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Current Changes in the Global Water Cycle. Richard P. Allan Diffusing 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. - PowerPoint PPT Presentation
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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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
• 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
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
NCAS-Climate Talk 15th January 2010
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
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
NCAS-Climate Talk 15th January 2010
Current changes in the water cycle As observed by satellite datasets and
simulated by models
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
Avoid reanalyses in defining wet/dry
regions
• Sample grid boxes:– 30% wettest– 70% driest
• Do wet/dry trends remain?
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
• 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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
• 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
NCAS-Climate Talk 15th January 2010
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?
NCAS-Climate Talk 15th January 2010
Precipitation in the Europe-Atlantic region (summer)
Dependence on NAO
NCAS-Climate Talk 15th January 2010
Wat
er v
apou
rP
reci
pita
tion
Current changes in the water cycle over 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
Extra Slides
NCAS-Climate Talk 15th January 2010
NCAS-Climate Talk 15th January 2010
• 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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
2008
Are the issues of cloud feedback and the water cycle linked?
NCAS-Climate Talk 15th January 2010
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
NCAS-Climate Talk 15th January 2010
How does UTH respond to warming?
Lindzen (1990) BAMS
Minschwaner et al. (2006) J Clim
Mitchell et al. (1987) QJRMS
NCAS-Climate Talk 15th January 2010
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)