Current Changes in Tropical Precipitation 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

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

Trenberth et al. (2009) BAMS Physical basis: energy balance Radiative Convective balance –Enhanced radiative cooling (Allen and Ingram, 2002) –Water vapour key: conserve RH, MALR (Stephens et al 94) –Modulated by GHG/aerosol forcings: fast and slow responses (Andrews et al JGR; Dong et al J Clim) –High cloud feedback (Lebsock et al J Clim) Temperature-dependent responses (2-3%/K) CO 2 increases mute the transient response somewhat (Andrews et al ERL; Wu et al GRL)

Physical basis: water vapour Clausius-Clapeyron –Low-level water vapour (~7%/K) –Intensification of rainfall OGorman & Schneider, 2009 PNAS –Moisture transport: enhanced P-E Held & Soden 2006 J Clim Radiative/thermodynamic constraints: –Wet get wetter, dry regions gets drier –Reduced mass flux: declining Walker circulation (Vecchi and Soden, 2006) P~Mq Water vapour (mm) Temperature (K)

Precip. (%) Allan and Soden (2008) Science Observed increase in water vapour and precipitation with warming

Tropical ocean column water vapour constrained by Clausius Clapeyron …despite inaccurate mean state, Pierce et al.; John and Soden (both GRL, 2006) …decreases in RH over land? (Simonds et al JGR; see also Joshi et al. 08) Reanalyses struggle to capture decadal changes in the water cycle. - see also Trenberth et al. (2005) Clim. Dyn., Soden et al. (2005) Science John et al. (2009) models Water Vapour (mm)

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

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?

Precipitation changes in wettest 30% and driest 70% of grid boxes 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

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

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 constrained by moist adiabatic lapse rate; responses highly sensitive to model-specific changes in upward velocities (OGorman &Schneider, 2009, PNAS; Gastineau & Soden 2009; Turner and Slingo, 2009 ASL).

Observations and simple physics help to confirm robust model projections –Increased precipitation (~2%/K) –Increased precipitation intensity (~7%/K) –Extratropics and wet regions of tropics get wetter –Dry regions of sub-tropics get drier –Transient response to GHG stabilisation Outstanding Issues –Inaccurate simulation of precipitation events –Limitations of satellite and gauge data –Detecting and attributing signals –Cloud Feedback –Aerosol Implications

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Add in more about obs. Add links to: –Lebsock et al. (2010) J Clim [rad/conv balance and feedback from obs]; Previdi (2010) ERL –Wu et al (2010) GRL; Andrews et al. (2010) ERL – transient changes in models –Enhanced moisture transport (dry to wet regions of tropics; tropics to extra-tropics) Held and Soden (2006) and implications for wet/dry region precipitation (Chou et al GRL) –Clausius Clapeyron contraint on intense precipitation (e.g. OGorman and Schneider 2009 PNAS; Allan and Soden 2008 Science; Lenderink and van Mijgaard 2010 ERL) –Changes in surface evaporation (Richter and Xie 2008 JGR); declining pan evaporation trends and wind speed (Roderick et al GRL) –Large-scale tropical circulation weakening (Vecchi and Soden, 2007 Nature) and variability (Park and Sohn, 2010 JGR in press)

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

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

What do we expect? Surface and atmosphere energy balance constraint (Allen and Ingram, 2002; Stephens and Ellis, 2008; Lambert and Webb, 2008, Andrews et al. 2009) Moisture transport constraint (Held and Soden, 2006) Moisture convergence constraint (OGorman and Schneider, 2009; Lenderink and Van Meijgaard, 2008) Mass flux and moist adiabat arguments (e.g. Vecchi and Soden, 2007; Held and Soden, 2006) Negative impact of greenhouse gases on transient precipitation response (Andrews et al. 2009; Wu et al. 2010) See also special focus issue of Environmental Research Letters (No. 2, April-June 2010)

Observations Daily estimates of column water vapour and precipitation from microwave retrievals (SSM/I) (e.g. Wentz et al Science) and TRMM (1998-present) microwave and radar Blended precipitation from infra-red and microwave radiance and rain gauge, Global Precipitation Climatology Project (GPCP; Huffman et al. (2009) GRL: monthly ( ) and daily ( ) CERES Earth Radiation Budget measurements ( ); ISCCP-based estimates ( )

Muted precip response achieved through reduced Walker circulation (mass flux) Enhanced moisture flux Enhanced rainfall intensity: MALR scaling Enhanced tropical to extra-tropical moisture flux Enhanced moisture flux from dry to wet regions of tropics Global precipitation changes constrained by radiative cooling Muted evaporation through subtle changes in BL Reduced rainfall intensity and/or frequency

Can we observe changes in atmospheric radiative heating/cooling? Changes in atmospheric longwave radiative radiative cooling (Wm -2 ) models John et al. (2009) GRL

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