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r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
Present day changes in tropical precipitation extremes
in models and observations
Richard P. Allan
Environmental Systems Science Centre, University of Reading
With thanks to:
Brian Soden (RSMAS, University of Miami)
Viju John (Hadley Centre)
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
Climate Impacts How the hydrological cycle responds to global warming is crucial for society (e.g. water supply, agriculture, severe weather)
Motivation
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Earth’s energy balance
Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94
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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
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How does clear-sky radiative cooling respond to warming?
Clear-sky Longwave shortwave
TOA SFC ATM ATM
1K increase in tropospheric T, constant RH
Greenhouse gas changes from 1980 to 2000 assuming different rates of warming
Clear-sky net cooling increases at ~3 Wm-2K-1
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AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1
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Increases in water vapour enhance clear-sky longwave radiative cooling of atmosphere to the surface
This is offset by enhanced absorption of shortwave radiation by water vapour
See Lambert and Webb (2008)
CMIP3 MODELS: Tropical oceans
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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; Lambert and Webb, 2008
Lambert and Webb (2008) submitted
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• But moisture observed & predicted to increase at greater rate ~7%K-1
(e.g. Soden et al. 2005, Science)
• Thus convective rainfall expected to increase at a faster rate than mean precipitation (e.g. Trenberth et al. 2003 BAMS)
1979-2002
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Held and Soden (2006) J. Clim
∆P
(%
)
“heavy rain”: ~7 % K-1
∆T (K)
Mean: ~2 % K-1
“light rain”: –XX % K-1
7 % K
-1
Contrasting precipitation response expected
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Changes in precipitation: “the rich get richer”?
precip trends
0-30oN
Rainy season: wetter
Dry season: drier
Chou et al. 2007 GRL
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IPCC 2007 WGI
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• Method: Analyse separately precipitation over the wet ascending and dry 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
Is this contrasting precipitation response borne out by observations?
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GPCP CMAP
AMIP3
• Model precipitation response smaller than the satellite observations
see also discussion in: Wentz et al. (2007) Science,Yu and Weller (2007) BAMS,Roderick et al. (2007) GRL,Chou et al. (2007) GRL,Zhang et al. (2007) NatureTrenberth and Dai (2007) GRLLambert and Webb (2008)
Tropical Precipitation ResponseAllan and Soden, 2007, GRL
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Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1
OCEAN LAND
AMIP SSM/I GPCP CMAP
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Projected changes in Tropical Precipitation
Allan and Soden, 2007, GRL
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Sensitivity to SST (top) and CWV (bottom): more consistent
with models
AMIP3
CMIP3 non-volcanicCMIP3 volcanic
Reanalyses/ Observations
Tropical ocean ascent
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Are observed trends sensitive to instrument/ algorithm?(Viju John)
Tropical ocean ascent
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Can precipitation response to ENSO be used as a test of model sensitivity?
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Histograms of the frequency of precipitation in bins of intensity (e.g. 0-10%, …, 80-90%, 90-95%, 99-100%).
Test model precipitation response to ENSO (+B.Soden)
Changes in tropical precipitation frequency
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r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
• Based on response to warming during ENSO, models:– Underestimate increases in frequency of heaviest precipitation– Produce spurious decrease in frequency of moderate
precipitation and increase frequency in lightest rainfall
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
• Based on response to warming during ENSO, models:– Underestimate increases in frequency of heaviest precipitation– Produce spurious decrease in frequency of moderate
precipitation and increase frequency in lightest rainfall
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
• Based on response to warming during ENSO, models:– Underestimate increases in frequency of heaviest precipitation– Produce spurious decrease in frequency of moderate
precipitation and increase frequency in lightest rainfall
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
Satellite data suggests that mean precipitation and evaporation changes appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates
Yu and Weller (2007) BAMS
(Wentz et al. 2007, Science)This appears to require super-Clausius Clapeyron changes in moist-region precipitation?
