24SMOI Fog Structure and Stability at Sable Island in July 2022

The Fog And Turbulence In Marine Atmosphere (FATIMA) field project took place during July 01-31 2022 over Sable Island located 160 km off Nova Scotia. FATIMA measurements on the island included many special in-situ observations including radiosondes and the profiles from remote sensing platforms as ceilometers and lidars. Fog occurrences and their associated synoptic scale forcing were examined. A fog period was defined as when the visibility measured at the Environmental Canada/Parks Canada operation weather station was less than or equal to 1 km and any increases to above 1 km lasted less than 1 hour. In July 2022, fog at Sable Island occurred 17.6 % of the time in 28 periods with durations distributed from 22 minutes to 14.5 hours and a median of 3.3 hours. All but one of the fog periods occurred within a distinctive pattern of low visibility with a low cloud ceiling. Fog occurred in saturated surface layers with tops at 120 – 700 m. These saturated fog layers consisted of 1 to 3 sub layers. For fog periods initiating under a cyclonic setting, the lower portion of the saturated fog layer was an air temperature inversion based in the lower 50 m while the upper portion of the saturated fog layer was isothermal or had a near moist adiabatic lapse rate. Fog periods initiating under an anticyclonic setting had a single saturated fog/cloud layer that was lower than that for cyclonic cases and was isothermal or had a near moist adiabatic lapse rate. The Gradient Richardson numbers as well as the thermal component and the wind shear components in fog/cloud layers had great variability. Most fog layers tended be least stable in the lowest layers due to wind shear and more stable in the upper portion in the upper portion of the saturated inversion due to the greater thermal component. At intermediate levels within the fog/cloud layer, there could be bands where the thermal component was greater next to a band where the shear component was greater and thus appeared to be transient. The nature of the turbulence is discussed as a possible cause of mixing moisture vertically under stable conditions to convert unsaturated layers to saturated.