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. We analyse the airborne measurements of above-cloud aerosols fromthe AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) fieldcampaign performed in Namibia during August and September 2017. The studyaims to retrieve the aerosol above-cloud direct radiative effect (DRE) withwell-defined uncertainties. To improve the retrieval of the aerosol andcloud properties, the airborne demonstrator of the Multi-Viewing,Multi-Channel, Multi-Polarization (3MI) satellite instrument, called theObserving System Including PolaRisation in the Solar Infrared Spectrum(OSIRIS), was deployed on-board the SAFIRE (Service des Avions Français Instrumentés pour la Rechercheen Environnement) Falcon 20 aircraft during 10flights performed over land, over the ocean, and along the Namibian coast.The airborne instrument OSIRIS provides observations at high temporal andspatial resolutions for aerosol above clouds (AACs) and cloud properties.OSIRIS was supplemented with the Photomètre Léger Aéroportépour la surveillance des Masses d'Air version 2 (PLASMA2). The combinedairborne measurements allow, for the first time, the validation of AAC algorithms previously developed for satellitemeasurements. The variations in the aerosol properties are consistent withthe different atmospheric circulation regimes observed during thedeployment. Airborne observations typically show strong aerosol opticaldepth (AOD; up to 1.2 at 550 nm) of fine-mode particles from biomass burning (extinction Ångström exponent varying between 1.6 and 2.2), transported above bright stratocumulus decks (mean cloud top around 1 km above mean sea level), with cloud optical thickness (COT) up to 35 at 550 nm. The above-cloud visible AOD retrieved with OSIRIS agrees within 10 % of the PLASMA2 sun photometer measurements in the same environment. The single scattering albedo (SSA) is one of the most influential parameters on the AAC DRE calculation that remains largely uncertain in models. During the AEROCLO-sA campaign, the average SSA obtainedby OSIRIS at 550 nm is 0.87, which is in agreement within 3 %, on average, with previous polarimetric-based satellite and airborne retrievals. The strong absorption of the biomass burning plumes in the visible range is generallyconsistent with the observations from the Aerosol Robotic Network (AERONET)ground-based sun photometers. This, however, shows a significantincrease in the particles' absorption at 440 nm in northern Namibia andAngola, which indicates more absorbing organic species within the observedsmoke plumes. Biomass burning aerosols are also vertically collocated, withsignificant amounts of water content up to the top of the plume at around 6 km height in our measurements. The detailed characterization of aerosol and cloud properties, water vapour, and their uncertainties obtained from OSIRIS and PLASMA2 measurements allows us to study their impacts on the AAC DRE. The high-absorbingload of AAC, combined with high cloud albedo, leads to unprecedentedDRE estimates, which are higher than previous satellite-based estimates. The average AAC DRE calculated from the airborne measurements in the visible range is +85 W m−2 (standard deviation of 26 W m−2), with instantaneousvalues up to +190 W m−2 during intense events. These high DREvalues, associated with their uncertainties, have to be considered as newupper cases in order to evaluate the ability of models to reproduce the radiative impact of the aerosols over the southeastern Atlantic region.

atmospheric chemistry and physics

Aerosol above-cloud direct radiative effect and properties in the Namibian region during the AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) field campaign – Multi-Viewing, Multi-Channel, Multi-Polarization (3MI) airborne simulator and sun photometer measurements