The Atmospheric water cycle
How will it change in a warmer climate?
Lennart Bengtsson
ESSC
ISSI
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Global surface temperature 1950-2011
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Credit:NOAA
Surface and 500 hPa temperature trends
1958-2011
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No trend but rather a low frequency variation with time
What is happening to the hydrological cycle?
Global precipitation 1900-2011
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Credit:NOAA
Annual precipitation for Sweden 1860-2011
credit: SMHI
A minor increase is indicated, some 50-75 mm
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Key scientific questions
•  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
The following will be highlighted:
•  The global water cycle
•  The greenhouse effect of water vapour
•  The different response of water vapour and precipitation to
temperature change and its consequences
•  The water cycle in polar regions
•  Consequences for weather systems (Example: tropical cyclones)
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The global water cycle
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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
Precipitation over land is about 3 times as high
Water vapour is the dominating greenhouse gas. Removing the effect of water
vapour in long wave radiation reduces climate warming at 2 x CO2 by a factor
of more than 3. (For the GFDL model from 3.38 K to 1.05 K).
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GPS-measurements compared with operational analyses from ECMWF
July 2000: IWV [mm] for station Tahiti (Polynesia)
dF = axln q
Water vapour forcing
We represent temporal
and areal mean by red.
We now have the
approx. relation:
ln q - ln q = -1/2(q /q)2
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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-milennia
•  The residence time of H2O in
the atmosphere is 7-8 days
CO2
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H 2O
•  H2O, albeit a more powerful
greenhouse gas, is driven my
temperature that in turn is
forced by the slower
components of the climate
system.
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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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Semenov and
Bengtsson, 2002
Fractional change in T, q and P. Note that there is no
increase in GHG after 100 years
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The atmospheric water cycle
•  The atmospheric water cycle follows closely ClausiusClapeyrons (C-C) relation. (6-7%/1°C)
•  That means that also 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%/1°C) ISSI/Bern 6.2.12
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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
even decrease! ISSI/Bern 6.2.12
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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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Horizontal transport of moisture,F
•  After Held and Soden (2006)
•  Horizontal transport of moisture
from the IPCC scenario A1B
(solid)
•  Transport by the simple formula
(2)
scaled by CC (dashed)
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Effect on P-E
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How well can it be modelled?
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Changes in the hydrological cycle
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IPCC 4th
assessment , 2007
Climate change experiment using ECHAM5
• 
We have investigated two periods:
• 
• 
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
• 
• 
• 
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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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 (Observation)
ECHAM5 (T213) for the period 1959-1990 (Model calculation of present
value)
ECHAM5 (IPCC scenario A1B)
2069-2100 (Model calculation of the
future value)
Bengtsson et al., 2011
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Arctic(60°-90°N) atmospheric water cycle in km3
Parameter/Data
Precipitation
Evaporation
Net transport
into the region
across 60N
ERA-Interim reanalysis 1989-2009
17408
8073
9335*
ECHAM5 T213
1959 - 1990, 20 C
17263
7720
9543*
ECHAM5 T213
2069 - 2100, C 21
A1B scenario
21584
(+25%)
9301
(+ 20%)
12283*
(+ 29%)
*( ca. 5% has been
added due to
underestimation in
transp. calculation)
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Energy transport across 60° N in a warmer climate
•  Moist energy transport is increasing but dry static energy
transport is decreasing with increasing temperature.
•  Comparing 20C and 21 C we have for 60-90°N: • 
20C
21C
•  Lq
21.3
27.0
+5.8 W/m2
•  CpT + gz
64.9
59.2
-5.7 W/m2
•  Total
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86.1
86.2
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+0.1 W/m2
Mass balance change C21-­‐C20 GREENLAND - 519 km3/year
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A mechanism for Arctic warming
• 
Net LW radiation in summer is reduced mainly due to increased water vapour
in the lower troposphere. -25.8 to -18.0 W/m2
• 
Net SW radiation in summer is increasing due to reduced albedo (reduced sea
ice). +108 to +116.8 W/m2
• 
The combined effect is to increase net surface flux into the ocean. 21C - 20C
increase is 24 % ( 74.9 to 93.5 W/m2)
• 
During the autumn and winter the increased heat of the ocean is delaying
cooling and creation of sea ice. ISSI/Bern 6.2.12
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Energy transport across 60° S in a warmer climate
•  Moist energy transport is increasing but dry static energy
transport is decreasing with increasing temperature.
