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Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation

Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation. Richard P. Allan Environmental Systems Science Centre, University of Reading, UK. Thanks to Brian Soden. Climate Impacts.

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Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation

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  1. Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation Richard P. Allan Environmental Systems Science Centre, University of Reading, UK Thanks to Brian Soden

  2. Climate Impacts How thehydrological cycle responds to aradiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather)

  3. Changing character of precipitation 1979-2002 • Convective rainfall draws in moisture from surroundings • Moisture is observed & predicted to increase with warming ~7%K-1 (e.g. Soden et al. 2005, Science) • Thus convective rainfall also expected to increase at this rate (e.g. Trenberth et al. 2003 BAMS)

  4. 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) 7 % K-1 - Changes in P with warming estimated to be ~3%K-1 - Consistent with model estimates (~2%K-1) ∆P (%) ∆T (K) Held and Soden (2006) J. Clim

  5. Precipitation linked to clear-sky longwave radiative cooling of the atmosphere

  6. Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP dSNLc/dCWV ~ 1 ─ 1.5 Wkg-1 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour Allan (2006) JGR 111, D22105

  7. Increase in clear-sky longwave radiative cooling to the surface ∆SNLc (Wm-2) CMIP3 CMIP3 volcanic NCEP ERA40 SSM/I-derived ~ +1 Wm-2 per decade

  8. Tropical Oceans dCWV/dTs ~2 ─ 4 mm K-1 dSNLc/dTs ~3 ─ 5 Wm-2K-1 AMIP3 CMIP3 non-volcanic CMIP3 volcanic Reanalyses/ Observations

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

  10. Increased moisture (~7%/K) •  increased convective precipitation • Increased radiative cooling •  smaller mean rise in precipitation (~3%/K) • Implies reduced precipitation in subsidence regions (less light rainfall?) • Locally, mixed signal from the above • Method: Analyse separately precipitation over the ascending and descending branches of the tropical circulation

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

  12. Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1) OCEAN LAND AMIPSSM/I GPCP CMAP Allan and Soden, 2007, GRL

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

  14. Conclusions • Heavy rainfall and areas affected by drought expected to increase with warming [IPCC 2007] • Heavy precipitation increases with moisture ~7%K-1 • Mean Precipitation constrained by radiative cooling • Models simulate increases in moisture (~7%K-1) and clear-sky LW radiative cooling (3-5 Wm-2K-1) • But large discrepancy between observed and simulated precipitation responses… • Model inadequacies or satellite calibration/algorithm problems? • Changes in evaporation and wind-speed over ocean at odds with models? (Yu and Weller, 2007 BAMS; Wentz et al. 2007, Science; Roderick et al. 2007 GRL) • Observing systems: capturing decadal variability problematic

  15. Extra slides…

  16. Outline • Clear-sky radiative cooling: • radiative convective balance • atmospheric circulation • Earth’s radiation budget • Understand clear-sky budget to understand cloud radiative effect • Method: • analyse relationship between water vapour, clear-sky radiative cooling and precipitation • Satellite observations, reanalyses, climate models (atmosphere-only/fully coupled)

  17. Models reproduce observed increases in total column water vapour

  18. ERA40 NCEP SRB Tropical Oceans HadISST Ts CWV LWc SFC SMMR, SSM/I Derived:SMMR, SSM/I, Prata) 1980 1985 1990 1995 2000 2005 Allan (2006) JGR 111, D22105

  19. Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB Calibration or sampling?

  20. ERA40 NCEP SRB Tropical Oceans Surface Net LWc Clear-sky OLR Clear-sky Atmos LW cooling QLWc HadISST ERBS, ScaRaB, CERES Derived Allan (2006) JGR 111, D22105

  21. Linear least squares fit ERA40 NCEP • Tropical ocean: descending regime • Dataset dQLWc/dTs Slope • ERA-40 3.7±0.5 Wm-2K-1 • NCEP 4.2±0.3 Wm-2K-1 • SRB 3.6±0.5 Wm-2K-1 • OBS 4.6±0.5 Wm-2K-1

  22. Implications for tropical precipitation (GPCP)? GPCP P ERA40 QLWc OBS QLWc Pinatubo?

  23. Comparison of AMIP3 models, reanalyses and observations over the tropical coeans

  24. Also considering coupled model experiments including greenhouse gas and natural forcings

  25. Clear-sky vs resolution

  26. Sensitivity study • Based on GERB- SEVIRI OLR and cloud products over ocean: • dOLRc/dRes ~0.2 Wm-2km-0.5 • Suggest CERES should be biased low by ~0.5 Wm-2relative to ERBS

  27. Links to precipitation

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