7.5.2.2 Aerosol Impacts on Mixed-Phase Clouds

As satellite observations of aerosol effects on mixed-phase clouds are not conclusive (Mahowald and Kiehl, 2003), this section only refers to model results and field studies. Studies with GCMs suggest that if, in addition to mineral dust, hydrophilic black carbon aerosols are assumed to act as IN at temperatures between 0°C and –35°C, then increases in aerosol concentration from pre-industrial to present times may cause a glaciation indirect effect (Lohmann, 2002). Increases in IN can result in more frequent glaciation of super-cooled stratiform clouds and increase the amount of precipitation via the ice phase, which could decrease the global mean cloud cover leading to more absorption of solar radiation. Whether the glaciation effect or warm cloud lifetime effect is larger depends on the chemical nature of the dust (Lohmann and Diehl, 2006). Likewise, the number and size of ice particles in convective mixed-phase clouds is sensitive to the chemical composition of the insoluble fraction (e.g., dust, soot, biological particles) of the aerosol particles (Diehl and Wurzler, 2004).

Rosenfeld (1999) and Rosenfeld and Woodley (2000) analysed aircraft data together with satellite data suggesting that pollution aerosols suppress deep convective precipitation by decreasing cloud droplet size and delaying the onset of freezing. This hypothesis was supported by a cloud-resolving model study (Khain et al., 2001) showing that super-cooled cloud droplets down to –37.5°C could only be simulated if the cloud droplets were small and numerous. Precipitation from single-cell mixed-phase convective clouds is reduced under continental and maritime conditions when aerosol concentrations are increased (Yin et al., 2000; Khain et al., 2004; Seifert and Beheng, 2006). In the modelling study by Cui et al. (2006), this is caused by drops evaporating more rapidly in the high aerosol case (see also Jiang et al., 2006), which eventually reduces ice mass and hence precipitation. Khain et al. (2005) postulate that smaller cloud droplets, such as those originating from human activity, would change the thermodynamics of convective clouds. More, smaller droplets would reduce the production of rain in convective clouds. When these droplets freeze, the associated latent heat release would then result in more vigorous convection and more precipitation. In a clean cloud, on the other hand, rain would have depleted the cloud so that less latent heat is released when the cloud glaciates, resulting in less vigorous convection and less precipitation. Similar results were obtained by Koren et al. (2005), Zhang et al. (2005) and for the multi-cell cloud systems studied by Seifert and Beheng (2006). For a thunderstorm in Florida in the presence of Saharan dust, the simulated precipitation enhancement only lasted two hours after which precipitation decreased as compared with clean conditions (van den Heever et al., 2006). Cloud processing of dust particles, sulphate particles and trace gases can lead to an acceleration of precipitation formation in continental mixed-phase clouds, whereas in maritime clouds, which already form on rather large CCN, the simulated effect on precipitation is small (Yin et al., 2002). This highlights the complexity of the system and indicates that the sign of the global change in precipitation due to aerosols is not yet known. Note that microphysical processes can only change the temporal and spatial distribution of precipitation while the total amount of precipitation can only change if evaporation from the surface changes.