Monitoring satellite observations and model simulations of changes in the atmospheric hydrological cycle since 1979

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Monitoring satellite observations and model
simulations of changes in theatmospheric
hydrological cycle since 1979
Richard P. Allan Environmental Systems Science
Centre, University of Reading, UK Brian
Soden RSMAS, University of Miami, USA Viju
John Mat Office, UK
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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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Tropical ocean variability
SST Water vapour Clear LW net down at surface
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Increased moisture enhances atmospheric radiative
cooling to surface
SNLc clear-sky surface net down longwave
radiation CWV column integrated water vapour
ERA40 NCEP
dSNLc/dCWV 1 - 1.5 W kg-1
dCWV (mm)
Allan (2006) JGR 111, D22105
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Linear fit
dCWV/dTs 3.01.0 mm K-1
dSNLc/dTs 3.51.5 Wm-2K-1
Models, reanalyses and observations show
increased surface net downward longwave with
warming due to increased water vapour
CMIP3 non-volcanic CMIP3 volcanic Reanalyses/
Obs AMIP3
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Clear-sky outgoing longwave radiation (Wm-2)
ERA40 NCEP-1 AMIP ensemble ERBS/ScaRaB/CERES GISS
_E_R volcanic ensemble
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Clear-sky outgoing longwave radiation (Wm-2)
ERA40 NCEP-1 AMIP ensemble ERBS/ScaRaB/CERES GISS
_E_R volcanic ensemble
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Radiative cooling/Latent heating
- OBS - ERA40 --- NCEP
Clear-sky atmospheric longwave cooling
Precipitation
- SSM/I AMIP3 GISSvolc
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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 3K-1
• But convective rainfall supplied by moisture
  convergence which increases at rate 7K-1
• e.g. Allen and Ingram (2002) Nature Trenberth et
  al. (2003) BAMS

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Tropical Precipitation Response
Allan and Soden, 2007, GRL
GPCP CMAP

• 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) Nature
• Trenberth and Dai (2007) GRL

AMIP3
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Projected changes in Tropical Precipitation
Allan and Soden, 2007, GRL
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Are observed trends sensitive to instrument/
algorithm?
Tropical ocean ascent
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Changes in tropical precipitation frequency
Histograms of the frequency of precipitation in
bins of intensity (e.g. 0-10, 90-95, 99-100,
etc). Test model precipitation response to ENSO
(B.Soden)
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• 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

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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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Increase in clear-sky longwave radiative cooling
to the surface
?SNLc (Wm-2)
CMIP3 CMIP3 volcanic NCEP ERA40 SSM/I-derived
0.7 Wm-2 decade-1
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Increase in atmospheric cooling over tropical
ocean descent 4 Wm-2K-1
CMIP3 volcanic Reanalyses/ Observations
AMIP3 CMIP3 non-volcanic
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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)