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Richard P. Allan Environmental Systems Science Centre, University of Reading, UK Brian Soden

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.

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Richard P. Allan Environmental Systems Science Centre, University of Reading, UK Brian Soden

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  1. 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

  2. Climate ImpactsHow the hydrological cycle responds to global warming is crucial for society (e.g. water supply, agriculture, severe weather) Motivation

  3. Tropical ocean variability SST Water vapour Clear LW net down at surface

  4. 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

  5. dCWV/dTs ~ 3.0±1.0 mm K-1 Linear fit dSNLc/dTs ~ 3.5±1.5 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

  6. Clear-sky outgoing longwave radiation (Wm-2) ERA40 NCEP-1 AMIP ensemble ERBS/ScaRaB/CERES GISS_E_R volcanic ensemble

  7. Clear-sky outgoing longwave radiation (Wm-2) ERA40 NCEP-1 AMIP ensemble ERBS/ScaRaB/CERES GISS_E_R volcanic ensemble

  8. Radiative cooling/Latent heating ─ OBS ─ ERA40 --- NCEP Clear-sky atmospheric longwave cooling ─ SSM/I AMIP3GISSvolc Precipitation

  9. 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

  10. 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

  11. Projected changes in Tropical Precipitation Allan and Soden, 2007, GRL

  12. Are observed trends sensitive to instrument/ algorithm? Tropical ocean ascent

  13. 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)

  14. 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

  15. 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

  16. Extra slides…

  17. 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

  18. Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1 CMIP3 volcanic Reanalyses/ Observations AMIP3 CMIP3 non-volcanic

  19. Links to precipitation

  20. 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

  21. Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1 OCEAN LAND AMIPSSM/I GPCP CMAP

  22. Are the results sensitive to the reanalysis data? • Changes in the reanalyses cannot explain the bulk of the trends in precipitation

  23. 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)

  24. 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)

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