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r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
• Vecchi and Soden (2006) Nature
• Evidence for weakening of Walker circulation in models and observations
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• Vecchi and Soden (2006) Nature
• Evidence for weakening of Walker circulation in models and observations
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Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation?
Mishchenko et al. (2007) Science
Also: Liepert and Prevedi (2008) submitted to J Clim
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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
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Extra slides
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Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere
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Increased moisture enhances atmospheric radiative cooling to surface
ERA40 NCEP
Allan (2006) JGR 111, D22105
SNLc = clear-sky surface net down longwave radiation
CWV = column integrated water vapour
dSNLc/dCWV ~ 1 ─ 1.5 W kg-1
dCWV (mm)
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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)
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
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
• But convective rainfall supplied by moisture convergence which increases at rate ~7%K-1
e.g. Allen and Ingram (2002) Nature; Trenberth et al. (2003) BAMS
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Are the results sensitive to the reanalysis data?
• Changes in the reanalyses cannot explain the bulk of the trends in precipitation
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Tropical ocean variabilitySST
Water vapour
Clear LW net down at surface
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
Increased moisture enhances atmospheric radiative cooling to surface
ERA40 NCEP
Allan (2006) JGR 111, D22105
SNLc = clear-sky surface net down longwave radiation
CWV = column integrated water vapour
dSNLc/dCWV ~ 1 ─ 1.5 W kg-1
dCWV (mm)
r.p.allan@reading.ac.uk © University of Reading 2007www.nerc-essc.ac.uk/~rpa
Linear fit
dSNLc/dTs ~ 3.5±1.5 Wm-2K-1
dCWV/dTs ~ 3.0±1.0 mm K-1
CMIP3 non-volcanic CMIP3 volcanic
Reanalyses/ Obs AMIP3
Models, reanalyses and observations show increased surface net downward longwave with warming due to increased water vapour
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ERA40 NCEP-1 AMIP ensemble
ERBS/ScaRaB/CERES GISS_E_R volcanic ensemble
Clear-sky outgoing longwave radiation (Wm-2)
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ERA40 NCEP-1 AMIP ensemble
ERBS/ScaRaB/CERES GISS_E_R volcanic ensemble
Clear-sky outgoing longwave radiation (Wm-2)
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Clear-sky atmospheric longwave cooling
Precipitation─ SSM/I AMIP3 GISSvolc
─ OBS ─ ERA40 --- NCEP
Radiative cooling/Latent heating
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Summary• Global water and energy cycles coupled• Satellite data and models agree on rate of moisture
increase with temperature (~7%/K) increased radiative cooling of atmosphere to the surface• Theoretical changes in clear-sky radiative cooling of
atmosphere implies “muted” precipitation response• Models simulate muted response, observations show
larger response• Models severely underestimate precipitation response in
ascending and descending branches of tropical circulation– Possible artifacts of data? – Implications for climate change prediction
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Extra slides…
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But water vapour is rising at a faster rate (~7%/K)
Convective rainfall draws in moisture from surroundings
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Increase in clear-sky longwave radiative cooling to the surface
CMIP3
CMIP3 volcanic
NCEP ERA40
SSM/I-derived~ +0.7 Wm-2 decade-1
∆SNLc (Wm-2)
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Links to precipitation
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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
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Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1
OCEAN LAND
AMIP SSM/I GPCP CMAP
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Are the results sensitive to the reanalysis data?
• Changes in the reanalyses cannot explain the bulk of the trends in precipitation
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Microwave estimates of precipitation and evaporation over the ocean appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates (Wentz et al. 2007, Science)
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Observed increases in evaporation over ocean larger than climate model simulations
Yu and Weller (2007) BAMS
- increased surface humidity gradient (Clausius Clapeyron)
- little trend in wind stress changes over ocean (Yu and Weller, 2007; Wentz et al., 2007) although some evidence over land (Roderick et al. 2007 GRL)