TABLE 2-2 Overview of the Different Aerosol Indirect Effects Associated with Clouds

Effect

Cloud Type

Description

Sign of TOA Radiative Forcing

First indirect aerosol effect (cloud albedo or Twomey effect)

All clouds

For the same cloud water or ice content, more but smaller cloud particles reflect more solar radiation

Negative

Second indirect aerosol effect (cloud lifetime or Albrecht effect)

All clouds

Smaller cloud particles decrease the precipitation efficiency, thereby prolonging cloud lifetime

Negative

Semidirect effect

All clouds

Absorption of solar radiation by soot leads to evaporation of cloud particles

Positive

Glaciation indirect effect

Mixed-phase clouds

An increase in ice nuclei increases the precipitation efficiency

Positive

Thermodynamic effect

Mixed-phase clouds

Smaller cloud droplets inhibit freezing, causing supercooled droplets to extend to colder temperatures

Unknown

Surface energy budget effect

All clouds

The aerosol-induced increase in cloud optical thickness decreases the amount of solar radiation reaching the surface, changing the surface energy budget

Negative

ciency and therefore enhance cloud lifetime and, hence, cloud reflectivity, which is referred to as the second indirect aerosol or cloud lifetime effect (Albrecht, 1989). Absorption of solar radiation by black carbon leads to heating of the air, which can result in an evaporation of cloud droplets. This is referred to as the semidirect effect (Hansen et al., 1997). The absorption of soot is 2 to 2.5 times greater if soot is present in the cloud droplets (Chylek et al., 1996); thus the magnitude of the semidirect effect depends crucially on the location of black carbon with respect to the cloud. This warming can partially offset cooling due to the indirect aerosol effect. Both



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