16 Oct 2020

16 Oct 2020

Review status: a revised version of this preprint was accepted for the journal ACP.

Aerosol and cloud properties in the Namibian region during AEROCLO-sA field campaign: 3MI airborne simulator and sun-photometer measurements

Aurélien Chauvigné1, Fabien Waquet1, Frédérique Auriol1, Luc Blarel1, Cyril Delegove1, Oleg Dubovik1, Cyrille Flamant2, Marco Gaetani2,3,6, Philippe Goloub1, Rodrigue Loisil1, Marc Mallet4, Jean-Marc Nicolas1, Frédéric Parol1, Fanny Peers5, Benjamin Torres1, and Paola Formenti6 Aurélien Chauvigné et al.
  • 1Univ. Lille, CNRS, UMR 8518 – LOA – Laboratoire d'Optique Atmosphérique, F-59000 Lille, France
  • 2LATMOS, UMR CNRS 8190, Sorbonne Université, Université Paris-Saclay, Institut Pierre Simon Laplace, Paris, France
  • 3Scuola Universitaria Superiore IUSS, Pavia, Italy
  • 4CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 5CEMPS, University of Exeter, Exeter, EX4 4QE, UK
  • 6LISA, UMR CNRS 7583, Université Paris-Est-Créteil, Université de Paris, Institut Pierre Simon Laplace, Créteil, France

Abstract. We analyse of the airborne measurements of above-cloud aerosols from the AEROCLO-sA field campaign performed in Namibia during August and September 2017. 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 OSIRIS, was deployed on-board the Safire 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 AAC and cloud properties, with well-defined uncertainties. OSIRIS was supplemented with the airborne multi-wavelength sun-photometer PLASMA2. The application of the algorithm developed for the POLDER spaceborne instrument in the visible range to the OSIRIS measurements allowed to characterise the Aerosol Above Cloud (AAC) properties. The variations of the aerosol properties are consistent with the different atmospheric circulation regimes observed during the deployment. Airborne observations typically a show strong Aerosol Optical Depth (AOD, up to 1.2 at 550 nm) of fine mode particles from biomass burning (extinction Angström exponent varying between 1.6 and 2.2), transported above a stratocumulus deck (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 % with the PLASMA2 sun-photometer measured in the same environment.

The AEROCLO-sA campaign-average Single Scattering Albedo (SSA) obtained by OSIRIS at 550 nm is 0.87. The strong absorption of the biomass burning plumes in the visible is consistent with the observations from the AERONET ground-based sun-photometers. The latter indicate a significant increase of the absorption at 440 nm, showing possible additional presence of absorbing organic species within the smoke plumes. Biomass burning aerosols are also vertically collocated with significant amounts of water content up to the top of the plume around 6 km height.

The average AAC Direct Radiative Effect (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 +200 W m−2 during intense events. Combination between water vapour and the strong positive aerosol forcing over the region explains possible feedbacks on cloud development. This new set of data represents a new opportunity to better constrain climate models and to study aerosol–cloud–radiation interactions over the South-East Atlantic region.

Aurélien Chauvigné et al.

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Status: final response (author comments only)
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Aurélien Chauvigné et al.

Aurélien Chauvigné et al.


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Short summary
The work presents aerosol above cloud properties close to the Namibian coast from a combination of airborne passive remote sensings. The complete analyse of aerosol and cloud optical properties as well as their microphysical and radiative properties allow us to better identify impacts of biomass burning emissions. This work also give a complete view of key-parameters to constrain climat models in case aerosol and cloud coexist in the troposphere.