Satellite observations of ship emission induced transitions from broken to closed cell marine stratocumulus over large areas

Documentation of the evolution of ship tracks during 42 h demonstrated that ship emissions are able to convert a marine stratocumulus regime of open cells into closed cells, along with significant negative radiative forcing. This was possible by examining continuous day and night geostationary satellite data that allowed for an uninterrupted documentation of the full life cycle of ship tracks. After nearly one day the ship tracks lost their linear appearance and expanded to cover large areas. These areas, when viewed out of sequence and context, would not be attributable to aerosol perturbations. A rejuvenation of previously dissipated ship tracks in the form of extensive closed cells was also observed. It is suggested that ship emissions may undergo photochemical reactions which nucleate new aerosols that, along with remaining ultrafine particles, grow to CCN that are activated hours later and close the open cells. The added radiative forcing from the closed cells that can be related to the ship tracks, which is mainly coming from the cloud cover effect, may exceed - 100 Wm^-2 depending on the season and latitude. This implies that anthropogenic aerosols that can be transported from continents through the boundary layer, or travel in the free troposphere and mix with the boundary layer from above, may explain the formation of large closed cells areas that are presently not recognized as originated by aerosol perturbations. The observations reported here pose a demanding test of the ability of cloud resolving models to replicate cloud-aerosol interactions.