A case study of boundary-layer development downstream of a small, remote island

Marine fog remains a challenging process for environmental prediction; in coastal areas, these challenges are complicated by the heterogeneity of the land-sea boundary. Here, the findings from an investigation into the boundary-layer development downstream of a remote, low-relief island will be presented. This work arose from an intense observing period (IOP) during the Fog And Turbulence Interactions in the Marine Atmosphere (FATIMA) 2022 field campaign near Sable Island (SI) in the North Atlantic. The premise was to examine a unique fog dissipation mechanism known as a fog shadow, hypothesized to be caused by fog-laden boundary interaction with the island's surface. Using coordinated measurements from the island observing station and a heavily instrumented research vessel, we have conducted an extensive analysis into the spatio-temporal evolution of the atmospheric boundary-layer state downstream of SI. A low-level jet during the IOP prevented the development of an extensive fog layer in the region, and hence localized dissipation in the lee of the island. In terms of boundary-layer development, the measurements captured the rough-smooth internal boundary-layer formation, with some secondary impacts of the temperature adjustment from land to water surfaces. Analysis of the fetch-limited wave growth followed the expected self-similarity, but at a much higher intensity than predicted for the stability regime; this was linked to spatial inhomogeneity in wind acceleration and stress downstream, which may have increased the lateral extent of island impacts on the boundary layer.