7.5.2.1 Aerosol Effects on Water Clouds and Warm Precipitation

Aerosols are hypothesised to increase the lifetime of clouds because increased concentrations of smaller droplets lead to decreased drizzle production and reduced precipitation efficiency (Albrecht, 1989). It is difficult to devise observational studies that can separate the cloud lifetime from the cloud albedo effect (see Section 2.4). Thus, observational studies usually provide estimates of the combined effects. Similarly, climate models cannot easily separate the cloud lifetime indirect effect once the aerosol scheme is fully coupled to a cloud microphysics scheme, but also predict the combined cloud albedo, lifetime and semi-direct effect.

Evidence for the absence of a drizzle mode due to anthropogenic emissions of aerosols and their precursors comes, for instance, from ship tracks perturbing marine stratus cloud decks off the coast of California (Ferek et al., 1998) as well as from analysing polluted compared with clean clouds off the Atlantic coast of Canada (Peng et al., 2002). One problem is that most climate models suggest an increase in liquid water when adding anthropogenic aerosols, whereas newer ship track studies show that polluted marine water clouds can have less liquid water than clean clouds (Platnick et al., 2000; Coakley and Walsh, 2002). Ackerman et al. (2004) attribute this effect to enhanced entrainment of dry air in polluted clouds in these instances with subsequent evaporation of cloud droplets. Similarly, when cloud lifetime is analysed, an increase in aerosol concentration from very clean to very polluted does not increase cloud lifetime, even though precipitation is suppressed (Jiang et al., 2006). This effect is due to competition between precipitation suppression and enhanced evaporation of the more numerous smaller cloud droplets in polluted clouds. Observed lower aerosol concentrations in pockets of open cells (Stevens et al., 2005) and in rifts of broken clouds surrounded by solid decks of stratocumulus with higher aerosol concentrations (Sharon et al., 2006) are manifestations of two stable aerosol regimes (Baker and Charlson, 1990). The low aerosol concentration regimes maintain themselves by higher drizzle rates. However, it is hard to disentangle cause and effect from these studies.

Smoke from burning vegetation reduces cloud droplet sizes and delays the onset of precipitation (Warner and Twomey, 1967; Rosenfeld, 1999; Andreae et al., 2004). In addition, desert dust suppresses precipitation in thin low-altitude clouds (Rosenfeld et al., 2001; Mahowald and Kiehl, 2003). Contradictory results have been found regarding the suppression of precipitation by aerosols downwind of urban areas (Givati and Rosenfeld, 2004; Jin et al., 2005) and in Australia (Rosenfeld, 2000; Ayers, 2005).

Models suggest that anthropogenic aerosols suppress precipitation in the absence of giant CCN and aerosol-induced changes in ice microphysics (e.g., Lohmann, 2002; Menon and DelGenio, 2007) as well as in mixed-phase clouds where the ice phase only plays a minor role (Phillips et al., 2002). A reduction in precipitation formation leads to increased cloud processing of aerosols. Feingold et al. (1998) and Wurzler et al. (2000) showed that cloud processing could either lead to an increase or decrease in precipitation formation in subsequent cloud cycles, depending on the size and concentration of activated CCN. Giant sea salt nuclei, on the other hand, may override the precipitation suppression effect of the large number of small pollution nuclei (Johnson, 1982; Feingold et al., 1999; Rosenfeld et al., 2002). Likewise, Gong and Barrie (2003) predict a reduction of 20 to 60% in marine cloud droplet number concentrations and an increase in precipitation when interactions of sulphate with sea salt aerosols are considered. When aerosol effects on warm convective clouds are included in addition to their effect on warm stratiform clouds, the overall indirect aerosol effect and the change in surface precipitation can be larger or smaller than if just the aerosol effect on stratiform clouds is considered (Nober et al., 2003; Menon and Rotstayn, 2006). Besides changes in the distribution of precipitation, the frequency of extreme events may also be reduced by the presence of aerosols (Paeth and Feichter, 2006).

Observations show that aerosols can decrease or increase cloud cover. Kaufman et al. (2005) conclude from satellite observations that the aerosol indirect effect is likely primarily due to an increase in cloud cover, rather than an increase in cloud albedo. In contrast, model results of Lohmann et al. (2006) associate the increase in cloud cover with differing dynamic regimes and higher relative humidities that maintain higher aerosol optical depths. On the other hand, the semi-direct effect of absorbing aerosols can cause evaporation of cloud droplets and/or inhibit cloud formation. In a large area with absorbing biomass-burning aerosol, few low-lying clouds were observed when the aerosol optical depth exceeded 1.2 (Koren et al., 2004). Increasing emissions of absorbing aerosols from the late 1980s to the late 1990s in China also reduced cloud amount leading to a decrease in local planetary albedo, as deduced from satellite data (Krüger and Grassl, 2004). When the combined effect of pollution and smoke aerosols is considered from ground-based observations, the net effect seems to be an increase in cloud cover with increasing aerosol column concentrations (Kaufman and Koren, 2006).