Analysis of ship emission effects on clouds over the southeastern Atlantic using geostationary satellite observations

This study investigates the impact of ship emissions on clouds over a shipping corridor in the southeastern Atlantic, employing geostationary-based observations, which have not been previously used in studies of this kind. Based on CLAAS-3, the 20-year (2004–2023) CLoud property dAtA set using SEVIRI (the geostationary Spinning Enhanced Visible and InfraRed Imager), the diurnal, seasonal and long-term corridor effects on clouds are examined. Results show a significant impact of ship emissions on cloud microphysics, consistent with the Twomey effect: an increase in cloud droplet number concentration (Nd) and a decrease in effective radius (re). Additionally, cloud liquid water path (W) decreases, though changes in cloud fraction are more subtle. No clear impact on cloud optical thickness is found, implying an overall minor radiative effect of the ship emissions, although methodological limitations to detect changes in the corridor cannot be excluded. Seasonal and diurnal variations of the impact are evident, influenced by regional conditions and by the cloud thinning during the day, respectively. The long-term analysis reveals a weakening of the shipping corridor effect on Nd and re, presumably following the International Maritime Organization’s 2020 stricter regulations on sulfur emissions, and broader regional changes in W and cloud fraction, associated with sea surface temperature variations. Focusing on a climatically important cloud regime, and including novel aspects, namely the diurnal and full seasonal cycle analyses, this study highlights the advantages and potential of geostationary satellite-based cloud observations for studying aerosol–cloud interactions.