richard p. allan environmental systems science centre, university of reading, uk brian soden
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Monitoring satellite observations and model simulations of changes in the atmospheric 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. - PowerPoint PPT PresentationTRANSCRIPT
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Monitoring satellite observations and model simulations of changes in the
atmospheric 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
[email protected] © 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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Tropical ocean variabilitySST
Water vapour
Clear LW net down at surface
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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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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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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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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) GRL
Tropical Precipitation ResponseAllan and Soden, 2007, GRL
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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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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)
Changes in tropical precipitation frequency
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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
CMIP3
CMIP3 volcanic
NCEP ERA40
SSM/I-derived~ +0.7 Wm-2 decade-1
∆SNLc (Wm-2)
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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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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)