Ice fog observations and new parameterization using the Weather Research and Forecasting (WRF) model

Description

Ice fog occurs frequently in aerosol-laden environments during the cold season at high latitudes, affecting the physical and chemical properties of the atmospheric boundary layer. It reduces visibility and is therefore a risk factor for aviation and traffic on the ground. We observed ice fog in interior Alaska at the Eielson Air Force Base and in Fairbanks and modified the Thompson microphysical scheme employed within the Weather Research and Forecasting (WRF) model.

The NCAR Video Ice Particle Sampler probe was used for continuous observations of ice crystal shape, size, and concentrations. We also used microscope slides with formvar (polyvinyl formal) to collect ice fog particles during especially heavy ice fog events during the winter 2012. Ice fog particles were generally smaller and droxtal-shaped (quasi-spherical) for sub 10-micron particles during the heaviest fog periods, while plate-shaped crystals dominated the larger sizes during lighter fog events. Fog ice water content and extinction were calculated and visibility was derived from the extinction values. The measurements further allowed conclusions on particle terminal velocities and mass weighted fall speeds.

The Thompson microphysics scheme was modified with an improved ice nucleation parameterization, size distribution, and gravitational settling. The new ice fog parameterization accounts for significantly higher ice crystal number concentrations; the size distribution of the crystals was changed into a Gamma distribution with the shape factor of -0.5, using the observed distribution. Furthermore, the gravitational settling was adjusted to the time scale of the ice crystals to take to settle to the surface.