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 t he Monterey Area Ship Tracks experiment. On 13 June 1994 a merchant vessel, the Sanko Peace, was observed steaming below a layer of broken stratus clo uds in a shallow (300 m deep) boundary layer. These clean maritime clouds w ere 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% reduc tion in average droplet size and drizzle flux. Less than an hour downwind o f the Sanko Peace, the cloud top in the ship track was observed to be eleva ted by 100 m above the surrounding cloud tops. A one-dimensional (1-D) model with size-resolved cloud microphysics, a deta iled radiation code, and a turbulence closure scheme is used to simulate th e observed ship track. The model predicts the observed microphysical change s reasonably well and also predicts a deepening of the boundary layer, thou gh the simulated deepening is considerably slower than was observed. The dr iving force in the modelled response is an increased buoyancy flux due to e nhanced radiative cooling in the cloud layer. The modelled boundary layer d eepens in response to an increase in cloud water, which results from suppre ssion of the drizzle flux. Differences between the modelled and observed pr ofiles 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.