Aerosol dynamics in ship tracks
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Authors
Russell, Lynn M.
Seinfeld, John H.
Flagan, Richard C.
Ferek, Ronald J.
Hegg, Dean A.
Hobbs, Peter V.
Wobrock, Wolfram
Flossmann, Andrea I.
O'Dowd, Colin D.
Nielsen, Kurt E.
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Date of Issue
1999-12-27
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Abstract
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.
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Meteorology
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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.
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Journal of Geophysical Research, Vol. 104, No. D24, pp. 31,077-31,095, December 27, 1999.
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.