2.4.5 Aerosol Influence on Clouds (Cloud Albedo Effect)

As pointed out in Section 2.4.1, aerosol particles affect the formation and properties of clouds. Only a subset of the aerosol population acts as cloud condensation nuclei (CCN) and/or ice nuclei (IN). Increases in ambient concentrations of CCN and IN due to anthropogenic activities can modify the microphysical properties of clouds, thereby affecting the climate system (Penner et al., 2001; Ramanathan et al., 2001a, Jacob et al., 2005). Several mechanisms are involved, as presented schematically in Figure 2.10. As noted in Ramaswamy et al. (2001), enhanced aerosol concentrations can lead to an increase in the albedo of clouds under the assumption of fixed liquid water content (Junge, 1975; Twomey, 1977); this mechanism is referred to in this report as the ‘cloud albedo effect’. The aerosol enhancements have also been hypothesised to lead to an increase in the lifetime of clouds (Albrecht, 1989); this mechanism is referred to in this report as the ‘cloud lifetime effect’ and discussed in Section 7.5.

The interactions between aerosol particles (natural and anthropogenic in origin) and clouds are complex and can be nonlinear (Ramaswamy et al., 2001). The size and chemical composition of the initial nuclei (e.g., anthropogenic sulphates, nitrates, dust, organic carbon and BC) are important in the activation and early growth of the cloud droplets, particularly the water-soluble fraction and presence of compounds that affect surface tension (McFiggans et al., 2006 and references therein). Cloud optical properties are a function of wavelength. They depend on the characteristics of the droplet size distributions and ice crystal concentrations, and on the morphology of the various cloud types.

The interactions of increased concentrations of anthropogenic particles with shallow (stratocumulus and shallow cumulus) and deep convective clouds (with mixed phase) are discussed in this subsection. This section presents new observations and model estimates of the albedo effect. The associated RF in the context of liquid water clouds is assessed. In-depth discussion of the induced changes that are not considered as RFs (e.g., semi-direct and cloud cover and lifetime effects, thermodynamic response and changes in precipitation development) are presented in Section 7.5. The impacts of contrails and aviation-induced cirrus are discussed in Section 2.6 and the indirect impacts of aerosol on snow albedo are discussed in Section 2.5.4.