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© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Tropospheric Water Vapour and the Hydrological Cycle
Richard Allan
University of Reading, UK
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Tony Slingo1950-2008
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
How does water vapour impact climate change?
– Amount of warming– Changes in water cycle
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Spectral cooling rate (H2O, CO2, O3) Clough & Iacono (1995) JGR
K d-1 (cm-1)-1
MLS
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Climate sensitivity and water vapour feedback = ─
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Climate sensitivity and water vapour feedback
0.2 Wm-2%-1
= ─Kernals: Soden et al. (2008)
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Climate sensitivity and water vapour feedback
0.2 Wm-2%-1 7%K-1
= ─
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Climate sensitivity and water vapour feedback
0.2 Wm-2%-1 7%K-1
λBB~ -4σT3 ~ -3.2 Wm-2K-1
λWV~(0.2)(7)=1.4 Wm-2K-1
= ─
λWV+λBB~1.4 - 3.2 = -1.8 Wm-2K-1
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
• Low level moisture over the ocean seems to behave…
• Does moisture really vary with temperature at ~ 7%/K?
• Do models capture the essential relationships?
- Land, upper troposphere?
- Reanalyses, surface measurements or satellite data?
How does moisture respond to warming?
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Does moisture rise at 7%/K over land?
Specific humidity trend correlation (left) and time series (right)
Willett et al. (2007) Nature; Willet et al. (2008) J Clim
But some contradictory results (e.g., Wang et al. (2008) GRL)
Land Ocean
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Is moisture at higher levels constrained by Clausius Clapeyron?
Soden et al. (2005) Science
Moi
sten
ing
Trend in water vapour radiance channels: 1983-2004
Observations
Model
Constant RH model
Constant water vapour model
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Is the mean state important?• Models appear to
overestimate water vapour– Pierce et al. (2006) GRL;
John and Soden (2006) GRL– But not for microwave data?
[Brogniez and Pierrehumbert (2007) GRL]
• This does not appear to affect feedback strength– Held and Soden (2006),
John and Soden (2006)• What about the
hydrological cycle?– Inaccurate mean state?
Pierce et al. (2006)
GRL
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Soden et al. (2002) Science; Forster/Collins (2004) Clim Dyn; Harries/Futyan (2006) GRL
What time-scales do different processes operate on?
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Bates and Jackson (2001) GRL
Trends in UTH (above)
Sensitivity of OLR to UTH (right)
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Reduction in UTH with warming
Lindzen (1990) BAMS
Minschwaner et al. (2006) J Clim
Mitchell et al. (1987) QJRMS
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Moistening processes: diurnal cycle (SEVIRI)
Condensation Evaporation
Convergence Divergence
DryingMoistening
Evaporation
UTH tendency
Divergence
Sohn et al.(2008)JGR
See also Soden et al. (2004) GRL
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Evaporation cannot explain moistening
John and Soden (2006) GRL; Luo and Rossow (2004)
350
250
180
120
90
63
45
30
g m-3
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Cloud feedback: a more complex problem
Non-trivial relationship between cloud and temperature
Response of cloud to warming is highly uncertain
• Depends on:– Type of cloud– Height of cloud– Time of day/year– Surface characteristics
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Spread in cloud feedback in models appears to relate to tropical low altitude clouds
IPCC (2007), after Sandrine Bony and colleagues
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Is cloud feedback an indirect forcing?
• • Clouds respond to – direct forcing from CO2
– Climate response to ∆SST
• • Does cloud feedback uncertainty stem from direct response rather than climate feedback response?
Andrews and Forster (2008) GRL (above); Gregory and Webb (2008) J Clim
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
How should precipitation respond to climate change?
Allen and Ingram (2002) Nature
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Surface Temperature (K)
Models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) and a resulting increase in precipitation to balance (~2%K-1) e.g., Allen & Ingram, 2002; Lambert & Webb (2008) GRL
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
• But moisture observed & predicted to increase at greater rate ~7%K-1
• Thus convective rainfall expected to increase at a faster rate than mean precipitation (e.g. Trenberth et al. 2003 BAMS)
1979-2002
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Intensification of heaviest rainfall with warming
Allan and Soden (2008) Science
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Contrasting precipitation response expectedP
reci
pita
tion
Heavy rain follows moisture (~7%/K)
Mean Precipitation linked to
radiation balance (~3%/K)
Light Precipitation (-?%/K)
Temperature e.g. see Held and Soden (2006) J. Clim
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
IPCC 2007 WGI
Mean projected precipitation changes
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Contrasting precipitation response in ascending and descending portions of the tropical circulation
GPCP Models
ascent
descent
Allan and Soden (2007) GRL
Pre
cipi
tatio
n ch
ange
(m
m/d
ay)
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Could changes in aerosol be driving recent changes in the hydrological cycle?
Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim
© University of Reading 2008 r.p.allan@reading.ac.ukChapman Conference on
Atmospheric Water Vapor and Its Role in Climate
Unanswered questions• How does UTH really respond to warming?• Do we understand the upper tropospheric moistening processes?• Is moisture really constrained by Clausius Clapeyron over land?• What time-scales do feedbacks operate on?• Apparent discrepancy between observed and simulated changes
in precipitation– Is the satellite data at fault?
– Are aerosol changes short-circuiting the hydrological cycle?
– Could model physics/resolution be inadequate?
• Could subtle changes in the boundary layer be coupled with decadal swings in the hydrological cycle?
• How do clouds respond to forcing and feedback including changes in water vapour?