How smoke aerosols influence convective clouds
Koren et al. (2008): Science
Indirect Effect
• Enhanced Cloud Albedo due to Added CCN by Pollution
Figure from NASA MODIS
38N
136W
134W
34N
MODIS 2003 06 26 19:40
From: Daniel Rosenfeld
New Hypothesis: Anthropogenic Aerosols cause Regime Change
Rosenfeld et al. (2006):Atm.Chem.Phys.
Cloud Susceptibility
Hobbs (1993: Academic Press)
Semi‐Direct Effect
Suppressed Cloud Formation due to Absorption of Solar Radiation
Absorption Fraction Feedback (AFF)
• Aerosol absorption of solar radiation: (1) heats the aerosol layer; (2) cools the surface
• (1) => The temperature profile is stabilized => moist convection is suppressed => reduced cloudiness
• (2) => Surface fluxes are reduced => moist convection is suppressed => reduced cloudiness
• Reduced cloudiness leads to more exposure of the aerosol layer to solar heating => Positive Feedback!
• (Will be balanced once the extra heating of the surface raises the surface temperature sufficiently to destabilize the temperature profile again, transferring humidity vertically, hence forming new clouds)
Background
• The aerosol indirect effect (microphysical) leads to an increased cloud extent with increasing pollution
• The semi‐direct effect (radiative) leads to a reduced cloud extent with increasing amount of aerosols which absorb solar radiation
• How are these two effects related?
Area of Study
Geographical location: 5°N‐14°S; 46°W‐72°W
Away from the Andes; over the rainforest
Dry season (Aug‐Sep 2005)
High‐pressure => Stable meteorological conditions, little precipitation, human‐induced biomass burning
• 50% of moisture from the Atlantic (trade winds); 50% from evapotranspiration
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A combination of two opposing effects of aerosols on clouds
Cfo = 0.9
Cfo = 0.5
Cfo = 0.2
Koren et al. (2008: Science)
• Microphysical: For small optical depths, cloud fraction increases due to suppressed precipitation release (indirect effect)
• Radiative: For larger optical depths (denser smoke), cloud fraction decreases due to absorption (semi‐
direct effect)
• The latter effect is most pronounced for low cloud fractions (Cf0)
Sensitivity to the choice of ‘b’
MODIS Observations Confirm Proposed Relationships
Pressure vs. optical depth
Cloud fraction vs. optical depth
All data
Cloud fraction
< 0.5
Very strong
absorption
effect
Koren et al. (2008: Science)
The Influence of Pollution on Cloud Fraction and Vertical Extent of Cloud
All data
Cloud fraction
and cloud-top
height well
correlated
Cloud fraction
< 0.5
The most
polluted clouds
have the lowest
cloud fraction
(‘AFF’)
(The most
polluted clouds
extend highest)
Koren et al. (2008: Science)
Main Findings
• A smooth transition is found from: • A logarithmic microphysical effect consisting of increasing cloudiness with increasing pollution for low optical depths, to:
• A (radiative) absorption effect for larger optical depths causing a boomerang shape between cloud fraction and cloud optical depth
• Final cloud fraction critically dependent on initial cloud fraction, because the absorption effect is weak for large initial cloud fractions, but dominates for low initial cloud fractions Consequences
• A polarization of cloud regimes in the region, such that:
• The overcast mode will last longer with thicker clouds
• Scattered cloud fields will be suppressed, resulting in a smaller coverage of thinner clouds
Agreement between approximative expressions and a full radiative transfer code
Heating profiles and rate of temperature change vs. optical depth
Shallow
clouds:
p > 800 hPa
Intermediate
clouds:
400 hPa < p <
800 hPa
Deep clouds:
p < 400 hPa