Clouds and Climate: Forced Changes to Clouds SOEE3410 Ken Carslaw Lecture 4 of a series of 5 on...
-
date post
19-Dec-2015 -
Category
Documents
-
view
217 -
download
2
Transcript of Clouds and Climate: Forced Changes to Clouds SOEE3410 Ken Carslaw Lecture 4 of a series of 5 on...
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)