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.

NCAS-Climate Talk 15th January 2010

Current Changes in the Global Water Cycle

Richard P. AllanDiffusing 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


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


Models simulate robust response of clear-sky radiation to warming (~2-3 Wm-2K-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

Rad

iativ

e co

olin

g, c

lear

(W

m-2K

-1)

Allan (2009) J Clim


Physical basis: water vapour

1979-2002• Clausius-Clapeyron

– Low-level water vapour (~7%/K)– Intensification of rainfall– Moisture transport– Enhanced P-E patterns

See Held and Soden (2006) J Clim


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


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

simulated by models


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

Wat

er V

apou

r (m

m)

ER

A40

N

CE

P

ER

AIN

T

SS

M/I


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


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


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


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 satellite data, 1988-2008– 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


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

No apparent link to convection scheme? What about CMIP experiments? e.g. Turner and Slingo (2009) ASL


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?


Precipitation in the Europe-Atlantic region (summer)

Dependence on NAO


Wat

er v

apou

rP

reci

pita

tion

Current changes in the water cycle over 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

Extra Slides



• 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


A=0.4 (1-A)=0.7

dPw/dT=7%/K dPd/dT

dP/dT=3%/K

Assume wet region follows Clausius Clapeyron (7%/K) and mean precip follows radiation constraint (~3%/K)

Pw=6 mm/day Pd=1 mm/day

P=3 mm/day

Wet Dry

A is the wet region fractional area

P is precipitation

T is temperature

DISCUSSION


A=0.4 (1-A)=0.7

dPw/dT=7%/K dPd/dT

dP/dT=3%/K

Assume wet region follows Clausius Clapeyron (7%/K) and mean precip follows radiation constraint (~3%/K)

dP/dT= A(dPw/dT)+(1-A)(dPd/dT) dPd= (dP-AdPw)/(1-A)

Pw=6 mm/day Pd=1 mm/day

P=3 mm/day

Wet Dry

A Pw Pd dPd/dTs

(mm/day/K)

“

(%/K)

0.4

0.2

6

9

1

1.5

-0.1

-0.05

-10

-4.5

0.1 10.5 2.2 +0.02 +0.9

A is the wet region fractional area

P is precipitation

T is temperature


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

SMOSESA’s SMOS (Soil Moisture and Ocean Salinity) launched November 2009

Courtesy of Ian Davenport


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


2008

Are the issues of cloud feedback and the water cycle linked?


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


How does UTH respond to warming?

Lindzen (1990) BAMS

Minschwaner et al. (2006) J Clim

Mitchell et al. (1987) QJRMS


Pre

cip.

(%

)

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)

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.

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