1. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Present day changes in tropical precipitation extremes in models and observations Richard P. Allan Environmental Systems Science Centre, University of Reading With thanks to: Brian Soden (RSMAS, University of Miami) Viju John (Hadley Centre)

2. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Climate Impacts How the hydrological cycle responds to global warming is crucial for society (e.g. water supply, agriculture, severe weather) Motivation

3. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Earths energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

4. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Earths energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94 Precip: +78 Wm -2 SW heating +67 Wm -2 LW cooling -169 Wm -2

5. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa How does clear-sky radiative cooling respond to warming? Clear-sky Longwave shortwave TOA SFC ATM ATM 1K increase in tropospheric T, constant RH Greenhouse gas changes from 1980 to 2000 assuming different rates of warming Clear-sky net cooling increases at ~3 Wm-2K-1

6. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations Increase in atmospheric cooling over tropical ocean descent ~4 Wm -2 K -1

7. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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 See Lambert and Webb (2008) CMIP3 MODELS: Tropical oceans

8. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Models simulate robust response of clear-sky radiation to warming (~2 Wm -2 K -1 ) and a resulting increase in precipitation to balance (~2%K -1 ) e.g., Allen and Ingram, 2002; Lambert and Webb, 2008 Lambert and Webb (2008) submitted

9. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa But moisture observed & predicted to increase at greater rate ~7%K -1 (e.g. Soden et al. 2005, Science) Thus convective rainfall expected to increase at a faster rate than mean precipitation (e.g. Trenberth et al. 2003 BAMS) 1979-2002

10. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Held and Soden (2006) J. Clim P (%) heavy rain: ~7 % K -1 T (K) Mean: ~2 % K -1 light rain: –XX % K -1 7 % K -1 Contrasting precipitation response expected

11. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Changes in precipitation: the rich get richer? precip trends 0-30 o N Rainy season: wetter Dry season: drier Chou et al. 2007 GRL

12. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa IPCC 2007 WGI

13. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Method: Analyse separately precipitation over the wet ascending and dry 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 Is this contrasting precipitation response borne out by observations?

14. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa GPCP CMAP AMIP3 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 Lambert and Webb (2008) Tropical Precipitation Response Allan and Soden, 2007, GRL

15. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Tropical Subsidence regions dP/dt ~ -0.1 mm day -1 decade -1 OCEANLAND AMIP SSM/IGPCPCMAP

16. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Projected changes in Tropical Precipitation Allan and Soden, 2007, GRL

17. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Sensitivity to SST (top) and CWV (bottom): more consistent with models AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations Tropical ocean ascent

18. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Are observed trends sensitive to instrument/ algorithm? (Viju John) Tropical ocean ascent

19. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Can precipitation response to ENSO be used as a test of model sensitivity?

20. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Histograms of the frequency of precipitation in bins of intensity (e.g. 0-10%, …, 80- 90%, 90-95%, 99-100%). Test model precipitation response to ENSO (+B.Soden) Changes in tropical precipitation frequency

21. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa

22. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

23. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

24. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

25. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Satellite data suggests that mean precipitation and evaporation changes appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates Yu and Weller (2007) BAMS (Wentz et al. 2007, Science) This appears to require super- Clausius Clapeyron changes in moist- region precipitation?

26. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa

27. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Vecchi and Soden (2006) Nature Evidence for weakening of Walker circulation in models and observations

28. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Vecchi and Soden (2006) Nature Evidence for weakening of Walker circulation in models and observations

29. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation? Mishchenko et al. (2007) Science Also: Liepert and Prevedi (2008) submitted to J Clim

30. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

31. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Extra slides

32. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere

33. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP Allan (2006) JGR 111, D22105 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour dSNLc/dCWV ~ 1 1.5 W kg -1 dCWV (mm)

34. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q) Changes in Q expected to be ~3 Wm -2 K -1 (e.g. Allen and Ingram, 2002) If so, changes in P with warming 3%K -1 …substantially lower than changes in moisture (~7%K -1 )

35. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q) Changes in Q expected to be ~3 Wm -2 K -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

36. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Are the results sensitive to the reanalysis data? Changes in the reanalyses cannot explain the bulk of the trends in precipitation

37. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Tropical ocean variability SST Water vapour Clear LW net down at surface

38. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP Allan (2006) JGR 111, D22105 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour dSNLc/dCWV ~ 1 1.5 W kg -1 dCWV (mm)

39. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Linear fit dSNLc/dTs ~ 3.5±1.5 Wm -2 K -1 dCWV/dTs ~ 3.0±1.0 mm K -1 CMIP3 non-volcanicCMIP3 volcanic Reanalyses/ ObsAMIP3 Models, reanalyses and observations show increased surface net downward longwave with warming due to increased water vapour

40. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa ERA40 NCEP-1AMIP ensemble ERBS/ScaRaB/CERESGISS_E_R volcanic ensemble Clear-sky outgoing longwave radiation (Wm -2 )

41. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa ERA40 NCEP-1AMIP ensemble ERBS/ScaRaB/CERESGISS_E_R volcanic ensemble Clear-sky outgoing longwave radiation (Wm -2 )

42. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Clear-sky atmospheric longwave cooling Precipitation SSM/I AMIP3 GISSvolc OBS ERA40 --- NCEP Radiative cooling/Latent heating

43. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

44. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Extra slides…

45. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa But water vapour is rising at a faster rate (~7%/K) Convective rainfall draws in moisture from surroundings

46. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Increase in clear-sky longwave radiative cooling to the surface CMIP3 CMIP3 volcanic NCEP ERA40 SSM/I-derived ~ +0.7 Wm -2 decade -1 SNLc (Wm -2 )

47. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Links to precipitation

48. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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

49. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Tropical Subsidence regions dP/dt ~ -0.1 mm day -1 decade -1 OCEANLAND AMIP SSM/IGPCPCMAP

50. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa Are the results sensitive to the reanalysis data? Changes in the reanalyses cannot explain the bulk of the trends in precipitation

51. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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)

52. r.p.allan@reading.ac.uk© University of Reading 2007www.nerc-essc.ac.uk/~rpa 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)