7.5.2 Indirect Effects of Aerosols on Clouds and Precipitation
Aerosols can interact with clouds and precipitation in many ways, acting either as CCN or IN, or as absorbing particles, redistributing solar energy as thermal energy inside cloud layers. These indirect effects (in contrast to the direct interaction with radiation, see Chapter 2) are the subject of this subsection. They can be subdivided into different contributing processes, as summarised in Table 7.10 and shown in Figure 7.20. Cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates and the relatively poor simulation of boundary layer clouds in the present climate is a reason for some concern (see Chapter 8). Therefore the results discussed below need to be considered with caution.
Table 7.10a. Overview of the different aerosol indirect effects and their sign of the net radiative flux change at the top of the atmosphere (TOA).
| Effect | Cloud Types Affected | Process | Sign of Change in TOA Radiation | Potential Magnitude | Scientific Understanding |
|---|
| Cloud albedo effect | All clouds | For the same cloud water or ice content more but smaller cloud particles reflect more solar radiation | Negative | Medium | Low |
| Cloud lifetime effect | All clouds | Smaller cloud particles decrease the precipitation efficiency thereby presumably prolonging cloud lifetime | Negative | Medium | Very low |
| Semi-direct effect | All clouds | Absorption of solar radiation by absorbing aerosols affects static stability and the surface energy budget, and may lead to an evaporation of cloud particles | Positive or negative | Small | Very low |
| Glaciation indirect effect | Mixed-phase clouds | An increase in IN increases the precipitation efficiency | Positive | Medium | Very low |
| Thermodynamic effect | Mixed-phase clouds | Smaller cloud droplets delay freezing causing super-cooled clouds to extend to colder temperatures | Positive or negative | Medium | Very low |
Table 7.10b. Overview of the different aerosol indirect effects and their implications for the global mean net shortwave radiation at the surface, Fsfc (Columns 2-4) and for precipitation (Columns 5-7).
| Effect | Sign of Change in Fsfc | Potential Magnitude | Scientific Understanding | Sign of Change in Precipitation | Potential Magnitude | Scientific Understanding |
|---|
| Cloud albedo effect | Negative | Medium | Low | n.a. | n.a. | n.a. |
| Cloud lifetime effect | Negative | Medium | Very low | Negative | Small | Very low |
| Semi-direct effect | Negative | Large | Very low | Negative | Large | Very low |
| Glaciation indirect effect | Positive | Medium | Very low | Positive | Medium | Very low |
| Thermodynamic effect | Positive or negative | Medium | Very low | Positive or negative | Medium | Very low |
The cloud-albedo effect, that is, the distribution of the same cloud liquid water content over more, hence smaller, cloud droplets leading to higher cloud reflectivity, is a purely radiative forcing and is therefore treated in Chapter 2. The other effects involve feedbacks in the climate system and are discussed here. The albedo effect cannot be easily separated from the other effects; in fact, the processes that decrease the cloud droplet size per given liquid water content also decrease precipitation formation, presumably prolonging cloud lifetime (cloud lifetime effect, Section 7.5.2.1 and Figure 7.20). In turn, an increase in cloud lifetime also contributes to a change in the time-averaged cloud albedo. The semi-direct effect refers to the absorption of solar radiation by soot, re-emitted as thermal radiation, hence heating the air mass and increasing static stability relative to the surface. It may also cause evaporation of cloud droplets (see Sections 2.4 and 7.5.4.1 and Figure 7.20). The glaciation effect refers to an increase in IN resulting in a rapid glaciation of a super-cooled liquid water cloud due to the difference in vapour pressure over ice and water. Unlike cloud droplets, these ice crystals grow in an environment of high super-saturation with respect to ice, quickly reaching precipitation size, with the potential to turn a non-precipitating cloud into a precipitating cloud (Section 7.5.2.2 and Figure 7.20). The thermodynamic effect refers to a delay in freezing by the smaller droplets causing super-cooled clouds to extend to colder temperatures (Section 7.5.2.2 and Figure 7.20). In addition to aerosol-induced changes at the top of the atmosphere (TOA), aerosols affect the surface energy budget (Table 7.10b; Section 7.5.2) with consequences for convection, evaporation and precipitation (Figure 7.20).