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ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Intensification of the tropical hydrological cycle?
Richard P. Allan
Environmental Systems Science Centre, University of Reading, UK
Thanks to Brian Soden
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Climate Impacts How the hydrological cycle responds to aradiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather)
Motivation
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
IPCC 2007 WGI
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Introduction
• Hydrological cycle linked to radiative energy balance
• How will global water cycle respond to warming?
• Method: compare observations and reanalysis products with model simulations of the present day climate over the period 1979-2006
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Earth’s energy balance
Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Earth’s energy balance
Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94
Precip: +78 Wm-2
SW heating +67 Wm-2
LW cooling -169 Wm-2
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Changing character of precipitation
Convective rainfall draws in moisture from surroundings
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Changing character of precipitation
• Moisture is observed & predicted to increase with warming ~7%K-1
(e.g. Soden et al. 2005, Science)
• Thus convective rainfall also expected to increase at this rate (e.g. Trenberth et al. 2003 BAMS)
1979-2002
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q)
• Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 2002)
• If so, changes in P with warming ≈3%K-1
• …substantially lower than changes in moisture (~7%K-1)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Models also display a “muted” precipitation response (~1-3%K-1)
Held and Soden (2006) J. Clim
∆P
(%
) 7 % K
-1
∆T (K)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Held and Soden (2006) J. Clim
∆P
(%
)
“heavy rain”: ~7 % K-1
∆T (K)
Mean: ~2 % K-1
“light rain”: -? % K-1
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
But recent results suggest that the “muted” precipitation response is not found in the observations
Is the global water cycle/radiation budget not captured by models?
Wentz et al. (2007) Science
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Are model simulated changes in clear-sky radiative cooling robust?
σT4 εAσT4
↓OLR
Ts
T, H2O, GHG ? Cloud
? Aerosol
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
How does atmospheric net radiative cooling respond to warming?
• Sensitivity test (Edwards/Slingo):
• Assume no change in clouds, aerosol, ozone
(1) Two warming scenarios (C1, C2)
(2) Greenhouse gas increase (1980-99)
(3) Shortwave absorption case
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
• Sensitivity test (Edwards/Slingo):
• Assume no change in clouds, aerosol, ozone
(1) Two warming scenarios (C1, C2)
(2) Greenhouse gas increase (1980-99)
(3) Shortwave absorption case
How does atmospheric net radiative cooling respond to warming?
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Increased moisture enhances atmospheric radiative cooling to surface
ERA40 NCEP
Allan (2006) JGR 111, D22105
dSNLc/dCWV ~ 1 ─ 1.5 W kg-1
SNLc = clear-sky surface net down longwave radiation
CWV = column integrated water vapour
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
What about in observations, reanalyses and models?
• Method: Compare monthly mean changes in temperature, water vapour and clear-sky longwave radiation, 1979-2006.
• Estimate water vapour driven changes in surface radiation using satellite and climatological ocean data, and an empirical model (Prata 1996)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Tropical ocean variability
SST
Water vapour
Clear net LW down at surface
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Increase in clear-sky longwave radiative cooling to the surface
CMIP3
CMIP3 volcanic
NCEP ERA40
SSM/I-derived
~ +1 Wm-2 per decade
∆SNLc (Wm-2)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Tropical Oceans
dCWV/dTs ~2 ─ 4 mm K-1
dSNLc/dTs ~3 ─ 5 Wm-2K-1
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Tropical ocean variability
LWc: TOA
LWc: ATM
Precip
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
• Increased moisture (~7%/K) increased convective precipitation
• Increased radiative cooling smaller mean rise in precipitation (~3%/K)
• Implies reduced precipitation away from convective regimes (less light rainfall?)
• Locally, mixed signal from the above
RECAP…
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
• Method: Analyse separately precipitation over the ascending and descending branches of the tropical circulation– Use reanalyses to sub-sample observed data– Employ widely used precipitation datasets– Compare with atmosphere-only and fully coupled
climate model simulations
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
GPCP CMAP
AMIP3
• Model precipitation response smaller than the satellite observations– see also Wentz et
al. (2007) Science
Tropical Precipitation Response
Allan and Soden, 2007, GRL
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1
OCEAN LAND
AMIP SSM/I GPCP CMAP
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Projected changes in Tropical Precipitation
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Calculated trends
• Models understimate mean precipitation response by factor of ~2-3
• Models severely underestimate precip response in ascending and descending branches of tropical circulation
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Questions
(1) Is the analysis flawed?
(2) Are the observed changes physically plausible? Are the observations wrong?
(3) Can decadal changes in cloud and aerosol explain the discrepancy?
(4) Are the models missing fundamental physics?
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
(1) Is the analysis flawed?
• Changes in the reanalyses cannot explain the bulk of the trends in precipitation
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
(2a) Are the observations wrong?
