7.5.4 Effects of Aerosols on Circulation Patterns

7.5.4.1 Effects on Stability

Changes in the atmospheric lapse rate modify the longwave emission and affect the water vapour feedback (Hu, 1996) and the formation of clouds (see, e.g., Section 8.6). Observations and model studies show that an increase in the lapse rate produces an amplification of the water vapour feedback (Sinha, 1995). As aerosols cool the Earth’s surface and warm the aerosol layer, the lapse rate will decrease globally and suppress the water vapour feedback (e.g., Feichter et al., 2004). The local change in atmospheric stability strongly depends on the altitude of the black carbon heating (Penner et al., 2003).

Absorption of solar radiation by aerosols can change the cloud amount (semi-direct effect; Grassl, 1975; Hansen et al., 1997; Ackerman et al., 2000; Ramanathan et al., 2001; Jacobson, 2006; Figure 7.20). The semi-direct effect has been simulated with GCMs and high-resolution cloud-resolving models, since it is implicitly accounted for whenever absorbing aerosols coupled to the radiation scheme are included (Hansen et al., 1997; Lohmann and Feichter, 2001; Jacobson, 2002; Menon et al., 2002b; Penner et al., 2003; Cook and Highwood, 2004; Hansen et al., 2005). Aerosol heating within cloud layers reduces cloud fractions, whereas aerosol heating above the cloud layer tends to increase cloud fractions. When diagnosed within a GCM framework, the semi-direct effect can also include cloud changes due to circulation effects and/or surface albedo effects. Moreover, the semi-direct effect is not exclusive to absorbing aerosol, as potentially any radiative heating of the mid-troposphere can produce a similar response in a GCM (Hansen et al., 2005; see also Section 2.8). Cloud-resolving models of cumulus and stratocumulus case studies also diagnose semi-direct effects indicating a similar relationship between the height of the aerosol layer relative to the cloud and the sign of the semi-direct effect (Ackerman et al., 2000; Ramanathan et al., 2001; Johnson et al., 2004; Johnson, 2005). Using a large eddy simulation, Feingold et al. (2005) show that the reduction in net surface radiation and in surface latent and sensible heat fluxes is the most simple explanation of the reduction in cloudiness associated with absorbing aerosols.