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
Résumé
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 20 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 25 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 30 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 35 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 40 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 SouthEast Atlantic region.
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