Global and regional modeling of absorbing aerosols in the southeast Atlantic during ORACLES 2016: comparisons to new observations

Biomass burning smoke influences regional climate through interactions with sunlight and clouds. A good place to observe this is the southeast Atlantic where smoke outflow and boundary-layer clouds are well-defined, abundant and coincident. We compare three model representations of smoke against the 160-hour airborne observations we made there in mostly September 2016. The observed vertical profiles of water vapor and carbon monoxide reveal that the smoke boosts humidity in the otherwise very dry air above sharp inversions at 500 - 1300 m altitudes. The in situ measurements of water isotopes and cloud condensation nuclei confirm that the smoke transported in the free troposphere had barely interacted with clouds. This explains the observed high black carbon and organic aerosol masses relative to carbon monoxide mixing ratio as well as the tight correlation between them. The WRF-CAM5 model skillfully reproduces the correlation. But it systematically underestimates light extinction by the aerosols by 20% and their wavelength dependence by 0.6. The implied error in the model aerosol size distribution can be explained partly by the omission of nitrate and ammonium. The representativeness of the flight days at capturing the monthly-mean is assessed by comparing values from WRF-Chem simulations from the 15 flight days to those from the entire month of September. These indicate that the in-situ measurements sampled slightly more pristine conditions to the northwest than the monthly-mean, but are reasonably representative. The top of smoke is found highest around 14oS by lidar backscattering measurements. This meridional trend is captured by the models near the coast, though less skillfully farther away from the land. Overall, this paper puts forth a framework for evaluating model performances against the observations of aerosols, clouds and radiation made during NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS).