TY - JOUR
T1 - Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer
AU - Abel, Steven J.
AU - Barrett, Paul A.
AU - Zuidema, Paquita
AU - Zhang, Jianhao
AU - Christensen, Matt
AU - Peers, Fanny
AU - Taylor, Jonathan W.
AU - Crawford, Ian
AU - Bower, Keith N.
AU - Flynn, Michael
N1 - Funding Information:
Acknowledgements. The CLARIFY deployment was jointly funded by the UK Natural Environment Research Council (NERC) through grant no. NE/L013479/1 and the Met Office. The LASIC field campaign was funded by the US Department of Energy’s Office of Science, Office of Biological and Environmental Research, as part of the Atmospheric Science Research Program. We thank the whole CLARIFY and LASIC operations and science teams for their efforts on deploying and maintaining the instruments and for pro- cessing and calibrating the campaign datasets. We acknowledge the use of imagery from the NASA Worldview application (https: //worldview.earthdata.nasa.gov/, last access: 31 March 2020), part of the NASA Earth Observing System Data and Information System (EOSDIS). The SEVIRI individual channel data used to generate infrared and RGB imagery was obtained from EUMETSAT. We thank Brent Holben for his effort in establishing and maintaining the Ascension Island AERONET site.
Funding Information:
Financial support. Fanny Peers was partly funded by the Research Council of Norway via the projects AC/BC (grant no. 240372) and NetBC (grant no. 244141). Paquita Zuidema and Jianhao Zhang were supported by funding from DOE ASR grant no. DESC0018272.
Publisher Copyright:
© 2020 Copernicus GmbH. All rights reserved.
PY - 2020/4/6
Y1 - 2020/4/6
N2 - 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 subcloud 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 of approximately a few span classCombining double low line"inline-formula">cm-3/span>) in the upper region of the boundary layer, which 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 span classCombining double low line"inline-formula">ĝˆ1/4/span> 1000 span classCombining double low line"inline-formula">cm-3/span>. Importantly, the air mass 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./ span idCombining double low line"page4060"/>We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (span classCombining double low line"inline-formula">>/span> 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./.
AB - 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 subcloud 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 of approximately a few span classCombining double low line"inline-formula">cm-3/span>) in the upper region of the boundary layer, which 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 span classCombining double low line"inline-formula">ĝˆ1/4/span> 1000 span classCombining double low line"inline-formula">cm-3/span>. Importantly, the air mass 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./ span idCombining double low line"page4060"/>We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (span classCombining double low line"inline-formula">>/span> 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./.
UR - http://www.scopus.com/inward/record.url?scp=85083116153&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85083116153&partnerID=8YFLogxK
U2 - 10.5194/acp-20-4059-2020
DO - 10.5194/acp-20-4059-2020
M3 - Article
AN - SCOPUS:85083116153
VL - 20
SP - 4059
EP - 4084
JO - Atmospheric Chemistry and Physics
JF - Atmospheric Chemistry and Physics
SN - 1680-7316
IS - 7
ER -