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

Abstract. This study investigates the impact of ship emissions on clouds over a shipping corridor in the southeastern Atlantic. Using 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 (N_d) and a decrease in effective radius (r_e). Additionally, cloud liquid water path (W) decreases, though changes in cloud fraction are more subtle. Seasonal and diurnal variations of the impact are also 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 N_d and r_e 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. The methodology developed for this analysis benefits from the spatially constrained emissions in the shipping corridor, which enhance the detectability of corresponding effects on clouds. Focusing on a climatically important cloud regime, this study highlights the potential of geostationary satellite-based cloud observations for similar analyses.