Biomass burning smoke aerosol during the Boreal summer typically is lofted to an altitude of between 3 km and 4 km and advected from the centre of the African continent and driven westwards on the prevailing winds. Below the trade wind inversion sits a semi-persistent layer of decoupled stratocumulus, with a cloud top height below 2 km. Cumulus are often found below with a cloud base below 500 m at the lifting condensation level. These clouds are driven from the south east where the oceanic boundary layer is often pristine in terms of aerosol concentrations.
During the airborne field campaign a wide range of conditions were encountered from ultra-clean marine boundary layers with cloud drop number concentrations as low as a few 10's cc-1, sometimes with smoke plumes aloft, to more polluted cases, where biomass burning aerosol appears to mix down from above the trade wind inversion, giving cloud droplet number concentrations of many hundreds cc-1. Cases of boundary layers containing diffuse pollution were observed, presumably from remote entrainment events, with intermediate cloud droplet number. Total aerosol number concentrations in the boundary layer were as low 100 cc-1 and as high as 1500 cc-1. A similar wide range in cloud cover and precipitation rates was also observed, with a subjective link observed between rainfall and aerosol concentration. Clean and polluted cases are contrasted with a view to establishing a direct link between the aerosol-cloud-precipitation interactions, as distinct from the large scale meteorology. This is explored by assessing the microphysical observations and driving meteorology alongside the long-term ground based and radar observations of precipitation.
Zuidema et al. Smoke and Clouds above the Southeast Atlantic: Upcoming Field Campaigns Probe Absorbing Aerosol's Impact on Climate,