https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL116904
*Authors: *Lucas A. McMichael, Peter N. Blossey, Robert Wood, Sarah J. Doherty First published: *15 October 2025* https://doi.org/10.1029/2025GL116904 *Abstract* Aerosol plumes emitted from ships can cause brightening of low clouds. The aerosol plume spreading rate controls what fraction of the cloud may experience brightening. Developing a deeper physical understanding of the mechanisms driving variations in spreading rate could inform the development of plume-spreading parameterizations in global climate models, which may be relevant for assessing the feasibility of Marine Cloud Brightening. In this study, we employ large-eddy simulations of two idealized precipitating stratocumulus cases to investigate the roles of collision-coalescence, cloud droplet sedimentation, and droplet effective radius in the ship track and quantify their individual and combined effects on plume buoyancy anomalies and spreading rates. Our results indicate that cloud droplet sedimentation and collision-coalescence are the primary mechanisms controlling buoyancy and horizontal spreading, whereas the influence of effective radius is negligible. Sensitivity tests indicate that mesoscale circulations can develop within the ship track even in the absence of precipitation suppression. *Plain Language Summary* Ship tracks are locally brightened streaks of low marine clouds evident in satellite imagery caused by particulate emissions from ships. The higher particle number in the aerosol plume increases the number of small droplets, reflecting more sunlight back to space (Twomey effect). Additionally, precipitation formation can be suppressed in ship tracks, which can increase the amount of liquid water and the lifetime of the cloud. It has been suggested that the deliberate creation of ship tracks using salt particles (Marine Cloud Brightening) could be used as a tool to limit climate warming. With such a strategy, particles would be injected from a surface-based point source and the aerosol plume would spread over time. However, plumes can spread at different rates depending on meteorological conditions. This study explores which physical mechanisms are driving spreading rates in precipitating environments. In contrast to previous studies, we find that the spreading rate is not solely determined by the suppression of precipitation in the ship track; the rate at which cloud droplets fall due to gravity is equally as important. These results will be helpful in informing the next generation of models developed to represent plume spreading. *Key Points* The spreading rate of aerosol plumes can be impacted by cloud-aerosol interactions and mesoscale circulations In-plume changes in cloud droplet sedimentation and collision-coalescence have a comparable influence on the plume spreading rate Mesoscale circulations can still develop in the absence of ship-track precipitation suppression *Source: AGU* -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/geoengineering/CAHJsh98%2BfRw3utz428xY4cTHExXxj%2BdC-hVVsErm4FLbTu05hA%40mail.gmail.com.
