Current Changes in Tropical Precipitation Richard P. Allan Department of Meteorology, University of...
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Transcript of Current Changes in Tropical Precipitation Richard P. Allan Department of Meteorology, University of...
Current Changes in Tropical Precipitation
Richard P. AllanDepartment 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 [email protected]
• 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. 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)
Physical basis: water vapour
1979-2002• Clausius-Clapeyron
– Low-level water vapour (~7%/K)– Intensification of rainfall O’Gorman &
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
Wat
er v
apou
r (m
m)
Temperature (K)
Pre
cip.
(%
)
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. 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
John et al. (2009)
models
Wat
er V
apou
r (m
m)
Contrasting precipitation response in wet and dry regions of the tropical circulation
Updated from Allan and Soden (2007) GRL
descent
ascentModelsObservations
Pre
cipi
tatio
n ch
ange
(%
)
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– 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
Models
DR
Y
WE
T
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 frequency Increased 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 (O’Gorman &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
END
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. O’Gorman 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)
Thermodynamic constraint
1979-2002• 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 -O’Gorman 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 cm2.1 2080-2099 minus 1980-1999 (% 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 (O’Gorman 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) 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)
“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 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?