NCAS-Climate Talk 15 th January 2010 Current Changes in the Global Water Cycle Richard P. Allan Diffusing slowly to Met Department/NCAS-Climate from ESSC Thanks to Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev, Mark Ringer and Tony Slingo

NCAS-Climate Talk 15 th January 2010 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

NCAS-Climate Talk 15 th January 2010 How should the water cycle respond to climate change? Precipitation Change (%) relative to : 2 scenarios, multi model (IPCC, 2001) See discussion in: Allen & Ingram (2002) Nature; Trenberth et al. (2003) BAMS

NCAS-Climate Talk 15 th January 2010 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

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

NCAS-Climate Talk 15 th January 2010 Models simulate robust response of clear-sky radiation to warming (~2-3 Wm -2 K -1 ) and a resulting increase in precipitation to balance (~3 %K -1 ) e.g. Allen and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim, Lambert and Webb (2008) GRL Radiative cooling, clear (Wm -2 K -1 ) Allan (2009) J Clim

NCAS-Climate Talk 15 th January 2010 Physical basis: water vapour Clausius-Clapeyron –Low-level water vapour (~7%/K) –Intensification of rainfall –Moisture transport –Enhanced P-E patterns See Held and Soden (2006) J Clim

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

NCAS-Climate Talk 15 th January 2010 Current changes in the water cycle As observed by satellite datasets and simulated by models

NCAS-Climate Talk 15 th January 2010 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 Allan (2009) AMIP3 CMIP3 CMIP3 volc John et al. (2009) models Water Vapour (mm) ERA40 NCEP ERAINT SSM/I

NCAS-Climate Talk 15 th January 2010 Tropical ocean precipitation dP/dSST: GPCP:10%/K ( ) AMIP:3-11 %/K ( ) dP/dt trend GPCP: 1%/dec ( ) AMIP: %/dec ( ) (land+ocean) SSM/I GPCP

NCAS-Climate Talk 15 th January 2010 Wet (ascent) and Dry (descent) regimes Robust response: wet regions become wetter at the expense of dry regions Large uncertainty in magnitude of change: satellite datasets and models & time period TRMM GPCP Ascent Region Precipitation (mm/day) John et al. (2009) GRL

NCAS-Climate Talk 15 th January 2010 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

NCAS-Climate Talk 15 th January 2010 Avoid reanalyses in defining wet/dry regions Sample grid boxes: –30% wettest –70% driest Do wet/dry trends remain?

NCAS-Climate Talk 15 th January 2010 Current trends in wet/dry regions of tropical oceans Wet/dry trends remain – GPCP record may be suspect for dry region –SSM/I dry region record: inhomogeneity 2000/01? GPCP trends –Wet: 1.8%/decade –Dry: -2.6%/decade –Upper range of model trend magnitudes Models DRY WET

NCAS-Climate Talk 15 th January 2010 Analyse daily rainfall over tropical oceans –SSM/I satellite data, –Climate model data (AMIP experiments) Create monthly PDFs of rainfall intensity 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

NCAS-Climate Talk 15 th January 2010 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

NCAS-Climate Talk 15 th January 2010 Increase in intense rainfall with tropical ocean warming (close to Clausius Clapeyron) SSM/I satellite observations at upper range of model range No apparent link to convection scheme? What about CMIP experiments? e.g. Turner and Slingo (2009) ASL

NCAS-Climate Talk 15 th January 2010 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?

NCAS-Climate Talk 15 th January 2010 Precipitation in the Europe- Atlantic region (summer) Dependence on NAO

NCAS-Climate Talk 15 th January 2010 Water vapourPrecipitation Current changes in the water cycle over Europe-Atlantic region

NCAS-Climate Talk 15 th January 2010 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

Extra Slides NCAS-Climate Talk 15 th January 2010

NCAS-Climate Talk 15 th January 2010 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 PDFs –Magnitude of change in precipitation in 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

NCAS-Climate Talk 15 th January 2010 A=0.4(1-A)=0.7 dP w /dT=7%/KdP d /dT dP/dT=3%/K Assume wet region follows Clausius Clapeyron (7%/K) and mean precip follows radiation constraint (~3%/K) P w =6 mm/dayP d =1 mm/day P=3 mm/day WetDry A is the wet region fractional area P is precipitation T is temperature DISCUSSION

NCAS-Climate Talk 15 th January 2010 A=0.4(1-A)=0.7 dP w /dT=7%/KdP d /dT dP/dT=3%/K Assume wet region follows Clausius Clapeyron (7%/K) and mean precip follows radiation constraint (~3%/K) dP / dT = A( dP w / dT )+(1-A)( dP d / dT ) dP d = (dP-AdP w )/(1-A) P w =6 mm/dayP d =1 mm/day P=3 mm/day WetDry APwPw PdPd dP d /dTs (mm/day/K) (%/K) A is the wet region fractional area P is precipitation T is temperature

NCAS-Climate Talk 15 th January 2010 Evaporation changes over land are not globally measured. New data on soil moisture could be vital in understanding changes in evaporation and regional water cycle feedbacks over land. The addition of ocean salinity measurements are also of potential value in understanding P-E changes and ocean circulating response SMOS ESAs SMOS (Soil Moisture and Ocean Salinity) launched November 2009 Courtesy of Ian Davenport

NCAS-Climate Talk 15 th January 2010 Cloud Feedback Can HadIR provide any information on cloud feedback For example, the FAT hypothesis (fixed anvil temperature): –Anvil outflow determined by position of zero radiative cooling –…which is determined by the rapid decline in water vapour with altitude –…which is determined by Clausius Clapeyron –Hypothesis: As temperature rises, outflow rises in altitude but not temperature which remains fixed –e.g. Hartmann and Larson (2003); Zelinka and Hartmann in press

NCAS-Climate Talk 15 th January Are the issues of cloud feedback and the water cycle linked?

NCAS-Climate Talk 15 th January 2010 Response of the hydrological cycle is sensitive to the type of forcing 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

NCAS-Climate Talk 15 th January 2010 How does UTH respond to warming? Lindzen (1990) BAMS Minschwaner et al. (2006) J Clim Mitchell et al. (1987) QJRMS

NCAS-Climate Talk 15 th January 2010 Precip. (%) Radiation budget, hydrological cycle and climate feedbacks Decadal changes in water vapour, precipitation and its extremes are beginning to be detected Allan and Soden (2008) Science Precipitation projections (IPCC)