Sea-salt aerosols over the north-east Atlantic: Model simulations of the ACE-2 Second Lagrangian experiment



A one-dimensional ensemble-average model is used in this study to simulate the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) Second Lagrangian experiment, where the same air mass was followed in the trade wind area over the north-east Atlantic during July 1997. The air mass was affected by increased sea surface temperatures and decreased synoptic-scale subsidence on its way southward towards the Canary Islands. This caused the marine boundary layer (MBL) to grow from a shallow layer capped by a stratocumulus deck to a decoupled layer with cumulus clouds forming below the stratocumulus layer.

Several of the meteorological parameters obtained during a control run show reasonable agreement with observations. The model essentially captures the MBL growth, but the increase of the cloud top height occurs more slowly in the simulation than is indicated by the in situ observations. The sea surface temperature increase seems to be the most important factor for the deepening of the MBL.

The model simulates relatively steep vertical gradients in the sea-salt mass concentrations both in the shallow and deeper MBLs, consistent with in situ measurements. Furthermore, the sea-salt particle mass concentrations estimated by the model for most of the altitude levels in the MBL are within 22% of the observed concentrations of sea-salt-related ions. Turbulent mixing by wind shear did not create well-mixed MBLs based on the sea-salt mass concentrations. Vertical gradients in sea-salt number and mass concentrations suggest a reduced influence of sea-salt particles as cloud condensation nuclei. © Royal Meteorological Society, 2004. S. R. Obsborne's contribution is Crown copyright.