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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. - PowerPoint PPT Presentation
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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 [email protected]
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 R
elea
se, L
ΔP
(Wm
-2)
Rad
iativ
e co
olin
g, c
lear
(Wm
-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 S
SM
/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
(Wm
-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 trendGPCP: 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 W
ET
• 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
rTe
mpe
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