Droplet Growth and Its Impact on Visibility During Freezing Fog Events from CFACT

Freezing fog is a type of cold fog that forms when the air temperature (Ta) is below 0℃. Although Ta is below 0℃, the water droplets can remain in a liquid state rather than freezing. Freezing-fog conditions can pose a significant hazard to aviation and marine operations because it can reduce visibility severely, and ice accumulates rapidly on the surfaces such as aircraft, ship, and roads. Observations collected during the CFACT (Cold Fog Amongst Complex Terrain) Project from 7 January – 24 February, representing cold-fog events over Heber Valley of Utah, are used in the analysis. The objectives of this study are to characterize freezing fog microstructure in detail with respect to droplet size distribution, critical diameter related to activation, and visibility. In the analysis, freezing fog (FZFG) and droplet size spectra will be examined theoretically and experimentally. The droplet activation and critical diameter forming in frozen-fog droplets will be revealed using the Köhler curve. The effect of the droplet-growth process on visibility changes for two cold-fog cases is examined and results are discussed. Preliminary analysis suggests that freezing-fog droplet growth strongly depends on environmental conditions, including Ta, relative humidity (RH), and liquid water content (LWC) as well as droplet number concentration (Nd). It is concluded that microphysical parameterizations should investigate freezing-fog droplet formation and growth in more detail because presently it is lacking in NWP predictions.