Clouds and Climate: Forced Changes to Clouds

SOEE3410Ken 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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

...

.

....

.

..

. .

unperturbedcloud

Increased CDN(constant LWC)

Albedo effectTwomey effect

1st Indirect effect

Drizzle suppression(increased

LWC)

Increasedcloud height

Increasedcloud

lifetime

Heating increases

cloud burn-off

Cloud lifetime effectAlbrecht effect

2nd Indirect effect

Semi-direct effect

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

An Additional Forced Change

• Not yet considered by IPCC

liquid

Cumulonimbus

Change in ice formation,latent heating

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

Aerosol Number (cm-3)

CD

N (

cm-3)

Observational data from Gultepe and Isaac (1999)

•Why doesn’t CDN increase linearly with aerosol number?

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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

CDN Smax

Aerosol

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 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

Droplet number vs. aerosol size and number

• Fixed updraught speed

log(N)

DiameterSolid contours = CDN; colours = aerosol mass (g m-3)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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)

Land cloud drop radiuys

Ocean cloud drop radius

Ocean Aerosol Optical Depth

Aerosol index

Clo

ud d

rop

radi

us (m

)

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

• Collision and coalescence suppressed in deep convective clouds

• Refer to Lecture 2

From Ramanathan et al., Science, 2001

polluted clouds clean clouds

1

2

3

4

5 Approx altitude (km

)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

The Semi-Direct Effect

Clo

ud

Fra

ctio

n

Smoke Optical DepthColumbia Shuttle imageMEIDEX, January 12, 2003

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Treatment of CDN in Climate Models

Jones (1994) (Met Office Model)

Gultepe and Isaac (2004)

Continental

Marine

Global

• Single fit equations describing CDN vs. model aerosol number

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 1st/2nd 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 (1/2)

• Observations– Limited quantitative information from satellites

• Aerosol and cloud drop optical properties (no aerosol chemistry, no direct microphysics measurement)

• Cloud top only

– Difficult to determine cause and effect• What would clouds look like without increased aerosol?

– Multiple changes• No “control experiment”• Increased aerosol loading is often associated with drier air• 1st indirect effect never observed without other changes

–

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Uncertainties (2/2)

• Models– Aerosol schemes too simplistic

• Particle size/composition

– Cloud physics incomplete• Highly parameterized

• 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)