WRF Modeling of Ice Fog Formation at Fairbanks and Fort Wainwright, Alaska

Ice fog formation in the Fairbanks, Alaska vicinity has typically been observed during wintertime conditions when surface air temperatures are below -20 C with relative humidity close to saturation (100%), low wind speed, and a persistent low-level inversion. Simulations of ice fog in Fairbanks were conducted with the Weather Research and Forecast (WRF) model, with a modified Thompson cloud-physics scheme including homogeneous ice particle formation, particle size distribution, and settling rate modifications. The simulated temperatures in January 2000 ranged from -42 C to -30 C, with some ice fog produced from the open water source of the Tanana River. Added water vapor from simulated external sources near the surface produced an increase in ice fog particle density within a drift radius under 2 km from the source. Increasing the vapor input by a factor of 8 produced a comparable increase in ice fog density in the immediate vicinity. Distributing an equal volume of water vapor over a larger source area, up to 5 km x 5 km, reduced the peak fog density, but spread out the fog over the larger area. Hourly observed weather data for Fairbanks International Airport and Fort Wainwright stations over years 1995-2014 were also analyzed to determine the frequency of occurrence of meteorological conditions associated with ice-fog formation. The conditions of air temperatures of -20 C or below occurred during 50% of the observations in the December through February timeframe. The meteorological conditions suitable for ice fog formation, if given a sufficient external source of water vapor, occurred in Fairbanks up to as much as 90 hours per year during this period. We will discuss the state of the science concerning such ice fog formation and climatology, including commentary on the WRF modifications and simulations in the context of this study as well as on possible desirable steps towards improving the state of the science and predictability of ice fog.