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
Authors: Chauvigné Aurélien , Waquet Fabien , Auriol Frédérique , Blarel Luc , Delegove Cyril , Dubovik Oleg , Flamant Cyrille , Gaetani Marco , Goloub Philippe , Loisil Rodrigue , Mallet Marc , Nicolas Jean-Marc , Parol Frédéric , Peers Fanny , Torres Benjamin , Formenti Paola 
Published: May 2021

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.