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Abstract. Estimates of the direct radiative effect (DRE) from absorbing smoke aerosolsover the southeast Atlantic Ocean (SAO) require simulation of themicrophysical and optical properties of stratocumulus clouds as well as ofthe altitude and shortwave (SW) optical properties of biomass burningaerosols (BBAs). In this study, we take advantage of the large number ofobservations acquired during the ObseRvations of Aerosols above Clouds and theirintEractionS (ORACLES-2016) and Layered Atlantic SmokeInteractions with Clouds (LASIC) projects duringSeptember 2016 and compare them with datasets from the ALADIN-Climate(Aire Limitee Adaptation dynamique DeveloppementInterNational) regional model. The model provides a good representation of the liquid waterpath but the low cloud fraction is underestimated compared to satellitedata. The modeled total-column smoke aerosol optical depth (AOD) and above-cloudAOD are consistent ( ∼0.7 over continental sources and ∼0.3 over the SAO at 550 nm) with the Modern-Era Retrospective analysis forResearch and Applications version 2 (MERRA-2), Ozone Monitoring Instrument (OMI)or Moderate Resolution Imaging Spectroradiometer (MODIS) data. Thesimulations indicate smoke transport over the SAO occurs mainly between 2 and 4 km,consistent with surface and aircraft lidar observations. The BBA singlescattering albedo is slightly overestimated compared to the Aerosol RoboticNetwork (AERONET) and more significantly when compared to Ascension Islandsurface observations. The difference could be due to the absence of internalmixing treatment in the ALADIN-Climate model. The SSA overestimate leads toan underestimation of the simulated SW radiative heating compared to ORACLESdata. ALADIN-Climate simulates a positive (monthly mean) SW DRE of about +6  W m −2 over the SAO (20 ∘  S–10 ∘  N and10 ∘  W–20 ∘  E) at the top of the atmosphere and in all-skyconditions. Over the continent, the presence of BBA is shown to significantlydecrease the net surface SW flux, through direct and semi-direct effects,which is compensated by a decrease (monthly mean) in sensible heat fluxes( −25  W m −2 ) and surface land temperature ( −1.5 ∘ C) overAngola, Zambia and the Democratic Republic of the Congo, notably. The surfacecooling and the lower tropospheric heating decrease the continental planetaryboundary layer height by about ∼200  m.

atmospheric chemistry and physics

Simulation of the transport, vertical distribution, optical properties and radiative impact of smoke aerosols with the ALADIN regional climate model during the ORACLES-2016 and LASIC experiments