Abstract
Abstract. We analyse the airborne measurements of above-cloud aerosols from the AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) field campaign performed in Namibia during August and September 2017. The study aims to retrieve the aerosol above-cloud direct radiative effect (DRE) with well-defined uncertainties. To improve the retrieval of the aerosol and cloud properties, the airborne demonstrator of the Multi-Viewing, Multi-Channel, Multi-Polarization (3MI) satellite instrument, called the Observing 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 10 flights performed over land, over the ocean, and along the Namibian coast. The airborne instrument OSIRIS provides observations at high temporal and spatial resolutions for aerosol above clouds (AACs) and cloud properties. OSIRIS was supplemented with the Photomètre Léger Aéroporté pour la surveillance des Masses d'Air version 2 (PLASMA2). The combined airborne measurements allow, for the first time, the validation of AAC algorithms previously developed for satellite measurements. The variations in the aerosol properties are consistent with the different atmospheric circulation regimes observed during the deployment. Airborne observations typically show strong aerosol optical depth (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 obtained by 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 generally consistent with the observations from the Aerosol Robotic Network (AERONET) ground-based sun photometers. This, however, shows a significant increase in the particles' absorption at 440 nm in northern Namibia and Angola, which indicates more absorbing organic species within the observed smoke plumes. Biomass burning aerosols are also vertically collocated, with significant 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-absorbing load of AAC, combined with high cloud albedo, leads to unprecedented DRE 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 instantaneous values up to +190 W m−2 during intense events. These high DRE values, associated with their uncertainties, have to be considered as new upper cases in order to evaluate the ability of models to reproduce the radiative impact of the aerosols over the southeastern Atlantic region.
Highlights
Aerosols from natural and anthropogenic sources impact the climate directly, by interacting with solar and terrestrial radiations, and indirectly, through interactions with cloud properties (IPCC, 2013)
A first increase in the aerosol loading is observed between 27 August and 1 September 2017 at the Namibe and Windpoort sites (AOD up to 0.88 and 0.73 at 550 nm, respectively), a second increase between 3 and 8 September at the Namibe, Windpoort, and São Tomé sites (AOD up to 1.80, 1.54 and 1.72 at 550 nm, respectively), and a third increase starting on 13 September at the Namibe and Wind-Figure 5
A new data set of cloud and above-cloud aerosol properties allows us to retrieve local aerosol above clouds (AACs) direct radiative effect (DRE) in the southeastern Atlantic region, where important biases persist between climate models for both the amplitude and the sign of the aerosol radiative perturbation
Summary
Aerosols from natural and anthropogenic sources impact the climate directly, by interacting with solar and terrestrial radiations, and indirectly, through interactions with cloud properties (IPCC, 2013). In addition to the aerosol–cloud interaction, the SEA region represents a unique opportunity to study the direct radiative effects of highly absorbing particles transported above the clouds In this region, smoke aerosol plumes can reach high altitudes (up to 6 km) and can be transported for several days in the atmosphere (Samset et al, 2014; Marenco et al, 2016) and overlay low-level clouds which are persistent over water. The regional atmospheric circulation on 8 September (Fig. 2a) represents the mean circulation during this period, with air masses coming from tropical Africa, moving westward until Ascension Island, which are deflected to the southeast due to the anticyclone centred over southern Africa These conditions were observed during most of the AEROCLO-sA campaign, except on 5 September (Fig. 2b) when the anticyclone was centred over the Indian Ocean between the South African coast and Madagascar. The climate model and the airborne lidar measurements (Formenti et al, 2019)
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