7.5.2.3 Aerosol Impacts on Cirrus Clouds

Cirrus clouds can form by homogeneous and heterogeneous ice nucleation mechanisms at temperatures below 235 K. While homogeneous freezing of super-cooled aqueous phase aerosol particles is rather well understood, understanding of heterogeneous ice nucleation is still in its infancy. A change in the number of ice crystals in cirrus clouds could exert a cloud albedo effect in the same way that the cloud albedo effect acts for water clouds. In addition, a change in the cloud ice water content could exert a radiative effect in the infrared. The magnitude of these effects in the global mean has not yet been fully established, but the development of physically based parametrization schemes of cirrus formation for use in global models led to significant progress in understanding underlying mechanisms of aerosol-induced cloud modifications (Kärcher and Lohmann, 2002; Liu and Penner, 2005; Kärcher et al., 2006).

A global climate model study concluded that a cloud albedo effect based solely on ubiquitous homogeneous freezing is small globally (Lohmann and Kärcher, 2002). This is expected to also hold in the presence of heterogeneous IN that cause cloud droplets to freeze at relative humidities over ice close to homogeneous values (above 130–140%) (Kärcher and Lohmann, 2003). Efficient heterogeneous IN, however, would be expected to lower the relative humidity over ice, so that the climate effect may be larger (Liu and Penner, 2005). In situ measurements reveal that organic-containing aerosols are less abundant than sulphate aerosols in ice cloud particles, suggesting that organics do not freeze preferentially (Cziczo et al., 2004). A model study explains this finding by the disparate water uptake of organic aerosols, and suggests that organics are unlikely to significantly modify cirrus formation unless they are present in very high concentrations (compared with sulphate-rich particles) at low temperatures (Kärcher and Koop, 2004).

With regard to aerosol effects on cirrus clouds, a strong link has been established between gravity wave induced, mesoscale variability in vertical velocities and climate forcing by cirrus (Kärcher and Ström, 2003; Hoyle et al., 2005). Hemispheric-scale studies of aerosol-cirrus interactions using ensemble trajectories suggest that changes in upper-tropospheric cooling rates and ice-forming aerosols in a future climate may induce changes in cirrus occurrence and optical properties that are comparable in magnitude with observed decadal trends in global cirrus cover (Haag and Kärcher, 2004). Optically thin and sub-visible cirrus are particularly susceptible to IN and therefore likely affected by anthropogenic activities.

Radiative forcing estimates and observed trends of aviation-induced cloudiness are discussed in Section 2.6. In terms of indirect effects of aircraft-induced aerosols on cirrus clouds, Lohmann and Kärcher (2002) show that the impact of aircraft sulphur emissions on cirrus properties via homogeneous freezing is small. The contribution from air traffic to the global atmospheric black carbon cycle was assessed by Hendricks et al. (2004). Assuming that black carbon particles from aviation serve as efficient IN, maximum increases or decreases in ice crystal number concentrations of more than 40% are simulated in a climate model study assuming that the ‘background’ (no aviation impact) cirrus cloud formation is dominated by heterogeneous or homogeneous nucleation, respectively (Hendricks et al., 2005). Progress in assessing the impact of aircraft black carbon on cirrus is hampered by the poor knowledge of natural freezing modes in cirrus conditions and the inability to describe the full complexity of cirrus processes in global models.