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

Observed and simulated changes in water vapour, precipitation and the clear-sky longwave radiation budget of the surface and atmosphere. Richard P. Allan Environmental Systems Science Centre, University of Reading, UK. Earth’s energy balance.

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

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  1. Observed and simulated changes in water vapour, precipitation and the clear-skylongwave radiation budget of the surface and atmosphere Richard P. Allan Environmental Systems Science Centre, University of Reading, UK

  2. Earth’s energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

  3. Earth’s energy balance SW heating +67 Wm-2 LW cooling -169 Wm-2 Precip: +78 Wm-2 Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

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

  6. Tropical oceans 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 Changes in greenhouse gases, aerosol and cloud alter this relationship…

  7. Sensitivity test: tropical oceans 1K increase in tropospheric T, constant RH Greenhouse gas changes from 1980 to 2000 assuming different rates of warming TOA SFC ATM ATM  Clear-sky Longwave  shortwave

  8. Increase in atmospheric LW cooling over tropical ocean descent ~4 ─ 5 Wm-2K-1 AMIP3 CMIP3 non-volcanic CMIP3 volcanic Reanalyses/ Observations

  9. 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

  10. 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

  11. Tropical Precipitation Response Model precipitation response smaller than the satellite observations (see also Wentz et al. 2007 Science; Chou et al. 2007 GRL, etc) GPCP CMAP AMIP3 Allan and Soden, 2007, GRL

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

  13. Could changes in aerosol and their indirect effect on cloud be driving changes in the tropical hydrological cycle through the surface radiation budget? Mishchenko et al. (2007) Science; Wild et al. (2005) Science

  14. 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

  15. Extra slides…

  16. 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

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

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

  19. 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)

  20. Links to precipitation

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