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| Titel |
Large eddy simulation of ship tracks in the collapsed marine boundary layer: a case study from the Monterey area ship track experiment |
| VerfasserIn |
A. H. Berner, C. S. Bretherton, R. Wood |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 10 ; Nr. 15, no. 10 (2015-05-27), S.5851-5871 |
| Datensatznummer |
250119760
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| Publikation (Nr.) |
 copernicus.org/acp-15-5851-2015.pdf |
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| Zusammenfassung |
For the first time, a large eddy simulation (LES) coupled to a bulk aerosol
scheme is used to simulate an aircraft-sampled ship track. The track was
formed by the M/V Sanko Peace on 13 June 1994 in a shallow drizzling
boundary layer with high winds but very low background aerosol concentrations
(10 cm−3). A Lagrangian framework is used to simulate the evolution of
a short segment of track as it is advected away from the ship for 8 h
(a downwind distance exceeding 570 km).
Using aircraft observations for initialization, good agreement is obtained
between the simulated and observed features of the ambient boundary layer
outside the track, including the organization of the cloud into mesoscale
rolls. After 8 h, a line of aerosol is injected to start the ship
track. The simulation successfully reproduces the significant albedo
enhancement and suppression of drizzle observed within the track. The aerosol
concentration within the track dilutes as it broadens due to turbulent
mixing. A sensitivity study shows the broadening rate strongly depends on the
alignment between the track and the wind-aligned boundary layer rolls, as
satellite images of ship tracks suggest. Entrainment is enhanced within the
simulated track, but the observed 100 m elevation of the ship track above
the surrounding layer is not simulated, possibly because the LES quickly
sharpens the rather weak observed inversion. Liquid water path within the
simulated track increases with time even as the ambient liquid water path is
decreasing. The albedo increase in the track from liquid water and cloud
fraction enhancement (second indirect effect) eventually exceeds that from
cloud droplet number increases (first indirect or Twomey effect). In a
sensitivity study with a higher initial ambient aerosol concentration,
stronger ship track aerosol source, and much weaker drizzle, there is less
liquid water inside the track than outside for several hours downwind,
consistent with satellite estimates for such situations. In that case, the
Twomey effect dominates throughout, although, as seen in satellite images,
the albedo enhancement of the track is much smaller. |
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