Transcript Clouds and Climate

Clouds and Climate: Cloud
Response to Climate Change
SOEEI3410
Ken Carslaw
Lecture 5 of a series of 5 on clouds and climate
• Properties and distribution of clouds
• Cloud microphysics and precipitation
• Clouds and radiation
• Clouds and climate: forced changes to clouds
• Clouds and climate: cloud response to climate
change
Content of this Lecture
• The importance of cloud feedbacks: Climate
sensitivity
• Cloud radiative forcing
• Factors affecting clouds
• Cloud feedback in climate models
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Reading
• Section 7.2.2 Cloud Processes and Feedbacks of IPCC 2001
– http://www.grida.no/climate/ipcc_tar/wg1/271.htm
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Climate Sensitivity
• Climate
sensitivity
determines
the global
temperature
when a
radiative
forcing is
applied
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Climate Sensitivity
DT 
Q
l
• DT = change in global mean temperature
• Q = radiative forcing (W m-2)
• l = climate sensitivity (W m-2 K-1)
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Sensitivity of Climate Models
NCAR
2xCO2 Sensitivity (K)
GFDL
Summer 2002
Sensitivity to forcing from doubled CO2 (~4 Wm-2)
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Cloud Changes
and Climate
Sensitivity
1/l=4.2 K Wm-2
% Change in low cloud amount
per 1K temperature change
1/l=1.8 K Wm-2
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Change in “Cloud Radiative
Forcing”
• Clouds cause a net cooling effect on climate (net
-20 Wm-2 forcing (equivalent to about 8*CO2)
– All models agree on sign (+/-) of CRF
• Cloud feedback is about how CRF changes as
greenhouse gases increase
– Models disagree greatly on this
• Some clouds warm, some cool. DT depends on
which clouds change
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Humidity and Temperature
• Increased T
• Increased water
vapour in atmosphere
• Increased
cloudiness?
Overall increase in atmospheric
water vapour and temperature
Overall increase in atmospheric
water vapour
100% RH
• NO
• Relative humidity is
the relevant quantity
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Cloud Radiative Forcing (CRF)
• Factors that determine CRF (or, what does a
climate model need to get right?)
–
–
–
–
Cloud location (solar intensity)
Depth/thickness
Coverage
Drop/ice concentrations
How would SW and LW
impact on climate change
for these two cloud field?
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CRF, dependence on location,
thickness and height
40
40
Winter 5o N
cloud height
0
high
med
low
-20
high
med
low
20
DTs (K)
DTs (K)
20
Winter 65o N
0
-20
-40
-40
0
50
100
150
liquid water path (g m-2)
0
50
100
150
liquid water path (g m-2)
Equilibrium surface temperature change due to presence of different clouds
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Reasons for Cloud Changes
• Large-scale dynamics/circulation
– Global circulation changes in response to changes in ocean
circulation, changes in ocean-atmosphere T contrast, etc
• Thermodynamic/cloud-scale changes
Changes to:
– vertical T profile,
– atmospheric stability,
– turbulence structure of boundary layer,
– water substance transport
– aerosol
• Very difficult to separate in observations
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Circulation/Dynamical Changes
Hadley/Walker
circulation
Tropical
convection
Equator
Tradewind
cumulus
Sub-tropical St/Sc
• Cloud
fields are
determined
by largescale
circulation
• Non-local
response
• El Nino
30oN
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Observed Clouds With Temperature
• Observations from the
International Satellite
Cloud Climatology
Project
d log(optical depth)/dT
Ocean low clouds
0.1
0
• Clouds become
optically thinner (less
reflective) at higher
temperatures
0.05
-0.1
-0.15
-60 -40 -20
• +ve or –ve feedback?
0 20 40
latitude
60
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Observed cloud with temperature:
Tropical Cirrus
Richard Lindzen, MIT
Cloud Amount
0.2
0.15
slope = 10-20%
change per 1 K SST
observations
0.1
0.05
0
25
26 27
28
29
30
sea surface temperature (K)
• Japan’s Geostationary
Meteorological Satellite
• 11 and 12 mm wavelength
radiometer
• 130oE-170oW, 30oS-30oN
(Pacific)
• 260 K brightness
temperature product is a
measure of “high thin
cloud” – cirrus
• Cirrus cover decreases
with increasing SST
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The Adaptive Infrared Iris as a
Climate Change Regulator
more IR to space
less cirrus
less water vapour
less water
transport
more rain
warm ocean
cold ocean
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Problems With the Infrared Iris Idea
• A hotly debated climate feedback
• See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html
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Net Cloud Feedbacks in GCMs
3
Change in CRF (W m-2)
2
Doubled CO2 experiments
WARMING
SW
1
0
LW
-1
-2
net
COOLING
-3
Different models
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Difficulties
• Different types of clouds have different effects and may
change in different ways – many separate problems
• Some aspects of clouds (thickness, ice content) are
difficult to observe
• Sub-grid scale problems
• Effects of temperature and circulation can be confused
• Changes observed on short time scales (e.g., El Niño)
may not always be good indicators of climate changeinduced changes
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Questions for this lecture
• On slide 6, what could explain the wide range of climate
model sensitivities to doubling of CO2?
• Based on slide 6, what would happen to our climate if
the coverage of high thin cirrus clouds increased (a) at
the equator, (b) 65oN? What explains the difference?
• On slide 14, explain whether the data indicate a positive
or negative climate feedback.
• For the first model shown on slide 19 explain what cloud
changes could account for the changes in global mean
SW and LW cloud radiative forcing.
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Competition
• Take a photograph of a cloudy scene. Send it to
me with a detailed explanation of what the
clouds are doing to climate.
• The winner will be decided based on beauty and
complexity of the cloud scene and accuracy of
the explanation
• Closing date: end of term
• Prize: A large tin of chocolates
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