• Many of the global precipitation datasets use satellite data– Changes in the observing system– Potential algorithm errors sensitive to temperature
and/or water vapour amount
• Evidence from land and ocean observations suggest models underestimate precipitation and evaporation response to warming– e.g. Wentz et al. 2007 Science; Zhang et al. 2007
Nature, Chou et al. 2007 GRL, …
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Zhang et al. 2007 Nature
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Surface Evaporation and wind speed
• Surface evaporation depends primarily upon– Wind stress– Surface humidity gradient (expected to change due to
Clausius Clapeyron equation ~7%/K)
• Globally, evaporation must equal precipitation– Therefore a muted precipitation response requires a
muted evaporation response (e.g. muted wind stress)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Changes in Evaporation over ocean
• Observed increases in surface evaporation • larger than climate model simulations; caused by
– increased surface humidity gradient (Clausius Clapeyron) – Little trend in wind stress changes over ocean (Yu and Weller,
2007; Wentz et al., 2007) but some evidence over land (Roderick et al. 2007 GRL)
Yu and Weller (2007) BAMS
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
(2b) Are the changes plausible?• Increases in tropical precipitation and
evaporation at ~7%/K plausible• But increases in ascent region precip ~ 30%K-1,
much larger than Clausius Clapeyron• Implies intensification of hydrological cycle
• At odds with observations and model simulations of a reduction in Walker circulation (Vecchi and Soden, 2007)
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
• Vecchi and Soden (2006) Nature
• Evidence for weakening of Walker circulation in models and observations
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
• Vecchi and Soden (2006) Nature
• Evidence for weakening of Walker circulation in models and observations
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
(3) What else could explain this apparent discrepancy?
• Changes in land/ocean tropics/extra-tropical transport of heat and moisture?
• Decadal variability in radiation balance and hydrological cycle?– Changes in clouds– Changes in aerosol
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.ukWong et al. 2006 J. Climate, Wielicki et al. 2002, Science
Climate models appear to underestimate variability in radiation budget
Does this relate to clouds and/or aerosol?
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Global dimming to Global BrighteningStanhill and Cohen EOS (below), Wild et al., Pinker et al. 2005 Science
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation?
See Mishchenko et al. (2007) Science
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Aerosol Hypothesis?• Could changes in aerosol directly and indirectly (through
cloud) alter the radiation balance and precipitation?
↓ Aerosol
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Aerosol Hypothesis?• Could changes in aerosol directly and indirectly (through
cloud) alter the radiation balance and precipitation?
↓ Aerosol
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Aerosol Hypothesis?• Could changes in aerosol directly and indirectly (through
cloud) alter the radiation balance and precipitation?
↓ Aerosol
Rad cooling
Solar heating
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Aerosol Hypothesis?• Could changes in aerosol directly and indirectly (through
cloud) alter the radiation balance and precipitation?
↓ Aerosol
Rad cooling Precip
Solar, Evap Circulation?
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Summary
• Global water and energy cycles coupled• Theoretical changes in clear-sky radiative
cooling of atmosphere implies “muted” precipitation response
• Models simulate muted response, observations show larger response
• Possible artifacts of data?• Possible mechanisms (aerosol, cloud)• Implications for climate change prediction
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
References
• Allen and Ingram (2002) Nature
• Trenberth et al. (2003) BAMS
• Wentz et al. (2007) Science
• My meagre contributions:– Allan (2006) JGR– Allan and Soden (2007) GRL
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Conclusions• Heavy rainfall and areas affected by drought expected to
increase with warming [IPCC 2007]• Heavy precipitation increases with moisture ~7%K-1
• Mean Precipitation constrained by radiative cooling– Models simulate increases in moisture (~7%K-1) and clear-sky LW
radiative cooling (3-5 Wm-2K-1)
• But large discrepancy between observed and simulated precipitation responses…– Model inadequacies or satellite calibration/algorithm problems?– Changes in evaporation and wind-speed over ocean at odds with
models? (Yu and Weller, 2007 BAMS; Wentz et al. 2007, Science; Roderick et al. 2007 GRL)
• Observing systems: capturing decadal variability problematic
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Implications for tropical precipitation (GPCP)?
ERA40 QLWc
GPCP P
OBS QLWc
Pinatubo?
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Comparison of AMIP3 models, reanalyses and observations over the tropical coeans
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Also considering coupled model experiments including greenhouse gas and natural forcings
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Clear-sky vs resolution
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Sensitivity study
• Based on GERB- SEVIRI OLR and cloud products over ocean:
• dOLRc/dRes ~0.2 Wm-2km-0.5
• Suggest CERES should be biased low
by ~0.5 Wm-2 relative to ERBS
ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk
Links to precipitation
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