Frontal Effects on the Rapid Formation of a Deep Layer of Marine Fog and Cloud in the NW Atlantic

A novel approach to the rapid formation of a deep layer of marine fog and cloud in the NW Atlantic is proposed. A deep layer of fog and cloud extending to 2 km height was observed over Sable Island offshore off Nova Scotia in Atlantic Canada on 13 – 14 July 2019. This fog and cloud event was dominantly caused by a fast intrusion of a trough from a cyclone over NE Canada. The trough quickly moved from SW-W to E-NE and encroached on Nova Scotia coastal waters. Due to a warm front in the trough, the air temperature increased up to 4 km height with increased stability while the sea-level pressure at Sable Island dropped by 11 hPa within twelve hours. There was no indication of advection-type fog mechanism since the surface air temperature was not adjusted to the colder sea-surface temperature that could have resulted in saturation of the surface air. The leading portion of a massive cloud band in the trough passing over Sable Island included precipitation before fog formation. The rain evaporated in the unsaturated and warm subcloud layer, effectively reduced the air temperature, and significantly increased the wet-bulb temperature in this layer. The cloud extended into the moist subcloud layer while fog formed at 14 UTC 13 July and lasted until 2327 UTC 13 July. Consequently, the development of this deep layer of fog and cloud in the NW Atlantic was caused by evolving synoptic conditions through the simultaneous effects of a rapid trough intrusion over the coastal waters with a massive cloud band, induced precipitation causing cloud lowering into the subcloud layer, and with surface wind convergence forcing vertical mixing in a stable marine boundary layer.