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Published December 27, 1999 | public
Journal Article

Aerosol dynamics in ship tracks


Ship tracks are a natural laboratory to isolate the effect of anthropogenic aerosol emissions on cloud properties. The Monterey Area Ship Tracks (MAST) experiment in the Pacific Ocean west of Monterey, California, in June 1994, provides an unprecedented data set for evaluating our understanding of the formation and persistence of the anomalous cloud features that characterize ship tracks. The data set includes conditions in which the marine boundary layer is both clean and continentally influenced. Two case studies during the MAST experiment are examined with a detailed aerosol microphysical model that considers an external mixture of independent particle populations. The model allows tracking individual particles through condensational and coagulational growth to identify the source of cloud condensation nuclei (CCN). In addition, a cloud microphysics model was employed to study specific effects of precipitation. Predictions and observations reveal important differences between clean (particle concentrations below 150 cm⁻³) and continentally influenced (particle concentrations above 400 cm⁻³) background conditions: in the continentally influenced conditions there is a smaller change in the cloud effective radius, drop number and liquid water content in the ship track relative to the background than in the clean marine case. Predictions of changes in cloud droplet number concentrations and effective radii are consistent with observations although there is significant uncertainty in the absolute concentrations due to a lack of measurements of the plume dilution. Gas-to-particle conversion of sulfur species produced by the combustion of ship fuel is predicted to be important in supplying soluble aerosol mass to combustion-generated particles, so as to render them available as CCN. Studies of the impact of these changes on the cloud's potential to precipitate concluded that more complex dynamical processes must be represented to allow sufficiently long drop activations for drizzle droplets to form.

Additional Information

This analysis was supported by NSF grant ATM-9732949 and ONR grant N00014-97-1-0673. The aerosol measurements on which this work was based were supported by ONR grant N00014-93-1-0872. Measurements by several coauthors on this work were supported under the ONR Accelerated Research Initiative entitled Surface Ship Cloud Effects. This work would not have been possible without the cooperation and support of the crew of the University of Washington C-131A aircraft and of the RAF crew of the MRF C-130 aircraft. The authors express their gratitude to all members of the MAST Science Team. In addition, the authors appreciate the comments of Doug Johnson, Bjorn Stevens, and two anonymous reviewers who provided helpful suggestions that have improved this work.

Additional details

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October 25, 2023