Current Changes in the Global Water Cycle

Richard P. AllanDepartment 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

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

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

• 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 15th January 2010 Trenberth et al. (2009) BAMS

Physical basis: energy balance

NCAS-Climate Talk 15th January 2010

CC Wind Ts-To RHo

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)

Richter and Xie (2008) JGR

Evaporation

Surface Temperature (K)

Lambert & Webb (2008) GRL

Late

nt H

eat

Rel

ease

, LΔ

P (

Wm

-2)

Rad

iativ

e co

olin

g, c

lear

(W

m-2)

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

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

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

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

P~Mq

Circulation response

Contrasting precipitation response expected

Pre

cipi

tatio

n 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

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, 0-30oN

The Rich Get Richer?

Current changes in the water cycle As observed by satellite datasets and

simulated by models

Focus on tropical oceans.

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

Wat

er V

apou

r (m

m)

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

Allan (2009) J . Climate

ER

A40

N

CE

P

ER

AIN

T

SS

M/I

ER

A40

N

CE

P

SR

B

SS

M/I

ER

A40

N

CE

P

SR

B

SS

M/I

NCAS-Climate Talk 15th January 2010

Rad

iativ

e co

olin

g, c

lear

(W

m-2K

-1)

Allan (2006) JGR

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

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

Variability in clear-sky radiative cooling

John et al. (2009) GRL

NCAS-Climate Talk 15th January 2010

Pre

cip.

(%

)

Allan and Soden (2008) Science

Tropical ocean variation in water vapour and 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

Updated from Allan and Soden (2007) GRL

descent

ascentModelsObservations

Pre

cipi

tatio

n ch

ange

(%

)

Sensitivity to reanalysis dataset used to define wet/dry regions

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?

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

• 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

DR

Y

WE

T

• 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

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) ScienceReduced frequency Increased frequency

• 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).

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?

Precipitation in the Europe-Atlantic region (summer)

Dependence on NAO

NCAS-Climate Talk 15th January 2010

Wat

er v

apou

rT

empe

ratu

reCurrent changes water cycle

variables: Europe-Atlantic region

NCAS-Climate Talk 15th January 2010

Eva

pora

tion

Pre

cipi

tatio

nCurrent changes water cycle

variables: Europe-Atlantic region

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?

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

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

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”

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”

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”

• 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