Aerosol Indirect Effects

Aerosol particles affect radiative fluxes by scattering solar radiation and absorbing solar and thermal radiation (direct effect). In addition, they interact with clouds and the hydrological cycle by acting as CCN and ice nuclei. For a given cloud LWC, a greater concentration of CCN increases cloud albedo (indirect cloud albedo effect) and is supposed to reduce the precipitation efficiency (indirect cloud lifetime effect), both of which are likely to result in a reduction of the global, annual mean net radiation at the top of the atmosphere (TOA). These effects may be partly offset through the evaporation of cloud droplets attributable to absorbing aerosols (semi-direct effect) and/or by more ice nuclei (glaciation effect). The influences of these processes on radiation at TOA and at the surface and on precipitation are summarized in Table 23.1. The following discussion is based on Denman et al. (2007), which also provides references to the studies noted.

Another aerosol influence on clouds and radiation that may be climatologi-cally important is the enhancement of downwelling longwave radiation from Arctic haze (Blanchet and Girard 1994) and thin Arctic stratus whose longwave optical thickness is augmented by increased droplet concentration (Lubin and Vogelmann 2006).

In addition to raising the number concentration of aerosol particles, there is evidence that increased particle concentrations can broaden the cloud drop size distribution (Liu and Daum 2002). This would have the effect of decreasing aerosol influences on shortwave radiation and inhibiting precipitation development (e.g., Peng and Lohmann 2003).

Table 23.1 Overview of known aerosol indirect effects on net radiative flux (at TOA and at the surface) and on precipitation, and an assessment of level of current scientific understanding. Modified from Dernnan et al. (2007).

Effect

Cloud albedo effect

Cloud lifetime effect

Semi-direct effect

Glaciation indirect effect

Thermodynamic effect

All; greatest for clouds

Cloud types affected

of intermediate optical thickness

All

All

Mixed-phase

Mixed-phase

For same cloud water

Smaller cloud particles decrease precipitation efficiency prolonging cloud lifetime

Absorption of solar

Smaller cloud droplets

or ice content more but

radiation by absorbing

Increase in ice nuclei

delay freezing and

Process

smaller cloud par

aerosols evaporates

increases precipitation

cause supercooled

ticles reflect more solar

cloud particles, in

efficiency

clouds to extend to

radiation

creases static stability

colder temperatures

Change in net TOA irradiation

-

-

+ /-

+

+ /-

Potential magnitude

medium

medium

small

medium

medium

Scientific understanding

low

very low

very low

very low

very low

Change in surface irradiation

-

-

+ /-

+

+ /-

Potential magnitude

medium

medium

large

medium

medium

Scientific understanding

low

very low

very low

very low

very low

Change in precipitation

N/A

-

-

+

+ /-

Potential magnitude

N/A

small

large

medium

medium

Scientific understanding

N/A

very low

very low

very low

very low

The increase in albedo of liquid water clouds attributable to anthropogenic aerosols has received much attention. Although uncertainties remain regarding the breadth of the cloud drop size distribution, more and probably larger uncertainties are related to aerosol effects on precipitation as well as on mixed- and ice-phase clouds, as discussed below.

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