Acceleration of the southern African easterly jet driven by the radiative effect of biomass burning aerosols and its impact on transport during AEROCLO-sA
Résumé
The direct and semi-direct radiative effects of biomass burning aerosols (BBA) are investigated over southern Africa and the southeast Atlantic during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign in September 2017. A reference convection-permitting simulation has been performed using the Meso-NH model with an on-line dust emission scheme, a BBA tracer emitted using the daily Global Fire Emissions Database and online-computed backward Lagrangian trajectories. The simulation captures both the aerosol optical depth and the vertical distribution of aerosols as observed from airborne and space-borne lidars. It also reproduces the occurrence of stratocumulus over the southeast Atlantic, deep convective clouds over equatorial Africa and the large-scale circulation. In contrast, a sensitivity experiment where the radiative effect of BBA is not taken into account shows the smoke plume that is predicted too low in altitude, low-cloud cover that is too weak, deep convective activity that is too frequent but not intense enough, a Benguela jet that is too strong and a southern African easterly jet that is too weak. The Lagrangian analysis indicates that BBA are transported to higher altitudes, farther southwest and with a stronger diurnal oscillation when accounting of the radiative effects of BBA. The higher smoke plume altitude can be explained by a combination of three factors: increased upward motion induced by the stronger southern African easterly jet, self-lofting of BBA and reduced subsidence associated with a less frequent deep convective activity over equatorial Africa.
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