The interaction of clouds, aerosols and radiation is highlighted as key climate uncertainties in the recent Intergovernmental Panel on Climate Change (IPCC) assessment report (Boucher et al., 2013). Aerosol-radiation interactions stem from direct scattering and absorption of solar and terrestrial radiation by aerosols, thereby changing the planetary albedo. Aerosol-cloud interactions, also termed indirect effects, arise from aerosols acting as cloud condensation (CCN) in warm clouds. An increase in the number of CCN translates into larger concentrations of smaller cloud droplets, increasing cloud albedo. Both aerosol-radiation and aerosol-cloud interactions trigger fast adjustments to the profiles of temperature, moisture, and cloud water content, which ultimately affect cloud formation and precipitation rates (e.g., Johnson et al., 2004). The quantification of interactions in the cloud-aerosol-radiation system remains elusive. The recent IPCC report (Boucher et al., 2013) stresses that aerosol climate impacts remain the largest uncertainty in driving climate change, with a global mean effective forcing of -0.50±0.40Wm-2 for the aerosol-radiation-interaction and in the range 0 to -0.9W m-2 for the aerosol-cloud-interaction thereby counteracting a significant, but poorly constrained, fraction of greenhouse gas-induced global warming which is estimated as +2.8±0.3W m-2 (Myhre et al., 2013a).
Our ability to provide reliable regional climate projections is further reduced downwind source regions, where aerosol radiative forcings are an order of magnitude larger than their global mean values. This holds particularly true for the South East (SE) Atlantic Ocean offshore Southern Africa, pointed as a regional hot-spot by the latest model exercise of the Aerosol Comparisons between Observations and Models (AEROCOM) project (Myhre et al., 2013b).