A case-study of pronounced perturbations to cloud properties and boundary-layer dynamics due to aerosol emissions

During June 1994 the UK Meteorological Office C-130 aircraft took part in the Monterey Area Ship Tracks experiment. On 13 June 1994 a merchant vessel, the Sanko Peace, was observed steaming below a layer of broken stratus clouds in a shallow (300 m deep) boundary layer. These clean maritime clouds were significantly modified by the aerosol emissions from the ship. Droplet concentrations increased from around 10 cm(-3) in the background cloud to a peak of 120 cm(-3) in the ship track. This resulted in a roughly 50% reduction in average droplet size and drizzle flux. Less than an hour downwind of the Sanko Peace, the cloud top in the ship track was observed to be elevated by 100 m above the surrounding cloud tops. A one-dimensional (1-D) model with size-resolved cloud microphysics, a detailed radiation code, and a turbulence closure scheme is used to simulate the observed ship track. The model predicts the observed microphysical changes reasonably well and also predicts a deepening of the boundary layer, though the simulated deepening is considerably slower than was observed. The driving force in the modelled response is an increased buoyancy flux due to enhanced radiative cooling in the cloud layer. The modelled boundary layer deepens in response to an increase in cloud water, which results from suppression of the drizzle flux. Differences between the modelled and observed profiles of cloud water in the ship track are difficult to reconcile with the modelled mechanism for the deepening, and are attributed to shortcoming in the 1-D model representation.