Fog Prediction by COAMPS during the C-FOG Field Experiment

C-FOG field campaign took place during September 2018 with ground instruments focused over the Avalon Peninsula in Newfoundland (Canada). NRL Marine Meteorology division provided support for intense observation period (IOP) planning activities by running COAMPS, the US Navy operational numerical weather prediction (NWP) model, in real time.

Several fog episodes occurred during the field campaign, which were captured by COAMPS with a varying degree of accuracy. In this presentation, we analyze IOPs in which fog has formed and dissipated, and discuss them from the model performance point of view. A NWP model ideally has to capture accurately physical processes on scales ranging over several orders of magnitude from synoptic scale forcing to fog droplets. We tested both single- and double-moment microphysics parameterizations in COAMPS to assess their impact on visibility parameterizations. In addition to comparing a hydrometeor- based visibility methods, we were able to test a method that uses number concentration and liquid water content and evaluate it against the ground measurements.

A novel synoptic mechanism triggering fog formation has been observed, namely an interaction of a mid-latitude baroclinic system, with a tropical cyclone transitioning to extra-tropical stage. Rapid changes in environmental conditions, especially low-level moisture advection over large sea-surface temperature gradient are essential for fog formation, and at the same time challenging accurate prediction of NWP models.