Current Changes in Tropical Precipitation

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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
• http//www.met.reading.ac.uk/sgs02rpa
  r.p.allan_at_reading.ac.uk

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Climate model projections (IPCC 2007)

• Increased Precipitation
• More Intense Rainfall
• More droughts
• Wet regions get wetter, dry regions get drier?
• Regional projections??

Precipitation Intensity
Dry Days
Precipitation Change ()
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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. 2009 JGR Dong et al.
  2009 J Clim)
• High cloud feedback
• (Lebsock et al. 2010 J Clim)

Temperature-dependent responses (2-3/K) CO2
increases mute the transient response somewhat
(Andrews et al. 2010 ERL Wu et al. 2010 GRL)
Trenberth et al. (2009) BAMS
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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)

1979-2002
Water vapour (mm)
Temperature (K)
PMq
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Observed increase in water vapour and
precipitation with warming
Precip. ()
Allan and Soden (2008) Science
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Tropical ocean column water vapour constrained by
Clausius Clapeyron
John et al. (2009)
Water Vapour (mm)
models
despite inaccurate mean state, Pierce et al.
John and Soden (both GRL, 2006) decreases in RH
over land? (Simonds et al. 2009 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
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Contrasting precipitation response in wet and dry
regions of the tropical circulation
ascent
Models
Observations
Precipitation change ()
descent
Sensitivity to reanalysis dataset used to define
wet/dry regions
Updated from Allan and Soden (2007) GRL
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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?

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Precipitation changes in wettest 30 and driest
70 of grid boxes

• 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
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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.
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• 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).
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Implications

• 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

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END
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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. 2007 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. 2007 GRL)
• Large-scale tropical circulation weakening
  (Vecchi and Soden, 2007 Nature) and variability
  (Park and Sohn, 2010 JGR in press)

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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

1979-2002
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Top GFDL cm2.1 2080-2099 minus 1980-1999 (
precipitation) Bottom GFDL-GPCP precipitation
()
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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)

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Observations

• Daily estimates of column water vapour and
  precipitation from microwave retrievals (SSM/I)
  1987-2009 (e.g. Wentz et al. 2007 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 (1979-2009) and daily
  (1997-2009)
• CERES Earth Radiation Budget measurements
  (2000-2006) ISCCP-based estimates (1983-2006)

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Global precipitation changes constrained by
radiative cooling
Muted precip response achieved through reduced
Walker circulation (mass flux)
Enhanced tropical to extra-tropical moisture flux
Enhanced moisture flux from dry to wet regions of
tropics
Enhanced moisture flux
Enhanced tropical to extra-tropical moisture flux
Muted evaporation through subtle changes in BL
Reduced rainfall intensity and/or frequency
Enhanced rainfall intensity MALR scaling
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Can we observe changes in atmospheric radiative
heating/cooling?
Changes in atmospheric longwave radiative
radiative cooling (Wm-2)
models
John et al. (2009) GRL
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Avoid reanalyses in defining wet/dry regions

• Sample grid boxes
• 30 wettest
• 70 driest
• Do wet/dry trends remain?