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Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation
Richard P. Allan
Environmental Systems Science Centre, University of Reading, UK
Thanks to Brian Soden
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How thehydrological cycle responds to aradiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather)
Climate Impacts
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Changing character of precipitation
• Convective rainfall draws in moisture from surroundings
• Moisture is observed & predicted to increase with warming ~7%K-1
(e.g. Soden et al. 2005, Science)
• Thus convective rainfall also expected to increase at this rate (e.g. Trenberth et al. 2003 BAMS)
1979-2002
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Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q)
• Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 2002)
- Changes in P with warming estimated to be ~3%K-1
- Consistent with model estimates (~2%K-1)
Held and Soden (2006) J. Clim
∆P
(%
)
7 % K
-1
∆T (K)
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Precipitation linked to clear-sky longwave radiative cooling of the atmosphere
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Increased moisture enhances atmospheric radiative cooling to surface
ERA40 NCEP
Allan (2006) JGR 111, D22105
dSNLc/dCWV ~ 1 ─ 1.5 W kg-1
SNLc = clear-sky surface net down longwave radiation
CWV = column integrated water vapour
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Increase in clear-sky longwave radiative cooling to the surface
CMIP3
CMIP3 volcanic
NCEP ERA40
SSM/I-derived
~ +1 Wm-2 per decade
∆SNLc (Wm-2)
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Tropical Oceans
dCWV/dTs ~2 ─ 4 mm K-1
dSNLc/dTs ~3 ─ 5 Wm-2K-1
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
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AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1
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• Increased moisture (~7%/K) increased convective precipitation
• Increased radiative cooling smaller mean rise in precipitation (~3%/K)
• Implies reduced precipitation in subsidence regions (less light rainfall?)
• Locally, mixed signal from the above• Method: Analyse separately precipitation over
the ascending and descending branches of the tropical circulation
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GPCP CMAP
AMIP3
• Model precipitation response smaller than the satellite observations– see also Wentz et
al. (2007) Science
Tropical Precipitation Response
Allan and Soden, 2007, GRL
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Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1)
OCEAN LAND
AMIP SSM/I GPCP CMAP
Allan and Soden, 2007, GRL
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Projected changes in Tropical Precipitation
Allan and Soden, 2007, GRL
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Conclusions• Heavy rainfall and areas affected by drought expected to
increase with warming [IPCC 2007]• Heavy precipitation increases with moisture ~7%K-1
• Mean Precipitation constrained by radiative cooling– Models simulate increases in moisture (~7%K-1) and clear-sky LW
radiative cooling (3-5 Wm-2K-1)
• But large discrepancy between observed and simulated precipitation responses…– Model inadequacies or satellite calibration/algorithm problems?– Changes in evaporation and wind-speed over ocean at odds with
models? (Yu and Weller, 2007 BAMS; Wentz et al. 2007, Science; Roderick et al. 2007 GRL)
• Observing systems: capturing decadal variability problematic
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Extra slides…
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Outline• Clear-sky radiative cooling:
– radiative convective balance– atmospheric circulation
• Earth’s radiation budget– Understand clear-sky budget to
understand cloud radiative effect
• Method:– analyse relationship between water vapour,
clear-sky radiative cooling and precipitation– Satellite observations, reanalyses, climate
models (atmosphere-only/fully coupled)
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Models reproduce observed increases in total column water
vapour
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Tropical Oceans
1980 1985 1990 1995 2000 2005
Ts
CWV
LWc
SFC
ERA40
NCEP
SRB
HadISST
SMMR, SSM/I
Derived:SMMR, SSM/I, Prata)
Allan (2006) JGR 111, D22105
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Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB
Calibration or sampling?
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Tropical Oceans
Surface Net LWc
Clear-sky OLR
Clear-sky Atmos LW cooling
QLWc
ERBS, ScaRaB, CERES
Derived
ERA40
NCEP
SRB
HadISST
Allan (2006) JGR 111, D22105
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Linear least squares fit
• Tropical ocean: descending regime
• Dataset dQLWc/dTs Slope
• ERA-40 3.7±0.5 Wm-2K-1
• NCEP 4.2±0.3 Wm-2K-1
• SRB 3.6±0.5 Wm-2K-1
• OBS 4.6±0.5 Wm-2K-1
ERA40 NCEP
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Implications for tropical precipitation (GPCP)?
ERA40 QLWc
GPCP P
OBS QLWc
Pinatubo?
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Comparison of AMIP3 models, reanalyses and observations over the tropical coeans
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Also considering coupled model experiments including greenhouse gas and natural forcings
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Clear-sky vs resolution
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Sensitivity study
• Based on GERB- SEVIRI OLR and cloud products over ocean:
• dOLRc/dRes ~0.2 Wm-2km-0.5
• Suggest CERES should be biased low
by ~0.5 Wm-2 relative to ERBS
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Links to precipitation