the hydrological response; this is being assessed in the PREPARE project

34. Could changes in aerosol be imposing direct and indirect changes in the hydrological cycle? e.g. Wild et al. (2008) GRL Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim Mishchenko et al. (2007) Science

35. Are the issues of cloud feedback and the water cycle linked? 2006 Allan et al. (2007) QJRMS How important are cloud microphysical processes in stratocumulus and large- scale processes involving cirrus outflow? e.g. Ellis and Stephens (2009) GRL; Stephens and Ellis (2008) J Clim. Zelinka and Hartmann (in prep) FAT/FAP hypothesis

36. Are the issues of cloud feedback and the water cycle linked? 2007 Allan et al. (2007) QJRMS How important are cloud microphysical processes in stratocumulus and large- scale processes involving cirrus outflow? e.g. Ellis and Stephens (2009) GRL; Stephens and Ellis (2008) J Clim. Zelinka and Hartmann (in prep) FAT/FAP hypothesis

37. Are the issues of cloud feedback and the water cycle linked? 2008 Allan et al. (2007) QJRMS How important are cloud microphysical processes in stratocumulus and large- scale processes involving cirrus outflow? e.g. Ellis and Stephens (2009) GRL; Stephens and Ellis (2008) J Clim. Zelinka and Hartmann (in prep) FAT/FAP hypothesis

38. Robust Responses –Low level moisture; clear-sky radiation –Mean and Intense rainfall –Observed precipitation response at upper end of model range? –Contrasting wet/dry region responses Less Robust/Discrepancies –Moisture at upper levels/over land and mean state –Inaccurate precipitation frequency distributions –Magnitude of change in precipitation from satellite datasets/models Further work –Decadal changes in global energy budget, aerosol forcing effects and cloud feedbacks: links to water cycle? –Precipitation and radiation balance datasets: forward modelling –Surface feedbacks: ocean salinity, soil moisture (SMOS?) –Boundary layer changes and surface fluxes Conclusions