clouds and climate: cloud response to climate change envi3410 : lecture 11 ken carslaw lecture 5 of...
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Clouds and Climate: Cloud Response to Climate Change
ENVI3410 : Lecture 11Ken 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
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Content of Lecture 11
• The importance of cloud feedbacks: Climate sensitivity
• Cloud radiative forcing
• Factors affecting clouds
• Cloud feedback in climate models
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Climate Sensitivity
• Climate sensitivity determines the global temperature when a radiative forcing is applied
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Climate Sensitivity
• T = change in global mean temperature
• Q = radiative forcing (W m-2)
• = climate sensitivity (W m-2 K-1)
Q
T
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Sensitivity of Climate Models
• Sensitivity to doubled CO2 (~4 Wm-2)
Summer 2002
NC
AR
GF
DL
2xC
O2 S
ensi
tivity
(K
)
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Cloud Changes and Climate Sensitivity
=4.2 K Wm-2
=1.8 K Wm-2
% Change in low cloud amount for 2xCO2
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Change in Cloud Radiative Forcing
• Today’s Earth is cooler because of clouds (net -20 Wm-2 forcing (= 4*CO2 doubling effect)– 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. T depends on which clouds change
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Humidity and Temperature
• Increased T• Increased water
vapour in atmosphere• Increased
cloudiness?
• NO
• Relative humidity is the relevant quantity
Overall increase in atmospheric water vapour
Overall increase in atmospheric water vapour and temperature
100% RH
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Cloud Radiative Forcing (CRF)
• Factors that determine CRF– Location (solar intensity)
– Depth/thickness
– Coverage
– Drop/ice concentrations
Very similar SW forcingVery different LW forcing
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Cloud Radiative Forcing
0 50 100 150 -40
-20
0
20
40
liquid water path (g m-2)
Ts (
K)
Winter 5o N
low
med
high
cloud height
Equilibrium surface temperature due to presence of different clouds
0 50 100 150 -40
-20
0
20
40
liquid water path (g m-2)
Ts (
K)
Winter 65o N
low
med
high
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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
• Very difficult to separate in observations
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Thermodynamic Changes
• Influence on water vapour feedback– water vapour is much more effective GHG in the
upper troposphere than near the surface
– Deep Cb clouds transport water vertically
high feedback
low feedback
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Tropical Cirrus – A Proposed “Adaptive Infrared Iris Effect”
25 26 27 28 29 300
0.05
0.1
0.15
0.2
sea surface temperature (K)
Clo
ud A
mou
nt
slope = 10-20% change per 1 K SST
observations
• Japan’s Geostationary Meteorological Satellite
• 11 and 12 m 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
Richard Lindzen, MIT
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
The Adaptive Infrared Iris as a Climate Change Regulator
warm ocean cold ocean
more IR to space
less cirrus
more rain
less watertransport
less water vapour
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Problems With the Infrared Iris Idea
• Observations of cloud IR radiance are not directly related to cirrus coverage
• Other observations from TRMM (Tropical Rainfall Measuring Mission) show that warm clouds rain more, but they also transport more water vertically
• See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Circulation/Dynamical Changes
Tropicalconvection
Tradewindcumulus
Sub-tropical St/Sc
Hadley/Walkercirculation
Equator 30oN
• Cloud fields are determined by large-scale circulation
• Non-local response
• El Nino
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Observed Clouds With Temperature
• Observations from the International Satellite Cloud Climatology Project (see lecture 7)
• Clouds become optically thinner (less reflective) at higher temperatures
• +ve or –ve feedback?
-60 -40 -20 0 20 40 60-0.15
-0.1
0.05
0
0.1
latitude
d ln
(opt
ical
dep
th)/
dT
Ocean low clouds
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Net Cloud Feedbacks in GCMs
-3
-2
-1
0
1
2
3
Cha
nge
in C
RF
(W m
-2)
Different models
SW
LW
netCOOLING
WARMING
Doubled CO2 experiments
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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 change-induced changes