Current Changes in the Tropical Precipitation and Energy

1. Current Changes in the Tropical Precipitation and Energy Richard P. Allan Department of Meteorology, University of Reading Thanks to Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev, Mark Ringer and Tony Slingo http://www.met.reading.ac.uk/~sgs02rpa r.p.allan@reading.ac.uk

2. Sea Fishing 101 Course Convener Final Grade: F What could have been 

3. Climate model projections (IPCC 2007) Precipitation Intensity • Increased Precipitation • More Intense Rainfall • More droughts • Wet regions get wetter, dry regions get drier? • Regional projections?? Dry Days Precipitation Change (%)

4. Physical basis: energy balance Trenberth et al. (2009) BAMS

5. 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 and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim Allan (2009) J. Clim Radiative cooling, clear (Wm-2K-1) NCAS-Climate Talk 15th January 2010

6. Trends in clear-sky radiation in coupled models Surface net clear-sky longwave Clear-sky shortwave absorption Can we derive an observational estimate of surface longwave? Prata (1996) QJRMS

7. The energy constraint on global precipitation Andrews et al. (2009) J Climate

8. Evaporation Richter and Xie (2008) JGR CC Wind Ts-To RHo Muted Evaporation changes in models are explained by small changes in Boundary Layer:1) declining wind stress2) reduced surface temperature lapse rate (Ts-To)3) increased surface relative humidity (RHo) NCAS-Climate Talk 15th January 2010

9. Current tropical ocean variation in water vapour and precipitation Precip. (%) Allan and Soden (2008) Science

10. Current changes in tropical ocean column water vapour John et al. (2009) Water Vapour (mm) models …despite inaccurate mean state, Pierce et al.; John and Soden (both GRL, 2006) - see also Trenberth et al. (2005) Clim. Dyn., Soden et al. (2005) Science

11. Thermodynamic constraint 1979-2002 • Clausius-Clapeyron • Low-level water vapour (~7%/K) • Intensification of rainfall: Trenberth et al. (2003) BAMS; Pall et al. (2007) Clim Dyn • Changes in intense rainfall also constrained by moist adiabat -O’Gorman and Schneider (2009) PNAS • Could extra latent heat release within storms enhance rainfall intensity above Clausius Clapeyron? • e.g. Lenderink and van Meijgaard (2008) Nature Geoscience

12. Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I) Reduced frequency Increased frequency Allan et al. (2010) Environ. Res. Lett.

13. Increase in intense rainfall with tropical ocean warming (close to Clausius Clapeyron) • SSM/I satellite observations at upper limit of model range Model intense precipitation dependent upon conservation of moist adiabatic lapse rate but responses are highly sensitive to model-specific changes in upward velocities (see O’Gorman and Schneider, 2009, PNAS; Gastineau & Soden 2009).

14. Clausius-Clapeyron Low-level water vapour (~7%/K) Enhanced moisture transport (F) Enhanced P-E patterns (below) See Held and Soden (2006) J Clim Large-scale water cycle response AR5 scaling

15. Circulation response P~Mq Models/observations achieve muted precipitation response by reducing strength of Walker circulation. Vecchi and Soden (2006) Nature But see also Park and Sohn (2010) JGR in press

16. 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.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature

17. Contrasting precipitation response in wet and dry regions of the tropical circulation ascent Observations Models Precipitation change (%) descent Sensitivity to reanalysis dataset used to define wet/dry regions Updated from Allan and Soden (2007) GRL

18. Is the contrasting wet/dry response robust? GPCP Ascent Region Precipitation (mm/day) John et al. (2009) GRL • Large uncertainty in magnitude of change: satellite datasets and models & time period TRMM • Robust response: wet regions become wetter at the expense of dry regions. Is this an artefact of the reanalyses?

19. Avoid reanalyses in defining wet/dry regions • Sample grid boxes: • 30% wettest • 70% driest • Do wet/dry trends remain?

20. Current trends in wet/dry regions of tropical oceans • Wet/dry trends remain • 1979-1987 GPCP record may be suspect for dry region • SSM/I dry region record: inhomogeneity 2000/01? • GPCP trends 1988-2008 • Wet: 1.8%/decade • Dry: -2.6%/decade • Upper range of model trend magnitudes DRY WET Models

21. Outstanding Issues Can we understand and predict regional climate change? Could aerosols short-circuit the changing water cycle? Are the cloud feedback and water cycle issues linked?

22. One of the largest challenges remains improving predictability of regional changes in the water cycle… Changes in circulation systems are crucial to regional changes in water resources and risk yet predictability is poor. How will catchment-scale runoff and crucial local impacts and risk respond to warming? What are the important land-surface and ocean-atmosphere feedbacks which determine the response?

23. Top: GFDL cm2.1 2080-2099 minus 1980-1999 (% precipitation) Bottom: GFDL-GPCP precipitation (%)

24. Current changes in precipitation for Europe-Atlantic region Precipitation

25. Could changes in aerosol be imposing direct and indirect changes in the hydrological cycle? e.g. Wild et al. (2008) GRL Mishchenko et al. (2007) Science Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim

26. Can we observe atmospheric radiative heating/cooling? John et al. (2009) GRL

27. Are the issues of cloud feedback and the water cycle linked? 2006 Allan et al. (2007) QJRMS How important are cloud microphysical processes in stratocumulus and large-scale processes involving cirrus outflow? e.g. Ellis and Stephens (2009) GRL; Stephens and Ellis (2008) J Clim. Zelinka and Hartmann (in prep) “FAT/FAP hypothesis”; Stephens et al. (2010) JAS in prep

28. Robust Responses Low level moisture; clear-sky radiation Mean and Intense rainfall Observed precipitation response at upper end of model range? Contrasting wet/dry region responses Less Robust/Discrepancies Moisture at upper levels/over land and mean state Inaccurate precipitation frequency distributions Magnitude of change in precipitation from satellite datasets/models Further work Decadal changes in global energy budget, aerosol forcing effects and cloud feedbacks: links to water cycle? Precipitation and radiation balance datasets: forward modelling Surface feedbacks: ocean salinity, soil moisture (SMOS?) Boundary layer changes and surface fluxes Conclusions

29. Radiative effects of persistent aircraft contrails: a case study Richard Allan Environmental Systems Science Centre

30. NOAA17 satellite image 20 March 2009 10:06 Met Office NAME model Courtesy of Jim Haywood

31. Courtesy of Jim Haywood

32. Courtesy of Jim Haywood

33. Courtesy of Jim Haywood

34. Courtesy of Jim Haywood

35. Courtesy of Jim Haywood

36. Courtesy of Jim Haywood

37. Using GERB-like/SEVIRI to quantify radiative effects of persistent contrail cirrus

38. More details in Haywood et al. (2009) JGR

39. Radiative Effect LW NET SW Estimated effect as large as 7% of radiative forcing of entire aircraft fleet for that day. Future work: Icelandic volcano influence on cirrus contrails?