Clouds and Climate: Forced Changes to Clouds

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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

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Content of Lecture 10

• Mechanisms
• Aerosol-cloud interaction
• Observational evidence for changes in clouds
• Climate models and estimated radiative forcings

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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-5-
  715.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
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unperturbed cloud
Increased CDN (constant LWC) Albedo
effect Twomey effect 1st Indirect effect
Drizzle suppression (increased LWC)
Increased cloud height
Increased cloud lifetime
Heating increases cloud burn-off
Cloud lifetime effect Albrecht effect 2nd
Indirect effect
Semi-direct effect
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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
Observational data from Gultepe and Isaac (1999)

• Why doesnt CDN increase linearly with aerosol
  number?

CDN (cm-3)
Aerosol Number (cm-3)
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Explanation for CDN-Aerosol Relationship
Aerosol

• Why doesnt CDN increase linearly with aerosol
  number?
• Maximum supersaturation (Smax) in cloud is
  reduced by droplet growth
• Figures show global model calculations

CDN
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 2002Quaas et al., JGR 2004
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Oceanic vs. Continental Regions

• Ocean clouds are more susceptible to changes in
  aerosol than over land
• Oceans also have lower albedo (larger change in
  reflectivity)

Ocean Aerosol Optical Depth
Cloud drop radius (mm)
Ocean cloud drop radius
Land cloud drop radiuys
Aerosol index
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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

• Collision and coalescence suppressed in deep
  convective clouds

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Approx altitude (km)
2
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polluted clouds
clean clouds
From Ramanathan et al., Science, 2001
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The Semi-Direct Effect
Koren et al. (2004) observational evidence for
semi-direct effect based on MODIS satellite
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

Gultepe and Isaac (2004)
Jones (1994) (Met Office Model)
Continental
Global
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)