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 1

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.

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Changes to Clouds Forced by Aerosol

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unperturbed cloud Increased CDN (constant LWC) Drizzle suppression (increased LWC) Increased cloud height

Albedo effect Twomey effect 1 st Indirect effect Cloud lifetime effect Albrecht effect 2 nd Indirect effect

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics Increased cloud lifetime Heating increases cloud burn-off

Semi-direct effect

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An Additional Forced Change

• Not yet considered by IPCC Cumulonimbus Change in ice formation, latent heating liquid ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Cloud Drop Number and Aerosol

• Composite of observations from many measurement sites ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

An Example of CDN-Aerosol Relationship 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 1

Aerosol CDN

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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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Other Factors Affecting CDN

• Updraught speed – Very difficult to quantify at global model spatial resolutions – Also affects response to D aerosol • Aerosol size distribution – Typically not simulated in a global model • Aerosol composition – Until recently, just sulphate mass ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

How aerosol size affects CDN

• Model calculations ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Satellite Observations

• • Polder satellite POL arization and D irectionality of the E arth's R eflectances radiometer • TOP: Aerosol index (measure of aerosol column number) • BOTTOM: Cloud droplet radius • Breon et al., (Science, 2002) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Satellite Observations of 1 st Indirect Effect

• Polder Satellite data • Cloud drop radius decreases with increasing aerosol number Bréon et al., Science 2002 Quaas et al., JGR 2004 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 1

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 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Inferred Changes in Precipitation

1 3 2 5 Approx a 4 • Collision and coalescence suppressed in deep convective clouds

polluted clouds

From Ramanathan et al., Science, 2001

clean clouds

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

The Semi-Direct Effect

Koren et al. (2004): observational evidence for semi-direct effect based on MODIS satellite

Smoke Optical Depth Columbia Shuttle image

MEIDEX, January 12, 2003 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Model Calculations of CDN 1860 emissions 2000 emissions

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Model Calculations of Change in Surface SW Energy Budget

• Due to aerosol direct effect and 1 st /2 nd indirect effects • Cloud effects significant ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Global Mean Forcings From Intergovernmental Panel on Climate Change Scientific Assessment

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 • 1 st indirect effect never observed without other changes – ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1