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Observed and simulated changes in water vapour, precipitation and the clear-sky
longwave radiation budget of the surface and atmosphere
Richard P. Allan
Environmental Systems Science Centre, University of Reading, UK
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Earth’s energy balance
Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94
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Earth’s energy balance
Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94
Precip: +78 Wm-2
SW heating +67 Wm-2
LW cooling -169 Wm-2
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Increased moisture enhances atmospheric radiative cooling to surface
ERA40 NCEP
Allan (2006) JGR 111, D22105
SNLc = clear-sky surface net down longwave radiation
CWV = column integrated water vapour
dSNLc/dCWV ~ 1 ─ 1.5 W kg-1
dCWV (mm)
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Tropical ocean variability
SST
Water vapour
Clear net LW down at surface
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Increases in water vapour enhance clear-sky longwave radiative cooling of atmosphere to the surface
This is offset by enhanced absorption of shortwave radiation by water vapour
Changes in greenhouse gases, aerosol and cloud alter this relationship…
Tropical oceans
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Sensitivity test: tropical oceans
Clear-sky Longwave shortwave
TOA SFC ATM ATM
1K increase in tropospheric T, constant RH
Greenhouse gas changes from 1980 to 2000 assuming different rates of warming
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Increase in atmospheric LW cooling over tropical ocean descent ~4 ─ 5 Wm-2K-1
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
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• Increased moisture (~7%/K) increased convective
precipitation
• Increased radiative cooling smaller mean rise in
precipitation (~3%/K)
• Implies reduced precipitation away from convective regimes (less light rainfall?)
• Locally, mixed signal from the above
[email protected] © University of Reading 2007www.nerc-essc.ac.uk/~rpa
• Method: Analyse separately precipitation over the ascending and descending branches of the tropical circulation– Use reanalyses to sub-sample observed data– Employ widely used precipitation datasets– Compare with atmosphere-only and fully coupled
climate model simulations
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GPCP CMAP AMIP3
Tropical Precipitation Response
Allan and Soden, 2007, GRL
Model precipitation response smaller than the satellite observations (see also Wentz et al. 2007 Science; Chou et al. 2007 GRL, etc)
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Projected changes in Tropical Precipitation
Allan and Soden, 2007, GRL
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Could changes in aerosol and their indirect effect on cloud be driving changes in the tropical hydrological
cycle through the surface radiation budget?
Mishchenko et al. (2007) Science; Wild et al. (2005) Science
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Summary
• Global water and energy cycles coupled• Theoretical changes in clear-sky radiative
cooling of atmosphere implies “muted” precipitation response
• Models simulate muted response, observations show larger response
• Possible artifacts of data?• Possible mechanisms (aerosol, cloud)• Implications for climate change prediction
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Extra slides…
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Calculated trends
• Models understimate mean precipitation response by factor of ~2-3
• Models severely underestimate precip response in ascending and descending branches of tropical circulation
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Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1
OCEAN LAND
AMIP SSM/I GPCP CMAP
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Are the results sensitive to the reanalysis data?
• Changes in the reanalyses cannot explain the bulk of the trends in precipitation
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Observed increases in evaporation over ocean larger than climate model simulations
Yu and Weller (2007) BAMS
- increased surface humidity gradient (Clausius Clapeyron)
- little trend in wind stress changes over ocean (Yu and Weller, 2007; Wentz et al., 2007) although some evidence over land (Roderick et al. 2007 GRL)
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Links to precipitation