Microphysics and Optical Attenuation in Coastal Fog

We examined 15 fog events observed in two field campaigns: the High Energy Laser in FOG (HELFOG) project conducted in Marina, California on 13-23 July 2018 and the Toward Improving Coastal Fog Prediction (C-FOG) project in Ferryland, Newfoundland on 1 September - 8 October 2018. In both projects, we made measurements of fog droplet number concentration and aerosol absorption and scattering, and high-rate environmental sampling for three-dimensional winds, temperature, pressure, water vapor, and CO₂ concentration. We also collected optical propagation data with a Wide Area Teleradiometric Transmissometer (WATT), a Vaisala Present Weather Detector (PWD) visibility meter, and a BLS900 scintillometer. Optical attenuation was also calculated using the Mie scattering theory with the measured fog droplet spectra as input. This work shows a comparison of the coastal fog events on both coasts of north America and examines the relationship between fog microphysics and optical extinction represented by the meteorological optical range (MOR), which is generally referred to as visibility.

Our results show consistent characteristics of the summertime advection fog in HELFOG with low number concentration, relatively large droplet size, and frequently a bimodal size distribution peaking at 4 and 10 μm. In contrast, the fog from fall season Newfoundland coast showed strong variabilities in fog microphysical properties where the fog events had high and variable droplet number concentration and small mean droplet sizes. The droplet spectra tend to peak at ~ 4 μm. The concurrent transmission measurements from the transmissometer and the visibility meter gave consistent results in HELFOG. Visibility derived from Mie theory using the measured droplet size distribution represent the general trend of visibility as a function of liquid water content (LWC) or number concentration. A power law fit of the measured relationship is also consistent with some previous research. Large scatter of data is seen in the relationship between visibility and LWC in C-FOG cases mainly due to the presence of rain. There are evidences that the light rain may have promoted fog formation because of the saturated environment associated with rain evaporation below the cloud base.