The impact of ship-produced aerosols on the microstructure and albedo of warm marine stratocumulus clouds: A test of MAST hypotheses 1i and 1ii

Anomalously high reflectivity tracks in stratus and stratocumulus sheets as sociated with ships (known as ship tracks) are commonly een in visible and near-infrared satellite imagery, Until now there have been only a limited n umber of in situ measurements made in ship tracks. The Monterey Area Ship T rack (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effec ts on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircr aft, the National Aeronautics and Space Administration ER-2 aircraft. the N aval Research Laboratory airship, the Research Vessel Glorita, and dedicate d U.S. Navy ships, participated in MAST in order to study processes governi ng the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that p roduce ship tracks are due to (a) particulate emission from the ship's stac k and/or (b) sea-salt particles from the ship's wake. it was found that shi ps powered by diesel propulsion units that emitted high concentrations of a erosols in the: accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concent rations of particles but at sizes tao small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships) , did not produce ship tracks. Statistics and case studies, combined with m odel simulations, show that provided a cloud layer is susceptible to an aer osol perturbation, and the atmospheric stability enables aerosol to be mixe d throughout the boundary layer. the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.