Current changes in the global water cycle
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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 [email protected] Introduction.

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Current Changes in the Global Water Cycle

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Current changes in the global water cycle

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

[email protected]


Introduction

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


Current changes in the global water cycle

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


Current changes in the global water cycle

Climate model projections (IPCC 2007)

Precipitation Intensity

  • Increased Precipitation

  • More Intense Rainfall

  • More droughts

  • Wet regions get wetter, dry regions get drier?

  • Regional projections??

Dry Days

Precipitation Change (%)


Current changes in the global water cycle

Physical basis: energy balance

Trenberth et al. (2009) BAMS


Evaporation

Evaporation

Richter and Xie (2008) JGR

CCWindTs-ToRHo

Muted Evaporation changes in models are explained by small changes in Boundary Layer:1) declining wind stress2) reduced surface temperature lapse rate (Ts-To)3) increased surface relative humidity (RHo)


Current changes in the global water cycle

Physical Basis: clear-sky radiative cooling: models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) & resulting precipitation increase

e.g. see Stephens and Ellis (2008); Lambert and Webb (2008) GRL

Radiative cooling, clear(Wm-2)

Latent Heat Release, LΔP (Wm-2)

Lambert & Webb (2008) GRL

Surface Temperature (K)


Current changes in the global water cycle

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

    -O’Gorman 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


Current changes in the global water cycle

Clausius-Clapeyron

Low-level water vapour (~7%/K)

Enhanced moisture transport (F)

Enhanced P-E patterns (below)

See Held and Soden (2006) J Clim

Physical basis: water vapour

AR5

scaling


Circulation response

Circulation response

P~Mq

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


Contrasting precipitation response expected

Contrasting precipitation response expected

Heavy rain follows moisture (~7%/K)

Mean Precipitation linked to radiation balance (~3%/K)

Precipitation 

Light Precipitation (-?%/K)

Temperature 

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


Current changes in the global water cycle

The Rich Get Richer?

Is there a contrasting precipitation responses in wet and dry regions? Some limited observational evidence, e.g. Zhang et al. (2007) Nature

Models ΔP [IPCC 2007 WGI]

Precip trends, 0-30oN

Rainy season: wetter

Dry season: drier

Chou et al. (2007) GRL


Current changes in the global water cycle

Current changes in the water cycleAs observed by satellite datasets and simulated by modelsFocus on tropical oceans.


Current changes in the global water cycle

Current changes in tropical ocean column water vapour

John et al. (2009)

Water Vapour (mm)

models

…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


Sensitivity of water vapour and clear sky radiation to surface temperature

Sensitivity of water vapour and clear-sky radiation to surface temperature

ERA40 NCEP ERAINT SSM/I

ERA40 NCEP SRB SSM/I

ERA40 NCEP SRB SSM/I

Allan (2009) J . Climate


Current changes in the global water cycle

Models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) and a resulting increase in precipitation to balance (~2 %K-1)e.g. Allen and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim

Allan (2006) JGR

Radiative cooling, clear (Wm-2K-1)

NCAS-Climate Talk

15th January 2010


Trends in clear sky radiation in coupled models

Trends in clear-sky radiation in coupled models

Surface net clear-sky longwave

Clear-sky shortwave absorption

Can we derive an observational estimate of surface longwave?

Prata (1996) QJRMS


Variability in clear sky radiative cooling

Variability in clear-sky radiative cooling

John et al. (2009) GRL


Current changes in the global water cycle

Tropical ocean variation in water vapour and precipitation

Precip. (%)

Allan and Soden (2008) Science

NCAS-Climate Talk

15th January 2010


Tropical ocean precipitation

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


Contrasting precipitation response in wet and dry regions of the tropical circulation

Contrasting precipitation response in wet and dry regions of the tropical circulation

ascent

Observations

Models

Precipitation change (%)

descent

Sensitivity to reanalysis dataset used to define wet/dry regions

Updated from Allan and Soden (2007) GRL


Is the contrasting wet dry response robust

GPCP Ascent Region Precipitation (mm/day)

Is the contrasting wet/dry response robust?

John et al. (2009) GRL

  • Large uncertainty in magnitude of change: satellite datasets and models & time period

TRMM

  • Robust response: wet regions become wetter at the expense of dry regions. Is this an artefact of the reanalyses?


Avoid reanalyses in defining wet dry regions

Avoid reanalyses in defining wet/dry regions

  • Sample grid boxes:

    • 30% wettest

    • 70% driest

  • Do wet/dry trends remain?


Current trends in wet dry regions of tropical oceans

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

DRY WET

Models


Current changes in the global water cycle

Precipitation Extremes

  • Trends in tropical wet region precipitation appear robust.

    • What about extreme precipitation events?

  • 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?

METHOD


Current changes in the global water cycle

Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I)

Reduced frequencyIncreased frequency

Updated from Allan and Soden (2008) Science


Current changes in the global water cycle

  • 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 O’Gorman and Schneider, 2009, PNAS; Gastineau & Soden 2009).


Current changes in the global water cycle

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?


Current changes in the global water cycle

Precipitation in the Europe-Atlantic region (summer)

Dependence on NAO


Current changes water cycle variables europe atlantic region

Current changes water cycle variables: Europe-Atlantic region

Water vapourTemperature

NCAS-Climate Talk

15th January 2010


Current changes water cycle variables europe atlantic region1

Current changes water cycle variables: Europe-Atlantic region

EvaporationPrecipitation


Outstanding issues

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


How does the hydrological cycle respond to different forcings

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


Current changes in the global water cycle

Mishchenko et al. (2007) Science

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


Current changes in the global water cycle

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”


Current changes in the global water cycle

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”


Current changes in the global water cycle

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”


Conclusions

Conclusions

  • 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


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