Understanding the reduction of low clouds in the smoky boundary layer of the remote SE Atlantic using LASIC observations and regional climate modeling

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Description

Many previous observational studies of the southeast Atlantic have emphasized an increase in low cloud cover when shortwave-absorbing aerosols are present in the free-troposphere, resulting in an overall climate cooling. Recent field measurements reveal that smoke is also often present in the boundary layer of the remote southeast Atlantic, most evident in August. We analyze measurements from the months of August 2016 and 2017, when near-surface black carbon loadings are at their annual peak and the cloud cover is not overcast. The composite analysis compares conditions between the upper and lower terciles of near-surface black carbon mass concentrations. The low cloud cover decreases on average, when more smoke is present in the boundary layer. The daily-mean surface-based mixed layer is warmer by approximately 0.5 K on average when the boundary layer is smokier, maintained through the night. We document the following diurnal cycle when more black carbon is measured near the surface: after sunrise, the sub-cloud layer dries while the smokier boundary layer becomes more well-mixed in temperatures and moistures, with increasing liquid water paths, increasing cloud top heights, and a rising trade-wind inversion indicating increased vertical transport of moisture to the cloud layer. The sub-cloud layer continues to warm during the day, for a diurnal range of 0.8K versus 0.55K for the less smoky tercile, with their difference most evident in the late afternoon. Late-afternoon cloud reductions are more pronounced under smokier conditions. After sunset, more sub-cloud moisture accumulates while the cloud base rises, indicating a stronger decoupling. This accumulated sub-cloud moisture is then able to ventilate after sunrise. The free troposphere is often also smoky when the boundary layer is smoky, with stronger, more easterly winds, and cooler, better-mixed potential temperatures near cloud top. The weakened cloud top inversion indicates a convolving meteorological influence, and will encourage entrainment of more smoke into the already smoky boundary layer, increasing the longevity of boundary layer smoke events. The top-of-atmosphere all-sky albedo in the remote southeast Atlantic decreases because of the low cloud reduction, despite a roughly estimated increase of 10 Wm-2 from aerosol alone. We then use regional climate model (WRF-CAM5) simulations to assess if models are able to reproduce the smoke impacts and what model configuration better represents these impacts. Simulations turning smoke emissions on and off will be analyzed to disentangle smoke effects from meteorological co-variability associated with smoke events. We expect this analysis will help reduce uncertainties in assessments of aerosol-cloud-radiation interactions over the SE Atlantic.