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 199 4, provides an unprecedented data set for evaluating our understanding of t he formation and persistence of the anomalous cloud features that character ize ship tracks. The data set includes conditions in which the marine bound ary layer is both clean and continentally influenced. Two case studies duri ng the MAST experiment are examined with a detailed aerosol microphysical m odel 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 specifi c effects of precipitation. Predictions and observations reveal important d ifferences between clean (particle concentrations below 150 cm(-3)) and con tinentally 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 effectiv e radii are consistent with observations although there is significant unce rtainty in the absolute concentrations due to a lack of measurements of the plume dilution. Gas-to-particle conversion of sulfur species produced by t he combustion of ship fuel is predicted to be important in supplying solubl e aerosol mass to combustion-generated particles, so as to render them avai lable as CCN. Studies of the impact of these changes on the cloud's potenti al to precipitate concluded that more complex dynamical processes must be r epresented to allow sufficiently long drop activations for drizzle droplets to form.