Modeling the smoky troposphere of the southeast Atlantic: a comparison to ORACLES airborne observations from September of 2016
Journal Article
Overview
abstract
Abstract. In the southeast Atlantic, well-defined smoke plumes from Africa advect over; marine boundary layer cloud decks; both are most extensive around September,; when most of the smoke resides in the free troposphere. A framework is put; forth for evaluating the performance of a range of global and regional; atmospheric composition models against observations made during the NASA; ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS); airborne mission in September 2016. A strength of the comparison is a focus; on the spatial distribution of a wider range of aerosol composition and; optical properties than has been done previously. The sparse airborne; observations are aggregated into approximately 2∘ grid boxes and into; three vertical layers: 3–6 km, the layer from cloud top to 3 km, and the; cloud-topped marine boundary layer. Simulated aerosol extensive properties; suggest that the flight-day observations are reasonably representative of; the regional monthly average, with systematic deviations of 30 % or less.; Evaluation against observations indicates that all models have strengths and; weaknesses, and there is no single model that is superior to all the others; in all metrics evaluated. Whereas all six models typically place the top of; the smoke layer within 0–500 m of the airborne lidar observations, the; models tend to place the smoke layer bottom 300–1400 m lower than the; observations. A spatial pattern emerges, in which most models underestimate; the mean of most smoke quantities (black carbon, extinction, carbon; monoxide) on the diagonal corridor between 16∘ S, 6∘ E, and; 10∘ S, 0∘ E, in the 3–6 km layer, and overestimate them; further south, closer to the coast, where less aerosol is present. Model; representations of the above-cloud aerosol optical depth differ more widely.; Most models overestimate the organic aerosol mass concentrations relative to; those of black carbon, and with less skill, indicating model uncertainties; in secondary organic aerosol processes. Regional-mean free-tropospheric; model ambient single scattering albedos vary widely, between 0.83 and 0.93; compared with in situ dry measurements centered at 0.86, despite minimal impact of; humidification on particulate scattering. The modeled ratios of the particulate; extinction to the sum of the black carbon and organic aerosol mass; concentrations (a mass extinction efficiency proxy) are typically too low; and vary too little spatially, with significant inter-model differences. Most; models overestimate the carbonaceous mass within the offshore boundary; layer. Overall, the diversity in the model biases suggests that different; model processes are responsible. The wide range of model optical properties; requires further scrutiny because of their importance for radiative effect; estimates.;
