Transcript Chou et al. (2007) GRL - University of Reading

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
“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
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 (%)
Physical basis: energy balance
NCAS-Climate Talk
15th January 2010
Trenberth et al. (2009) BAMS
Evaporation
Richter and Xie (2008) JGR
CC Wind Ts-To RHo
Muted Evaporation changes in models are
explained by small changes in Boundary Layer:
NCAS-Climate Talk
15th January 2010
1) declining wind stress
2) reduced surface temperature lapse rate (Ts-To)
3) increased surface relative humidity (RHo)
Physical Basis: clear-sky radiative cooling:
models simulate robust response of clear-sky radiation to
warming (~2 Wm-2K-1) & resulting precipitation increase
Radiative cooling, clear (Wm-2)
Latent Heat Release, LΔP (Wm-2)
e.g. see Stephens and Ellis (2008); Lambert and Webb (2008) GRL
Lambert & Webb (2008) GRL
Surface Temperature (K)
Physical basis: water vapour
• 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
1979-2002
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
Circulation response
P~Mq
Models/observations achieve muted precipitation response by
reducing strength of Walker circulation. Vecchi and Soden (2006) Nature
Precipitation 
Contrasting precipitation response expected
Temperature 
e.g.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature
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 water cycle
As observed by satellite datasets and
simulated by models
Focus on tropical oceans.
Water Vapour (mm)
Current changes in tropical ocean
column water vapour
John et al. (2009)
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
ERA40
NCEP
SRB
SSM/I
ERA40
NCEP
SRB
SSM/I
ERA40
NCEP
ERAINT
SSM/I
Sensitivity of water vapour and
clear-sky radiation to surface temperature
Allan (2009) J . Climate
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)
Radiative cooling, clear (Wm-2K-1)
e.g. Allen and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim
NCAS-Climate Talk
15th January 2010
Allan (2006) JGR
Trends in clear-sky radiation
in coupled models
Surface net clear-sky longwave
Can we derive an
observational
estimate of surface
longwave?
Clear-sky shortwave absorption
Prata (1996) QJRMS
Variability in clear-sky radiative cooling
John et al. (2009) GRL
Precip.
(%)
Tropical ocean variation in water
vapour and precipitation
Allan and Soden (2008) Science
NCAS-Climate Talk
15th January 2010
Tropical ocean precipitation
• dP/dSST:
GPCP: 10%/K
(1988-2008)
AMIP: 3-11 %/K
(1979-2001)
SSM/I
GPCP
• dP/dt trend
GPCP: 1%/dec
(1988-2008)
AMIP: 0.4-0.7%/dec
(1979-2001)
(land+ocean)
Precipitation change (%)
Contrasting precipitation response in wet
and dry regions of the tropical circulation
ascent
Observations
Models
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)
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
• 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
WET
– 1979-1987 GPCP
record may be suspect
for dry region
– SSM/I dry region
record: inhomogeneity
2000/01?
DRY
Models
• GPCP trends 1988-2008
– Wet: 1.8%/decade
– Dry: -2.6%/decade
– Upper range of model
trend magnitudes
Precipitation Extremes
Trends in tropical wet region
precipitation appear robust.
– What about extreme precipitation events?
METHOD
• 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?
Increases in the frequency of the heaviest rainfall with warming:
daily data from models and microwave satellite data (SSM/I)
Reduced frequency
Increased frequency
Updated from Allan and Soden (2008) Science
• 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 EuropeAtlantic region
(summer)
Dependence
on NAO
Water vapourTemperature
Current changes water cycle
variables: Europe-Atlantic region
NCAS-Climate Talk
15th January 2010
Evaporation
Precipitation
Current changes water cycle
variables: Europe-Atlantic region
NCAS-Climate Talk
15th 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
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
Mishchenko et al. (2007) Science
Wielicki et al. (2002) Science; Wong et al.
(2006) J. Clim; Loeb et al. (2007) J. Clim
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 largescale 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 largescale 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 largescale 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
• 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