Download The atmospheric water cycle How will it change in a

The atmospheric water cycle
How will it change in a warmer climate? –
Lennart Bengtsson
University of Reading
International Space Science Institute, Bern
Uppsala University
Seoul 30.5.2012
Lennart Bengtsson Global Water cycle
and Climate
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The global temperature depends on the state of the
climate system but a warming trend stands out.
(source :WMO)
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and Climate
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What is happening to the hydrological cycle?
The global precipitation during100 years
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and Climate
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Annual precipitation for Sweden
1860-2011
credit: SMHI
A minor increase is indicated, some 50-75 mm
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The atmospheric water cycle
- Some key questions • 
• 
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Why is the global precipitation not increasing in spite of a warming
temperature trend?
Is this an observational problem or is it because global precipitation is
not directly driven by temperature?
There are indications from some regions that precipitation is indeed
increasing. Is this an artefact because of unreliable reports or can it be
correct?
What is happening to weather systems such as tropical cyclones that
are driven by release of latent heat? Are they likely to intensify and will
they increase in number?
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The Atmospheric water cycle
- content of my talk •  The greenhouse effect on water vapour and
precipitation
•  The water cycle in polar regions
•  Tropical cyclones
•  Summary and conclusions
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The role of the water cycle in the climate
system
Precipitation is crucial for life on the planet
The largest warming factor of the atmosphere is through the release of latent
heat amounting to 80-90 Wm-2
The net transport of water from ocean to the land surfaces amounts to some
40000 km3/year
Water is re-circulating over land increasing land precipitation some 3 times
Water vapour is the dominant greenhouse gas. Water vapour provides a
positive feedback and enhances significantly the warming from the
anthropogenic greenhouse gases
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CO2 is a genuine forcing while H2O is a
part of the climate response system
•  The residence time of CO2 in
the atmosphere is from years
to multi-millennia
•  The residence time of H2O in
the atmosphere is 7-8 days
CO2
Seoul 30.5.2012
H2O
•  H2O, albeit a more powerful
greenhouse gas, is driven by
temperature that in turn is
forced by the slower
components of the climate
system.
Lennart Bengtsson Global
Water cycle and Climate
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Clausius-Clapeyron relation
Relation between temperature,T and
saturated water vapor, es
Atmospheric temperature determines water vapour
following the C-C relation
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Water vapour and temperature
For a temperature change, dT , the humidity change, dq, follows the
C-C relation seen as a conservation of relative humidity
dT + 0.4°C
Observations and
model
calculations from
observed SST
1979-2005
dq + 3%
Held and
Soden, 2006
dT+ 4°C
dq + 35%
Model 1860-2100
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Water cycle and Climate
Semenov and
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Bengtsson, 2002
The atmospheric water cycle
•  The atmospheric water cycle follows closely
Clausius-Clapeyrons (C-C) relation. (6-7%/ K)
•  It also follows that transport of water vapour scales
with the C-C relation.
•  That means more precipitation in areas of
convergence
•  The global precipitation increases much slower than
global water vapor. (1-2%/ K)
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Why is water vapour increasing faster
than precipitation in global mean and
what is the reason?
•  Water vapour is controlled by the 3-dimensional
atmospheric circulation.
•  Precipitation = Evaporation is determined by the
surface energy balance.
•  While water vapour always will increase in a warmer
climate, global precipitation can under certain
conditions, such as enhanced aerosol load, even
decrease!
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What is happening to the regional water
cycle?
•  Transport of water vapour is scaled by the CC relation
•  That means increased convergence and divergence of water
vapour
•  A crucial consequence is less precipitation in areas of
divergence and more in areas of convergence.
•  Increased transport of water vapour (latent heat) implies a
reduced transport of dry energy (weakening of the large scale
circulation)
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and Climate
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IPCC 4th assessment ,
Changes in the hydrological cycle 2007
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What might happen in a warmer climate?
A modeling approach
We have used the AIB scenario
And the coupled MPI model at T63 resolution
used in the IPCC 4th assessment
Higher resolution experiments use the
transient SST from T63 (time - window)
We study C20 (1961- 1990)
C21 ( 2071-2100)
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The Cryosphere
( picture by Frederic Edwin Church)
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Transport of water vapour across 60°N
Annual mean calculated for
every
6 hrs. T213 resolution ( ca 50
km)
ERA-Interim re-analysis
1989-2009
ECHAM5 (T213) for the period
1959-1990
ECHAM5 (IPCC scenario A1B)
2069-2100
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Water cycle and Climate
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Changed hydro production
in Sweden in a warmer
climate.
Credit: SMHI (S. Bergström)
Increased precipitation.
