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
Journal Article
Abstract. Estimates of the direct radiative forcing (DRF) from absorbing smoke aerosols over the Southeast Atlantic Ocean (SAO) requires simulation of the microphysical and optical properties of stratocumulus clouds (Sc) as well as of the altitude and shortwave (SW) optical properties of biomass burning aerosols (BBA). In this study, we take advantage of the large number of observations acquired during the ORACLES-2016 and LASIC projects during September 2016 and compare them with datasets from the ALADIN-Climate regional model. The model provides a good representation of the liquid water path (LWP) but the low cloud fraction (LCF) is underestimated compared to satellite data. The modeled total column smoke aerosol optical depth (AOD) and Above Cloud AOD (ACAOD) are consistent (~ 0.7 over continental sources and ~ 0.3 over SAO at 550 nm) with MERRA2, OMI or MODIS data. The simulations indicate smoke transport over SAO occurs mainly between 2 and 4 km, consistent with surface and aircraft lidar observations. The BBA single scattering albedo (SSA) is slightly overestimated compared to AERONET, and more significantly when compared to Ascension Island surface observations. The difference could be due to the absence of internal mixing treatment in the ALADIN-Climate model. The SSA overestimate leads to underestimate the simulated SW radiative heating compared to ORACLES data. For September 2016, ALADIN-Climate simulates a positive (monthly mean) SW DRF of about +6 W m−2 over SAO (20° S–10° N and 10° W–20° E) at the top of the atmosphere (TOA) and in all-sky conditions. Over the continent, the presence of BBA is shown to significantly decrease 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) over Angola, Zambia and Congo notably. The surface cooling and the lower tropospheric heating tends to decrease the continental planetary boundary layer (PBL) height by about ~ 200 m.;
