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Global Indirect Aerosol Effects

Global Indirect Aerosol Effects. Atul Kapur. Definitions. Direct effects Aerosols scatter & absorb solar radiation Scatter, absorb & emit thermal radiation Indirect effects: Aerosols can act as CCN & IN

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Global Indirect Aerosol Effects

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  1. Global Indirect Aerosol Effects Atul Kapur

  2. 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

  3. 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)

  4. Global Mean Twomey effect Lohmann & Feichter (2005)

  5. Twomey effect & lifetime effect Lohmann & Feichter (2005)

  6. 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 doesn’t 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%

  7. Global mean total indirect aerosols effects Lohmann & Feichter (2005)

  8. 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

  9. Lohmann (2002)

  10. Lohmann (2004)

  11. Different Approaches to the concept of “Forcing”

  12. 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

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