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

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.