changes in water vapour, clear-sky radiative cooling and precipitation richard p. allan...
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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
How thehydrological cycle responds to aradiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather)
Climate Impacts
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
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)
Precipitation linked to clear-sky longwave radiative cooling of the atmosphere
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
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)
Tropical Oceans
dCWV/dTs ~2 ─ 4 mm K-1
dSNLc/dTs ~3 ─ 5 Wm-2K-1
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
AMIP3
CMIP3 non-volcanic
CMIP3 volcanic
Reanalyses/ Observations
Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1
• 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
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
Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1)
OCEAN LAND
AMIP SSM/I GPCP CMAP
Allan and Soden, 2007, GRL
Projected changes in Tropical Precipitation
Allan and Soden, 2007, GRL
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
Extra slides…
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)
Models reproduce observed increases in total column water
vapour
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
Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB
Calibration or sampling?
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
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
Implications for tropical precipitation (GPCP)?
ERA40 QLWc
GPCP P
OBS QLWc
Pinatubo?
Comparison of AMIP3 models, reanalyses and observations over the tropical coeans
Also considering coupled model experiments including greenhouse gas and natural forcings
Clear-sky vs resolution
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
Links to precipitation