Observations of Boundary Layer Evolution Downwind of a Remote, Small Island during the FATIMA 2022 Campaign

The Fog and Turbulence in the Marine Atmosphere (FATIMA) Project executed a field campaign in the northwest Atlantic Ocean near Sable Island (SI), Nova Scotia in July and August 2022. SI is a low relief crescent-shaped, sandy island approximately 40 km long, 1.5 km wide (at its widest point), and is approximately 300 km east of Halifax on the edge of the continental shelf. The over-arching scientific objective of FATIMA is to characterize the upper ocean and atmospheric controls on marine fog and to better understand the physics and microphysics governing the fog life cycle. The remote observing station set-up on SI was complimented by a retrofitted supply vessel, the PSV Atlantic Condor, equipped for atmospheric and oceanographic sampling.

This presentation will focus on the island- and ship-coordinated sampling mission executed on July 25^th to investigate fog-island interactions within the marine atmospheric boundary layer (MABL). In the days preceding the 25^th, high resolution numerical forecasts predicted a fog event with reduced visibility across the Sable Banks portion of the Nova Scotian shelf. However, these forecasts also reported a small region of high visibility (clear surface conditions: no fog) on the lee side of SI and extending to the northeast several island widths downstream. This feature persisted over several forecast updates leading up to the 25^th; it was also supported by anecdotal evidence from SI in earlier fog events. It was hypothesized that this feature was caused by wind-fog-island interactions leading to enhanced turbulent mixing downstream of the island causing a localized evaporation of fog liquid water in the island’s lee and generating an apparent shadow, or wake, in the fog.

To investigate the atmosphere-island-fog interactions, the Atlantic Condor was deployed in the lee of SI to survey the downstream area for ~21 hours. This was complimented by continuous measurements on the island and a ship-shore coordinated rawindsonde launch schedule to probe the boundary layer profile. The atmospheric state on the 25^th was largely dominated by a low-level southwesterly jet driving winds >20 m/s at 200 m, with 10-m winds of 10 m/s. Localized shear in the horizontal mean wind speed at the surface and within the lower MABL was attributed to the island effect on the flow. This was likely caused by the change in surface roughness, and temperature as the day continued, but localized to within 20 km of shore and the lowest 50 meters of profile. Using the ship’s fog liquid water and microphysical sampling, fog was possibly observed before/after sunrise/sunset, but without a strong indication the ship crossed a fog-clear surface boundary condition in the lee of SI during its surveying. Anecdotal evidence and sounding profiles reveal multi-level cloud structure above the MABL.

Further analysis will focus on connecting the observed spatio-temporal variance observed from ship and island to typical internal boundary layer scaling theory. This presentation will also include a discussion of the results within the context of the larger scale MABL evolution across the continental shelf boundary, where regime changes in surface temperature, waves, and currents exert strong forcing on the MABL.