Atul Kapur

 Direct effects i. Aerosols scatter & absorb solar radiation ii. Scatter, absorb & emit thermal radiation  Indirect effects: Aerosols can act as CCN & IN i. Twomey effect: More but smaller cloud droplets lead to enhanced reflection of solar radiation ii. 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

Evidence of aerosol effect in warm clouds Evidence from Ship Tracks – conversion from open to closed cells Rosenfeld, Kaufman, and Koren (2005) Evidence from Satellite data Breon, Tanre, and Generoso (2002)

Lohmann & Feichter (2005)

 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%

Lohmann & Feichter (2005)

 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

Lohmann (2002)

Lohmann (2004)

 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