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Discussion papers
https://doi.org/10.5194/acp-2019-189
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-189
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Mar 2019

Submitted as: research article | 06 Mar 2019

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Satellite inference of water vapor and aerosol-above-cloud combined effect on radiative budget and cloud top processes in the Southeast Atlantic Ocean

Lucia T. Deaconu1,2, Nicolas Ferlay2, Fabien Waquet2, Fanny Peers3, François Thieuleux2, and Philippe Goloub2 Lucia T. Deaconu et al.
  • 1Department of Physics,University of Oxford, OX1 3PU, Oxford, UK
  • 2Université de Lille, CNRS, UMR 8518, LOA – Laboratoire d’Optique Atmosphérique, 59000 Lille, France
  • 3College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK

Abstract. Aerosols have a direct effect on the Earth's radiative budget and can also affect the cloud development and lifetime. The aerosols above clouds (AAC) are particularly associated with high uncertainties in global climate models. Therefore, it is prerequisite to improve the description and understanding of these situations. During the austral winter, large loadings of biomass burning aerosols originating from fires in the southern African subcontinent are lifted and transported long distances westwards, across the Southeast Atlantic Ocean. The negligible wet scavenging of these absorbing aerosols leads to a near-persistent smoke layer above one of the largest stratocumulus cloud deck on the planet. Therefore, the Southeast Atlantic region is a very important area for studying the impact of above cloud absorbing aerosols, their radiative forcing and their possible effects on clouds.

In this study we aim to analyse and quantify the effect of smoke loadings on cloud properties using a synergy of different remote sensing technics from A-Train retrievals (methods based on the passive instruments POLDER and MODIS and the operational method of the spaceborne lidar CALIOP), collocated with ERA-Interim re-analysis meteorological profiles. To analyse the possible mechanisms of AACs effects on cloud properties, we developed a high and low aerosol loading approach, that consists in evaluating the change in radiative quantities (i.e. cloud top cooling, heating rate vertical profiles) and cloud properties with the smoke loading. During this analysis, we account for the variation in the meteorological conditions over our sample area.

The results show that the region we focus on is primarily under the energetic influence of absorbing aerosols, leading to a significant positive shortwave direct effect at the top of the atmosphere. For larger loads of AACs, clouds are optically thicker, with an increase in liquid water path of 20 g m−2 and lower cloud top altitudes by 100 m. These results do not contradict the semi-direct effect of above cloud aerosols, explored in previous studies. Furthermore, we observe a strong correlation between the aerosol and the water vapor loadings, which has to be accounted for. A detailed analysis of the heating rates shows that the absorbing aerosols are 90 % responsible for warming the ambient air where they reside, with approximately + 5.7 K/day, while the accompanying water vapour above clouds has a longwave effect of + 4.7 K/day (equivalent to 7 % decrease) on the cloud top cooling. We infer that this decreased cloud top cooling in particular, in addition with the higher humidity above the clouds, might modify the cloud top entrainment rate and its effect, leading to thicker clouds. Therefore, smoke (the combination of aerosol and water vapor) events would have the potential to modify and probably reinforce the underlaying cloud cover.

Lucia T. Deaconu et al.
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Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Lucia T. Deaconu et al.
Lucia T. Deaconu et al.
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Short summary
We analyse and quantify the effect of above cloud aerosol (AAC) loadings on the underlying cloud properties in the South Atlantic Ocean. We use a synergy of remote sensing retrievals collocated with ERA-Interim meteorological profiles. The results show that for larger loads of AACs, clouds are optically thicker, with an increase in liquid water path of 20 g.m-2 and lower cloud top altitudes by 100 m. We also observe a strong correlation between the aerosol and the water vapor loadings.
We analyse and quantify the effect of above cloud aerosol (AAC) loadings on the underlying cloud...
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