Lightning used to follow ship-tracks in Eastern Mediterranean winter thunderstorms

The interaction between aerosol particles and thunderstorm evolution and properties is complex and was studied by direct observational campaigns, remote sensing from space and through numerical simulations. Aerosols invigorate convection and can lead to enhanced charging manifested in more lightning, but they can also lead to a “Boomerang Effect” where too large concentrations of particles lead to diminished vertical development and weaker electrical activity. The effects of ship exhaust on ocean cloudiness have been studied intensively in recent years, following the discovery of prolonged ship tracks in oceanic regions where maritime transportation is most heavy, leading to large-scale changes in albedo and reduced precipitation. Recently it was shown that aerosols emitted by ships also tend to increase lightning activity, by modifying the dynamics and microphysics of clouds formed directly above the busiest shipping lanes. Here, we study the effects of ship-emitted aerosols on thunderstorms in one of the busiest shipping routes in the world: the Mediterranean Sea between the Suez Canal and the Gibraltar Straights. This region hosts hundreds of ships daily, and space observations show considerable enhancement of the Aerosol Optical Depth (AOD) and sulphate concentrations there, some from land sources and others directly related to maritime transportation. The research utilized 14 winter months of lightning detection networks data (ENTLN, from 2018 to 2022) and studied the properties of lightning with respect to sulphate concentrations and cloud properties. The results were divided between before and after the International Maritime Organization (IMO) regulation change in January 2020 that curbed sulphate emissions from 3.5 % to 0.5 %. We show a marked increase in winter lightning activity over the main east-west shipping lanes. That spatial enhancement is all but gone following the reduction in sulphate emissions from ships from February 2020, and on average, clouds became shallower with a thinner charging layer, testifying to the role of aerosol particles in convective invigoration and lightning generation.