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Tropospheric Water Vapour and the Hydrological Cycle

Tropospheric Water Vapour and the Hydrological Cycle. Richard Allan University of Reading, UK. Tony Slingo. 1950-2008. How does water vapour impact climate change?. Amount of warming Changes in water cycle. Spectral cooling rate (H 2 O, CO 2 , O 3 ) Clough & Iacono (1995) JGR.

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Tropospheric Water Vapour and the Hydrological Cycle

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  1. Tropospheric Water Vapour and the Hydrological Cycle Richard Allan University of Reading, UK

  2. Tony Slingo 1950-2008

  3. How does water vapour impact climate change? • Amount of warming • Changes in water cycle

  4. Spectral cooling rate (H2O, CO2, O3) Clough & Iacono (1995) JGR K d-1 (cm-1)-1 MLS

  5. = ─ Climate sensitivity and water vapour feedback

  6. = ─ Climate sensitivity and water vapour feedback Kernals: Soden et al. (2008) 0.2 Wm-2%-1

  7. = ─ Climate sensitivity and water vapour feedback 0.2 Wm-2%-1 7%K-1

  8. = ─ Climate sensitivity and water vapour feedback 0.2 Wm-2%-1 7%K-1 λBB~ -4σT3 ~ -3.2 Wm-2K-1 λWV~(0.2)(7)=1.4 Wm-2K-1 λWV+λBB~1.4 - 3.2 = -1.8 Wm-2K-1

  9. How does moisture respond to warming? • Low level moisture over the ocean seems to behave… • Does moisture really vary with temperature at ~ 7%/K? • Do models capture the essential relationships? • Land, upper troposphere? • Reanalyses, surface measurements or satellite data?

  10. Does moisture rise at 7%/K over land? Specific humidity trend correlation (left) and time series (right) LandOcean Willett et al. (2007) Nature; Willet et al. (2008) J Clim But some contradictory results (e.g., Wang et al. (2008) GRL)

  11. Is moisture at higher levels constrained by Clausius Clapeyron? Trend in water vapour radiance channels: 1983-2004 Observations Model Moistening Constant RH model Constant water vapour model Soden et al. (2005) Science

  12. Is the mean state important? • Models appear to overestimate water vapour • Pierce et al. (2006) GRL; John and Soden (2006) GRL • But not for microwave data? [Brogniez and Pierrehumbert (2007) GRL] • This does not appear to affect feedback strength • Held and Soden (2006), John and Soden (2006) • What about the hydrological cycle? • Inaccurate mean state? Pierce et al. (2006) GRL

  13. What time-scales do different processes operate on? Soden et al. (2002) Science; Forster/Collins (2004) Clim Dyn; Harries/Futyan (2006) GRL

  14. Trends in UTH (above)Sensitivity of OLR to UTH (right) Bates and Jackson (2001) GRL

  15. Reduction in UTH with warming Minschwaner et al. (2006) J Clim Lindzen (1990) BAMS Mitchell et al. (1987) QJRMS

  16. Moistening processes: diurnal cycle (SEVIRI) Convergence Divergence Sohn et al.(2008)JGR Evaporation Condensation Moistening Drying Evaporation Divergence UTH tendency See also Soden et al. (2004) GRL

  17. Evaporation cannot explain moistening g m-3 350 250 180 120 90 63 45 30 John and Soden (2006) GRL; Luo and Rossow (2004)

  18. Cloud feedback: a more complex problem • Depends on: • Type of cloud • Height of cloud • Time of day/year • Surface characteristics Non-trivial relationship between cloud and temperature Response of cloud to warming is highly uncertain

  19. Spread in cloud feedback in models appears to relate to tropical low altitude clouds IPCC (2007), after Sandrine Bony and colleagues

  20. Is cloud feedback an indirect forcing? • • Clouds respond to • direct forcing from CO2 • Climate response to ∆SST • • Does cloud feedback uncertainty stem from direct response rather than climate feedback response? Andrews and Forster (2008) GRL (above); Gregory and Webb (2008) J Clim

  21. How should precipitation respond to climate change? Allen and Ingram (2002) Nature

  22. Models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) and a resulting increase in precipitation to balance (~2%K-1) e.g., Allen & Ingram, 2002; Lambert & Webb (2008) GRL Surface Temperature (K)

  23. But moisture observed & predicted to increase at greater rate ~7%K-1 • Thus convective rainfall expected to increase at a faster rate than mean precipitation (e.g. Trenberth et al. 2003 BAMS) 1979-2002

  24. Intensification of heaviest rainfall with warming Allan and Soden (2008) Science

  25. Contrasting precipitation response expected Heavy rain follows moisture (~7%/K) Mean Precipitation linked to radiation balance (~3%/K) Precipitation  Light Precipitation (-?%/K) Temperature  e.g. see Held and Soden (2006) J. Clim

  26. Mean projected precipitation changes IPCC 2007 WGI

  27. Contrasting precipitation response in ascending and descending portions of the tropical circulation ascent Precipitation change (mm/day) descent GPCP Models Allan and Soden (2007) GRL

  28. Could changes in aerosol be driving recent changes in the hydrological cycle? Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim

  29. Unanswered questions • How does UTH really respond to warming? • Do we understand the upper tropospheric moistening processes? • Is moisture really constrained by Clausius Clapeyron over land? • What time-scales do feedbacks operate on? • Apparent discrepancy between observed and simulated changes in precipitation • Is the satellite data at fault? • Are aerosol changes short-circuiting the hydrological cycle? • Could model physics/resolution be inadequate? • Could subtle changes in the boundary layer be coupled with decadal swings in the hydrological cycle? • How do clouds respond to forcing and feedback including changes in water vapour?

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