Characteristics of Summertime Marine Fog Observed from A Small Remote Island

This research is part of the Fog and Turbulence Interactions in the Marine Atmosphere (FATIMA) project focusing on the physical processes in marine fog and their impact on optical attenuation. This observational study is based on the measurements from Sable Island during the FATIMA Grand Banks field campaign. The analyses used the water droplet size distribution from a fog monitor (FM-120) and the meteorological optical range (MOR) measured by a present weather sensor (PWD22) as the main data source augmented by frequent radiosonde launches. Based on the 35 days of measurements, we identified nine reduced visibility events RVEs). The radiosonde profiles revealed the complex layered thermodynamic structure within and above the fog layers and the frequent presence of high surface winds up to 16 ms-1 and a low-level jet, while the droplet spectra indicated a bimodal distribution below droplet size of 50 m for most RVEs. Although the complete dataset under all weather conditions follows the same general trend of increasing MOR with decreasing liquid water content (LWC), the heavy fog events (visibility below 400 m) show different rates of decrease. We also found that representing both fog and mist using a single power function was unrealistic. Furthermore, the empirical relationship is sensitive to the length of averaging. Therefore, empirical functions derived from the nominal 1-min data may not be suitable for mesoscale models with a much coarser temporal output of LWC.