Simulations of Coastal Fog in the Canadian Atlantic with the Weather Research and Forecasting Model

We evaluate the efficacy of microphysics and planetary-boundary-layer (PBL) parametrizations of the Weather Research and Forecasting (WRF) model for simulation of the coverage and intensity (visibility) of fog during the Coastal Fog (C-FOG) Research Program. The C-FOG observations are used for model validation, particularly focusing on offshore and onshore fog events during 13–14 and 28–29 September 2018. Sensitivity experiments with high horizontal (1 km) and vertical (99 levels) resolution are conducted to elicit possible physical processes underlying the fog life-cycle. Various microphysical and PBL parametrizations, as well as empirical algorithms available for visibility calculations, are evaluated. The model coastal fog formation and characteristics strongly depend on the simulated local meteorology (e.g., temperature, relative humidity, mixing ratio, and wind field) and the microphysical parametrization employed. High space–time resolution simulations for fog coverage and visibility based on the Mansell et al. (J Atmos Sci 67(1):171–194, 2010) microphysical parametrization compare better with data vis-à-vis other microphysical parametrizations, although spatial coverage and visibility are still overestimated. The disparities are likely related to uncertainties of model fog microphysical parameters, including the liquid water content (LWC), droplet number concentration (Nd), and aerosol particle size. It is found that (i) visibility algorithms that use both the variables LWC and Nd, instead of only LWC, provide improved fog estimates; (ii) different PBL parametrizations mainly affect the fog onset and dissipation; (iii) the WRF model has improved performance over the ocean than over land, possibly due to homogeneity of the ocean-surface cover.