Global Indirect Aerosol Effects

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Global Indirect Aerosol Effects

• Atul Kapur

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Definitions

• Direct effects
• Aerosols scatter absorb solar radiation
• Scatter, absorb emit thermal radiation
• Indirect effects Aerosols can act as CCN IN
• Twomey effect More but smaller cloud droplets
  lead to enhanced reflection of solar radiation
• Lifetime effect More but smaller cloud droplets
  reduce the precipitataion efficiency therefore
  enhance the cloud lifetime
• Semi-direct effect
• Aerosols can absob solar radiation leading to
  heating of air, which can result in evaporation
  of cloud droplets

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Evidence of aerosol effect in warm clouds
Evidence from Satellite data Breon, Tanre, and
Generoso (2002)
Evidence from Ship Tracks conversion from open
to closed cells Rosenfeld, Kaufman, and Koren
(2005)
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Global Mean Twomey effect
Lohmann Feichter (2005)
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Twomey effect lifetime effect
Lohmann Feichter (2005)
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Considerations for Warm Clouds

• Twomey effect and Cloud lifetime cause net
  radiative cooling (both are comparable)
• Warming due to semi-direct effect can partially
  offset this cooling
• But both Cloud Lifetime and Semi-direct effect
  involve feedbacks
• (Hence not included in IPCC)
• Twomey effect holds only if cloud thickness
  doesnt change!
• But polluted clouds were thinner as they
  originated over continents
• (which can lower optical thickness)
• Dispersion effect Increasing no. of cloud
  droplets affect shape of cloud droplet spectrum.
• Twomey effect can be reduced by 10-80

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Global mean total indirect aerosols effects
Lohmann Feichter (2005)
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Mixed phase clouds

• Most precipitation occurs via ice phase
• Aerosol effects poorly understood because of
  variety of heterogenous nucleation modes
• Contact Nucleation Most efficient at slight
  coolings
• Immersion freezing Efficient at lower
  temperatures
• Deposition nuclei least efficient
• Bergeron-Findeisen process glaciation leads to
  shorter lifetime as precipitation formation is
  now more efficient
• If soot is also considered
• More frequent glaciation
• Leads to glaciation indirect effect
• Effects deep convective processes
• Smaller cloud droplets reduce drizzle leads to
  vigrous convection

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Lohmann (2002)
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Lohmann (2004)
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Different Approaches to the concept of Forcing
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Conclusions

• Aerosol Radiative effects
• Both Twomey and cloud lifetime effect lead to
  cooling
• Semi-direct effect more prominent when aerosols
  occur within cloud droplets
• Decrease in cloud amount due to precipitation
• Effects on hydrological cycle
• Suppression of drizzle increase in cloud
  lifetime
• Occurrence and frequency of precipitation
• Reduction in net solar radiation reaching the
  surface (in particular direct sunlight)
• Delay of freezing in mixed phase clouds due to
  smaller droplets