Download Clouds and Climate

Clouds and Climate: Forced
Changes to Clouds
ENVI3410 : Lecture 10
Ken Carslaw
Lecture 4 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 Lecture 10
•
•
•
•
Mechanisms
Aerosol-cloud interaction
Observational evidence for changes in clouds
Climate models and estimated radiative forcings
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Reading
• Global indirect aerosol effects: a review, U. Lohmann, J. Feichter,
Atmospheric Chemistry and Physics, 5, 715-737, 2005. Available
online at http://www.copernicus.org/EGU/acp/acp/5/715/acp-5715.htm
• The complex interaction of aerosols and clouds, H. Graf, Science,
303, 1309-1311, 27 February 2004.
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Changes to Clouds Forced by
Aerosol
..
. . ..
. .. . .
.
unperturbed
cloud
Increased CDN
(constant LWC)
Albedo effect
Twomey effect
1st Indirect effect
Drizzle
suppression
(increased
LWC)
Increased
cloud
height
Increased
cloud
lifetime
Cloud lifetime effect
Albrecht effect
2nd Indirect effect
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Heating
increases
cloud
burn-off
Semi-direct effect
1
An Additional Forced Change
• Not yet considered by IPCC
Cumulonimbus
Change in ice
formation,
latent heating
liquid
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Cloud Drop Number and Aerosol
• Composite of observations from many
measurement sites
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An Example of CDN-Aerosol
Relationship
CDN (cm-3)
Observational data from Gultepe and Isaac (1999)
•Why doesn’t CDN increase
linearly with aerosol number?
Aerosol Number (cm-3)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Aerosol
•
CDN
Explanation for
CDN-Aerosol
Relationship
Why doesn’t CDN increase
linearly with aerosol number?
•
Maximum supersaturation (Smax)
in cloud is reduced by droplet
growth
•
Figures show global model
calculations
Smax
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Other Factors Affecting CDN
• Updraught speed
– Very difficult to quantify at global model spatial
resolutions
– Also affects response to Daerosol
• Aerosol size distribution
– Typically not simulated in a global model
• Aerosol composition
– Until recently, just sulphate mass
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How aerosol size
affects CDN
• Model calculations
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Satellite
Observations
• Polder satellite
•
POLarization and
Directionality of the Earth's
Reflectances radiometer
• TOP: Aerosol
index (measure of
aerosol column
number)
• BOTTOM: Cloud
droplet radius
• Breon et al., (Science,
2002)
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Satellite Observations of 1st Indirect
Effect
• Polder Satellite data
• Cloud drop radius
decreases with
increasing aerosol
number
Bréon et al., Science 2002
Quaas et al., JGR 2004
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Cloud drop radius (mm)
Oceanic vs. Continental Regions
Ocean Aerosol Optical Depth
Ocean cloud drop radius
Land cloud drop radiuys
Aerosol index
• Ocean clouds
are more
susceptible to
changes in
aerosol than
over land
• Oceans also
have lower
albedo (larger
change in
reflectivity)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Localised Effects
• Aerosol point sources in the
Adelaide region of Australia
• Advanced Very High
Resolution Radiometer
(AVHRR) multi-wavelength
satellite observations
• Green/yellow implies
smaller/more numerous drops
in polluted regions
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Inferred Changes in Precipitation
5
3
2
1
polluted clouds
Approx altitude (km)
4
• Collision and
coalescence
suppressed in
deep convective
clouds
clean clouds
From Ramanathan et al.,
Science, 2001
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The Semi-Direct Effect
Cloud Fraction
Koren et al. (2004): observational evidence for
semi-direct effect based on MODIS satellite
Smoke Optical Depth
Columbia Shuttle image
MEIDEX, January 12, 2003
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Treatment of CDN in Climate Models
• Single fit equations
describing CDN vs.
model aerosol
number
Jones (1994) (Met Office Model)
Global
Gultepe and Isaac (2004)
Continental
Marine
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Model Calculations of CDN
1860 emissions
2000 emissions
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Model Calculations of Change in
Surface SW Energy Budget
• Due to
aerosol
direct
effect and
1st/2nd
indirect
effects
• Cloud
effects
significant
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Global Mean Forcings
From Intergovernmental Panel on Climate Change Scientific Assessment
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Uncertainties
• Observational
– Limited quantitative information from satellites
• Aerosol and cloud drop optical properties (no aerosol
chemistry)
• Cloud top only
– Difficult to determine cause and effect
• What would clouds look like without increased aerosol?
– Multiple changes
• Increased aerosol loading is often associated with drier air
• 1st indirect effect never observed without other changes
–
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Uncertainties
• Models
– Aerosol schemes too simplistic
• Particle size/composition
– Cloud physics incomplete
• Highly parametrised
• CDN-aerosol link too simplistic (improvement needs
information that is unreliable in models; e.g., updraught
speed)
• Rain formation
– Sub-grid processes (multi-cell clouds)
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