•  Comparing 20C and 21 C we have for 60-90°S: • 
20C
21C
•  Lq
28.0
34.0
+6.0 W/m2
•  CpT + gz
56.3
48.7
-7.6 W/m2
•  Total
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84.3
82.7
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-1.6 W/m2
Mass balance change C21-­‐C20 ANTARCTICA +289 km3/year
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Net surface radiation for Antarctic is essentially
unchanged
• 
• 
Net LW radiation cooling in summer is reduced mainly due to increased water
vapour in the lower troposphere. -48.5 to -43.2 W/m2
Net SW radiation in summer is in fact decreasing slightly .
+121.8 to +118.0 W/m2
• 
The combined effect is to increase net surface flux into the ocean. 21C - 20C
increase is 2 % ( 73.3 to 74.8 W/m2)
• 
In comparison to the Arctic region the total net surface radiation is only
increasing by 2%
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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
Total
Antarctica
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P Huybrechts
How does tropical cyclones change in a warmer
climate?
Bengtsson et al. 2007, Tellus
The role of the
water cycle
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Hurricane Katrina August 2005
ECMWF operational analyses, 850 hPa vorticity
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Hurricane Katrina Intensity at Landfall
29 Aug 2005 14 Z
4 km WRF, 62 h forecast
Mobile Radar
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Courtesy of P. Fox (NCAR)
Structure of modeled tropical cyclones
This shows the averaged structure of the 100 most
intense storms at the time when the reach their
maximum intensity
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Tangential (left) and Radial winds (right) for the T213
resolution. Negative values inflow. Average of 100 tropical
cyclones. Radius 5 degrees.
Observations:
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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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Tropical cyclones in different regions,
T213 resolution
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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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Characteristics of CO2 evolution in the given scenarios
Credit MPIM, Monika Esch
A recent
climate
change
experiment
with MPI
ECHAM6
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Tropical
cyclones at
2250-2300
Credit MPIM,
Monika Esch
Tropical cyclones number
and intensities (T63)
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Held and Soden, 2006
J. of Climate
•  Because the increase in strength of the global hydrological cycle is
constrained by the relatively small changes in radiative fluxes, it cannot
keep up with the rapid increase in lower tropospheric vapor. The
implication is that the exchange of mass between boundary layer and
the mid-troposphere must decrease, and, since much of this exchange
occurs in moist convection in the Tropics, the convective mass flux
must decrease. In many popular, and in some scientific, discussions of
global warming, it is implicitly assumed that the atmosphere will, in
some sense, become more energetic as it warms. By the fundamental
measure provided by the average vertical exchange of mass between
the boundary layer and the free troposphere, the atmospheric circulation
must, in fact, slow down. ISSI/Bern 6.2.12
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Model suggests that tropical cyclones intensify in a warmer
climate and the number of cyclones are reduced
•  That the tropical cyclones intensify in a warmer climate with higher
concentration of water vapour is intuitively straightforward.
•  But why do the number of cyclones diminish?
•  In a warmer climate the water vapour increases much faster than
precipitation that means that the residence time of water vapour in
the air increases. •  Another way to see this is that the atmosphere does not have to work
that hard to transport energy as more water vapour does the the
work more efficiently.
•  This reduces the intensity of the 3 dimensional large scale circulation
that is responsible for creating the conditions favourable for generating
tropical cyclones. ISSI/Bern 6.2.12
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Summary 1
•  The response of the global hydrological cycle to climate
warming is in many ways surprising and partly counterintuitive. •  The primary effect is to enhance the horizontal transport of
water vapour leading to more precipitation in wet areas
and reduced precipitation in dry areas.
•  The polar regions are expected to get more moisture and
precipitation and this is likely to play a role in changing
the climate of the polar regions but in different ways for
Arctic and Antarctic ISSI/Bern 6.2.12
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Summary 2
•  Tropical cyclones are expected to be affected in an
unexpected way.
•  Tropical cyclones are expected to be more intense but at
the same time less frequent.
•  The reason is the slowing down of the tropical circulation
that affects the large scale onset conditions for tropical
cyclones.
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END
Question time!
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