Seasonal and Microphysical Characteristics of Fog at a Northern Airport in Alberta, Canada

Reduction in visibility (Vis) due to fog is one of the deadliest severe weather hazards affecting aviation and public transportation. Nowcasting/forecasting of Vis reduction due to fog using current models is still problematic, with most using some type of empirical parameterization. To improve the models, further observational studies to better understand fog microphysics and seasonal variability are required. To help achieve these goals, the seasonal and microphysical characteristics of different fog types at Cold Lake airport (CYOD), Alberta, Canada were analyzed using hourly and sub-hourly METAR data. Microphysical and meteorological measurements obtained using the DMT Fog Monitor FM-120 and the Vaisala PWD22 were examined. The results showed that radiation fog (RF) dominates at CYOD in summer while precipitation, advection and cloud-base-lowering fogs mostly occur in fall and winter. All fog types usually form at night or early morning and dissipate after sunrise. The observed dense fog events (Vis < 400 m) were mainly caused by RF. The observed mean fog particle spectra (n(D)) for different fog types and temperatures showed bimodal n(D) (with two modes near 4 mu m and 17-25 mu m; the maximum total number concentration (N-d) was 100 cm(-3) and 20 cm(-3), respectively, corresponding to each mode). Parameterizations of Vis as a function of liquid water content (LWC) and N-d were developed using both the observed Vis and calculated Vis based on n(D). It was found that the observed Vis was higher than the calculated Vis for warm fog with LWC > 0.1 gm(-3) and most of the mass was contributed by the large drops. Based on the observed Vis, the relative error of the visibility parameterization as a function of both LWC and N-d (32%) was slightly lower than that (34%) using LWC alone for warm fogs.