Tropospheric Water Vapour and the Hydrological Cycle

1
Tropospheric Water Vapour and the Hydrological
Cycle
Richard Allan University of Reading, UK
2
Tony Slingo

• 1950-2008

3
How does water vapour impact climate change?

• Amount of warming
• Changes in water cycle

4
Spectral cooling rate (H2O, CO2, O3) Clough
Iacono (1995) JGR
K d-1 (cm-1)-1
MLS
5
Climate sensitivity and water vapour feedback
6
Climate sensitivity and water vapour feedback
Kernals Soden et al. (2008)
0.2 Wm-2-1
7
Climate sensitivity and water vapour feedback
0.2 Wm-2-1
7K-1
8
Climate sensitivity and water vapour feedback
0.2 Wm-2-1
7K-1
?BB -4sT3 -3.2 Wm-2K-1 ?WV(0.2)(7)1.4
Wm-2K-1
?WV?BB1.4 - 3.2 -1.8 Wm-2K-1
9
How does moisture respond to warming?

• 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?

10
Does moisture rise at 7/K over land?
Specific humidity trend correlation (left) and
time series (right)
Land Ocean
Willett et al. (2007) Nature Willet et al.
(2008) J Clim
But some contradictory results (e.g., Wang et al.
(2008) GRL)
11
Is moisture at higher levels constrained by
Clausius Clapeyron?
Trend in water vapour radiance channels 1983-2004
Observations
Model
Moistening
Constant RH model
Constant water vapour model
Soden et al. (2005) Science
12
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
13
What time-scales do different processes operate
on?
Soden et al. (2002) Science Forster/Collins
(2004) Clim Dyn Harries/Futyan (2006) GRL
14
Trends in UTH (above)Sensitivity of OLR to UTH
(right)
Bates and Jackson (2001) GRL
15
Reduction in UTH with warming
Minschwaner et al. (2006) J Clim
Lindzen (1990) BAMS
Mitchell et al. (1987) QJRMS
16
Moistening processes diurnal cycle (SEVIRI)
Convergence
Divergence
Sohn et al.(2008)JGR
Evaporation
Condensation
Drying
Moistening
Evaporation
Divergence
UTH tendency
See also Soden et al. (2004) GRL
17
Evaporation cannot explain moistening
g m-3
350 250 180 120 90 63 45 30
John and Soden (2006) GRL Luo and Rossow (2004)
18
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

19
Spread in cloud feedback in models appears to
relate to tropical low altitude clouds
IPCC (2007), after Sandrine Bony and colleagues
20
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
21
How should precipitation respond to climate
change?
Allen and Ingram (2002) Nature
22
(No Transcript)
23
Models simulate robust response of clear-sky
radiation to warming (2 Wm-2K-1) and a resulting
increase in precipitation to balance (2K-1)
e.g., Allen Ingram, 2002 Lambert Webb (2008)
GRL
Surface Temperature (K)
24

• But moisture observed predicted to increase at
  greater rate 7K-1
• Thus convective rainfall expected to increase at
  a faster rate than mean precipitation (e.g.
  Trenberth et al. 2003 BAMS)

1979-2002
25
Intensification of heaviest rainfall with warming
Allan and Soden (2008) Science
26
Contrasting precipitation response expected
Heavy rain follows moisture (7/K)
Mean Precipitation linked to radiation balance
(3/K)
Precipitation ?
Light Precipitation (-?/K)
Temperature ?
e.g. see Held and Soden (2006) J. Clim
27
Mean projected precipitation changes
IPCC 2007 WGI
28
Contrasting precipitation response in ascending
and descending portions of the tropical
circulation
ascent
Precipitation change (mm/day)
descent
GPCP
Models
Allan and Soden (2007) GRL
29

• 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
30
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?