More generation during
winter due to more rain
Estimated increase by
5-20 TWh/year at the
end of 21st century
More energy available
when the need is largest
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and Climate
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Mass balance change C21-­‐C20 GREENLAND - 519 km3/year
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Change in sea level from SMB only. Contribution from Greenland
(red), from Antarctica (blue).Total contribution (black). ECHAM5
model, IPCC Scenario A1B, MPI, Hamburg
Greenland
Does not include
changes in calving
glaciers
Total
Antarctica
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What might happen to tropical cyclones?
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Hurricane Katrina Intensity at Landfall
29 Aug 2005 14 Z
4 km WRF, 62 h forecast
Mobile Radar
IVA, 6 oktober 2010
Klimatmodellering
Lennart Bengtsson
Courtesy of P. Fox (NCAR)
Hurricane Katrina August 2005
ECMWF operational analyses, 850 hPa vorticity
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Super Typhoon 21 (1991) in ERA-40 (left)
and a selected simulated typhoon in ECHAM5 (right)
Intensity (vorticity at 850 hPa)
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Tangential (left) and Radial winds (right) for the T319
resolution. Negative values inflow. Average of 100
tropical cyclones. Radius 5 degrees. Unit m/s
The flow is predominantly inward to the rear and
left of the storm and outward to the front and right
(Frank 1977 MWR)
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Number of tropical cyclones simulated
C19 (black), C20 (red) and C21(blue)
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Number of TCs/year (T213) for C 20 and
C21 for wind speed and vorticity
All (6, 6, 4)
>2x10-4 s-1
>5x10-4 s-1
>1x10-3 s-1
20C (1961-1990)
104
97
40
6.0
21C (2071-2100)
94
90
49
9.8
T213
>18ms-1
>33ms-1
>50ms-1
20C (1961-1990)
100
33
3.7
21C (2071-2100)
92
36
4.9
T213
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Conclusions from the T319 resolution
(40 km)
•  Results confirm the finding from the T213 experiment
•  The number of tropical cylones are reduced by some 10% at
the end of the 21st century
•  The stronger storms are becoming more intense
(Cyclones with wind speeds >50m/s increase from 12 to 17 per
year compared to the end of the 20th century)
Intensification occurs in all tropical storm regions
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Why do we see a reduced number of
tropical cyclones in a warmer climate?
•  We suggest the following mechanism
•  There is a reduction in the large-scale tropical
circulation at climate warming due to increased static
stability and a rapid increase in the amount of water
vapor in the atmosphere.
•  This generally leads to less favorable conditions to
generate organized onset vortices which are seen as
key atmospheric conditions for the generation of
tropical storms.
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Climate
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Why do we see an intensification of the
tropical storms in a warmer climate?
•  We suggest the following process
•  Due to chance favorable conditions are likely to evolve
•  In such situations the mechanisms proposed by Emanuel may
be applied, that means that the maximum wind speed is
proportional to the sqr. of the available latent energy provided
to the storm
•  To demonstrate this with a GCM requires sufficient resolution
( say 50-100 km or less)
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END
Thanks for your attention!
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and Climate
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Result from present study:
More intense hurricanes
• 
Given favorable atmospheric condition we suggest the ideas put forward by
Emanuel and Holland comparing a hurricane with a Carnot cycle can be
applied.
• 
This will provide an energy input broadly proportional to the specific humidity at
a higher temperature (following SST)
• 
The intensification of the tropical cyclones depends on sufficient model
resolution to accurately describe the convergence of momentum which generate
the very high wind speeds at the core of the vortex.
• 
For TCs reaching 33ms-1 PDI (power dissipation index) increases by 16%
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Climate change experiment using
ECHAM5 (MPI, Hamburg)
• 
We have investigated two periods:
• 
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20 C: 1959-1990 using observed/estimated greenhouse gases and aerosols
21 C: 2069-2100 using scenario A1B
• 
A1B is a middle-of-the-line scenario
• 
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Carbon emission peaking in the 2050s (16 Gt/year)
CO2 reaching 450 ppm. in 2030
CO2 reaching 700 ppm. in 2100
• 
SO2 peaking in 2020 then coming done to 20% thereof in 2100
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Mass balance change C21-­‐C20 ANTARCTICA +289 km3/year
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Tropical cyclones in different regions, T319
resolution
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Number of tropical cyclones
Gualdi et al (2008)
30-years
PREIND
2CO2
4CO2
Total
2196
1839
1229
Mean
73.2
61.3
41.0
STD
6.8
8.3
7.6
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Result from present study:
Reduced number of hurricanes
• 
There is s reduction in the number of tropical cyclones in agreement
with most previous studies.
• 
We suggest this is due to a weakening of the tropical circulation. This
can best be seen as a slowing down in the circulation of water vapor by
16%
• 
The radiative cooling of the tropics increases due to more water vapor
in the upper troposphere and dynamical cooling due to increased static
stability. This processes can compensate warming from release of
latent heat (6%) with a less active tropical circulation.
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