For the first time, a large eddy simulation (LES) coupled to a bulk aerosolscheme is used to simulate an aircraft-sampled ship track. The track wasformed by the M/V on 13 June 1994 in a shallow drizzlingboundary layer with high winds but very low background aerosol concentrations(10 cm). A Lagrangian framework is used to simulate the evolution ofa 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 obtainedbetween the simulated and observed features of the ambient boundary layeroutside the track, including the organization of the cloud into mesoscalerolls. After 8 h, a line of aerosol is injected to start the shiptrack. The simulation successfully reproduces the significant albedoenhancement and suppression of drizzle observed within the track. The aerosolconcentration within the track dilutes as it broadens due to turbulentmixing. A sensitivity study shows the broadening rate strongly depends on thealignment between the track and the wind-aligned boundary layer rolls, assatellite images of ship tracks suggest. Entrainment is enhanced within thesimulated track, but the observed 100 m elevation of the ship track abovethe surrounding layer is not simulated, possibly because the LES quicklysharpens the rather weak observed inversion. Liquid water path within thesimulated track increases with time even as the ambient liquid water path isdecreasing. The albedo increase in the track from liquid water and cloudfraction enhancement (second indirect effect) eventually exceeds that fromcloud droplet number increases (first indirect or Twomey effect). In asensitivity study with a higher initial ambient aerosol concentration,stronger ship track aerosol source, and much weaker drizzle, there is lessliquid water inside the track than outside for several hours downwind,consistent with satellite estimates for such situations. In that case, theTwomey effect dominates throughout, although, as seen in satellite images,the albedo enhancement of the track is much smaller.
展开▼
