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Current Changes in the Tropical Precipitation and Energy Richard P. Allan Department of Meteorology, University of Reading Thanks to Brian Soden, Viju.

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Presentation on theme: "Current Changes in the Tropical Precipitation and Energy Richard P. Allan Department of Meteorology, University of Reading Thanks to Brian Soden, Viju."— Presentation transcript:

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

2 Sea Fishing 101 Course Convener

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

4 NCAS-Climate Talk 15 th January 2010 Trenberth et al. (2009) BAMS Physical basis: energy balance

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

6 Trends in clear-sky radiation in coupled models Clear-sky shortwave absorptionSurface net clear-sky longwave 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 NCAS-Climate Talk 15 th January 2010 CCWindT s -T o RH o Muted Evaporation changes in models are explained by small changes in Boundary Layer: 1) declining wind stress 2) reduced surface temperature lapse rate (T s -T o ) 3) increased surface relative humidity (RH o ) Richter and Xie (2008) JGR Evaporation

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

10 Current changes in tropical ocean column water vapour …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 John et al. (2009) models Water Vapour (mm)

11 Thermodynamic constraint 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 -OGorman 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) Allan et al. (2010) Environ. Res. Lett. Reduced frequencyIncreased frequency

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 OGorman and Schneider, 2009, PNAS; Gastineau & Soden 2009).

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

15 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 P~Mq Circulation response

16 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.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 Updated from Allan and Soden (2007) GRL descent ascent ModelsObservations Precipitation change (%) Sensitivity to reanalysis dataset used to define wet/dry regions

18 Is the contrasting wet/dry response robust? Large uncertainty in magnitude of change: satellite datasets and models & time period TRMM GPCP Ascent Region Precipitation (mm/day) John et al. (2009) GRL 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 – GPCP record may be suspect for dry region –SSM/I dry region record: inhomogeneity 2000/01? GPCP trends –Wet: 1.8%/decade –Dry: -2.6%/decade –Upper range of model trend magnitudes Models DRY WET

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 cm minus (% precipitation) Bottom: GFDL- GPCP precipitation (%)

24 Precipitation Current changes in precipitation for Europe-Atlantic region

25 Could changes in aerosol be imposing direct and indirect 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

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 Courtesy of Jim Haywood Met Office NAME model NOAA17 satellite image 20 March :06

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

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