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Discussion papers
https://doi.org/10.5194/acp-2019-738
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-738
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 13 Sep 2019

Submitted as: research article | 13 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Open cells can decrease the mixing of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

Steven J. Abel1, Paul A. Barrett1, Paquita Zuidema2, Jianhao Zhang2, Matt Christensen3, Fanny Peers4, Jonathan W. Taylor5, Ian Crawford5, Keith N. Bower5, and Michael Flynn5 Steven J. Abel et al.
  • 1Met Office, Fitzroy Road, Exeter, EX1 3PB, UK
  • 2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA
  • 3Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Oxford, UK
  • 4College of Engineering, Mathematics, and Physical Sciences, University of Exeter, Exeter, UK
  • 5Centre for Atmospheric Science, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK

Abstract. This work presents synergistic satellite, airborne and surface based observations of a Pocket of Open Cells (POC) in the remote south-east Atlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case-study is that an extensive free-tropospheric biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion over the POC and the surrounding closed cellular cloud regime. The in-situ measurements show marked contrasts in the boundary layer thermodynamic structure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field.

The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of the POC, with elevated carbon monoxide, black carbon mass loadings and accumulation mode aerosol concentrations measured beneath the trade-wind inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed across the boundary into and deep within the POC. The sub-cloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in the POC formed within an ultra-clean layer (accumulation mode aerosol concentrations ~ few cm−3) in the upper region of the boundary layer, that was likely to have been formed via efficient collision-coalescence and sedimentation processes. Enhanced Aitken mode aerosol concentrations were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer inversion and immediately above the ultra-clean layer, accumulation mode aerosol concentrations were ~ 1000 cm−3. Importantly, the airmass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the transport of aerosol from the free-troposphere down into the marine boundary layer.

We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September MODIS Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may often come into contact with the underlying boundary layer cloud. If the results from the observational case-study applied more broadly, then the apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important consequences for aerosol-cloud interactions in the region.

Steven J. Abel et al.
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Short summary
In-situ measurements of a free-tropospheric (FT) biomass burning aerosol plume in contact with the boundary layer inversion overriding a Pocket of Open Cells (POC) and surrounding stratiform cloud are presented. The data highlight the contrasting thermodynamic, aerosol and cloud properties in the two cloud regimes and further demonstrate that the cloud regime plays a key role in regulating the flow of FT aerosols into the boundary layer, which has implications for the aerosol indirect effect.
In-situ measurements of a free-tropospheric (FT) biomass burning aerosol plume in contact with...
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