1. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Agreement/discrepancies between models and observations on the large scale hydrological cycle Richard Allan ESSC/National Centre for Earth Observation, University of Reading, UK Thanks to: Brian Soden Viju John

2. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle What do we want to predict? –P-E, P, P-events; large-scale, regional –For agriculture, health, infrastructure, etc Physical constraints –Thermodynamic, dynamic –Radiation, water vapour, surface conditions How can observations help? –Observing system; monitoring, variability

3. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle

4. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Surface Temperature (K) 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 & Ingram, 2002; Lambert & Webb (2008) GRL

5. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle 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

6. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Contrasting precipitation response expected Precipitation  Heavy rain follows moisture (~7%/K) Mean Precipitation linked to radiation balance (~3%/K) Light Precipitation (-?%/K) Temperature  e.g. see Held and Soden (2006) J. Clim

7. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle What do observations tell us? Response of Moisture and Clear-sky Longwave Radiation to warming appears robust in models and observations

8. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle …despite inaccurate mean state Left: Pierce et al. (2006) GRL; see also John and Soden (2006) GRL

9. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Does moisture rise at 7%/K? Specific humidity trend correlation (left) and time series (right) Willett et al. (2007) Nature Robust relationships globally. Less coherent relationships regionally/ over land (right)? See also Wang et al. (2008) GRL LandOcean Willett et al. (2008) J Clim

10. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Does extreme precipitation follow moisture & Clausius Clapeyron? - Intensification of heaviest rainfall with warming in obs/models Allan and Soden (2008) Science

11. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Large spread in response of heaviest precipitation to warming between models and satellite data. Changes in extreme vertical motion may be key…

12. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Changes in Extreme Precipitation Determined by changes in low-level water vapour and updraft velocity Above: O’Gorman & Schneider (2008) J Clim Aqua planet experiment shows extreme precipitation rises with surface q, a lower rate than column water vapour Right: Gastineau and Soden, in prep: Reduced frequency of upward motion offsets extreme precipitation increases.

13. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle How are Observed Mean Precipitation and Evaporation Changing? Yu and Weller (2007) BAMS (Wentz et al. 2007, Science) Observed responses appear larger than models and closer to Clausius Clapeyron

14. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Muted Evaporation changes in models are not explained simply by declining wind stress Richter and Xie (2008) JGR in press; also Lorenz et al. J Climate under review Simulated changes in Evaporation are sensitive to small changes in the boundary layer CCWindT s -T o RH o

15. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle IPCC 2007 WGI Do observations confirm contrasting precipitation responses in the wet and dry regions? : “the rich get richer” or the wet get wetter? Rainy season: wetter Dry season: drier Chou et al. (2007) GRL Precip trends, 0-30 o N

16. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Contrasting precipitation response in ascending and descending portions of the tropical circulation GPCP Models ascent descent Allan and Soden (2007) GRL Precipitation change (mm/day)

17. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle TRMM ASCENT ALL DESCENT Contrasting ascent/descent reponse robust Magnitude of responses sensitive to time period/dataset Viju John et al. in prep

18. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Could changes in aerosol be driving recent changes in the hydrological cycle? e.g. Wild et al. (2008) GRL Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim Mishchenko et al. (2007) Science

19. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Precipitation response depends upon forcing Andrews et al. J Climate in press:

20. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Conclusions Low level moisture responses robust: –Less clear over land. Inaccurate mean state? Precipitation extremes linked to moisture –Moisture response at lowest level? Changes in updraft velocity? –Differences between individual models and observations Mean and regional precipitation response: a tug of war –Slow rises in radiative cooling –Faster rises in low-level moisture –Wet get wetter and dry get drier –Who cares about drought/flooding over the ocean? Recent Precipitation Responses appear larger in observations than models. –Could aerosol be influencing decadal variability in the hydrological cycle? –Are observing systems up to monitoring changes in the water cycle? Understanding changes in near surface conditions may be important AGU Fall meeting: GC08 Dynamic and Thermodynamic Controls of the Global Water Cycle in the 20th and 21st Centuries, Thurs 18 th Dec Future NERC projects PREPARE project proposal

21. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Extra Slides

22. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle

23. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Vecchi and Soden (2006) Nature Evidence for weakening of Walker circulation in models and observations

24. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle How should precipitation respond to climate change? Allen and Ingram (2002) Nature

25. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Changes in precipitation: “the rich get richer”? precip trends 0-30 o N Rainy season: wetter Dry season: drier Chou et al. (2007) GRL

26. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Water vapour –Instrumental in determining amount of warming and changes in precipitation –Agreement: tropical ocean, UTH –Land? Moistening processes/cloud? Precipitation –How should it change –Extreme precip –Regional responses Evaporation Sensible Heat Aerosol

27. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle 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?

28. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Is moisture at higher levels constrained by Clausius Clapeyron? Soden et al. (2005) Science Moistening Trend in water vapour radiance channels: 1983-2004 Observations Model Constant RH model Constant water vapour model

29. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Soden et al. (2002) Science; Forster/Collins (2004) Clim Dyn; Harries/Futyan (2006) GRL What time-scales do different processes operate on?

30. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle 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

31. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Reduction in UTH with warming Lindzen (1990) BAMS Minschwaner et al. (2006) J Clim Mitchell et al. (1987) QJRMS

32. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Evaporation cannot explain moistening John and Soden (2006) GRL; Luo and Rossow (2004) 350 250 180 120 90 63 45 30 g m -3

33. © University of Reading 2008r.p.allan@reading.ac.uk Hadley Centre Workshop: Improving predictions of the large- scale hydrological cycle Interanual Variability: Response of water vapour & clear-sky LW radiation at the surface and TOA in models, reanalyses & observations Water vapour Surface clear LW Clear-sky OLR