28 Jul 2022
28 Jul 2022
Status: this preprint is currently under review for the journal ACP.

Aerosol first indirect effect of African smoke in marine stratocumulus clouds over Ascension Island, south Atlantic Ocean

Martin de Graaf1, Karolina Sarna2, Jessica Brown3, Elma Tenner2, Manon Schenkels4, and Dave Donovan1 Martin de Graaf et al.
  • 1Royal Netherlands Meteorological Institute (KNMI) R&D Satellite Observations, De Bilt, The Netherlands
  • 2Geosciences & Remote Sensing Department, Delft University of Technology (TUD), Delft, The Netherlands
  • 3Wageningen University, Meteorology and Air Quality Department, Wageningen, The Netherlands
  • 4Utrecht University, Institute for Marine and Atmospheric Research, Utrecht, The Netherlands

Abstract. The first indirect or Twomey effect was measured in marine stratocumulus clouds over the south Atlantic Ocean. Measure- ments were collected over Ascension Island, a remote spot between the African and South American continents. This area is known for its persistent broken cloud cover and smoke intrusions from vegetation fires in Africa during the monsoonal dry period. The interactions between aerosols and clouds are among the least understood climatic processes and were studied over Ascension using a combination of in-situ and remote sensing instruments. Particularly, a new method using a ground- based UV-polarisation lidar to infer cloud droplet sizes and droplet number concentrations was tested against more traditional radar-radiometer measurements. The lidar measurements show to be robust and at least as accurate as the lidar-radiometer measurements and have the large advantage of depending on a single instrument. The UV-lidar was deployed on Ascension for one month in the summer of 2016 and one month in the summer of 2017. In 2016, the presence of smoke in the troposphere decreased the effective cloud droplet size and increased the average droplet number distribution. In 2017, alignment problems of the lidar prohibited conclusions about a Twomey effect. The cloud microphysical properties showed differences between the two years depending on the meteorological circumstances.

Martin de Graaf et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-473', Anonymous Referee #1, 30 Aug 2022
  • RC2: 'Comment on acp-2022-473', Anonymous Referee #2, 15 Oct 2022

Martin de Graaf et al.

Martin de Graaf et al.


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Short summary
Simultaneous measurements were performed of cloud droplet sizes and smoke particles in and near clouds over the Ascension Island, a remote island in the Atlantic Ocean to test the hypothesis that more smoke particles will result in more but smaller cloud droplets. This was done during the dry season in Africa when vegetation fires produce smoke plumes drifting over the ocean, affecting the cloud droplet sizes over the ocean and the island.