A Case Study of the Formation, Evolution and Dissipation of Ice Radiation Fog in a Mountain Valley

Radiation fog usually forms by radiative cooling of the Earth's surface under cloud-free skies and very weak wind conditions. It has been studied for decades due to its effect on transportation, traffic and air quality. However, owing to the complexity and heterogeneity of fogs, there are still numerous knowledge gaps remaining in this field. For instance, little is known about the impacts of turbulence and gravity waves on fog formation and dissipation. We present an ice fog case study from a field experiment conducted in Heber City, Utah during 5 January – 15 February 2015, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) project. MATERHORN was designed to better understand atmospheric fluid dynamics across all scales over realistic mountainous terrain as well as under transient and steady conditions.

We use data collected on 9 January 2015 to evaluate relationships between ice fog and turbulence variables. A very shallow (about 10 m) ice fog was observed due to radiative cooling along with the mountain circulation from 0710 MST to 0915 MST. This ice fog event occurred when air temperature were of -9 °C, which is relatively warm for the ice fog to occur. Relative humidity with respect to ice was ≈105% before and during the fog episode. The results show a constant longwave radiative cooling of -45 Wm-2 throughout the night. Observed particle concentrations indicate that that bigger particles (0.1 – 10.0 μm) were formed through the transformation of smaller particles (0.3 – 1.0 μm). An extremely low wind speed (mostly < 0.5 m s-1) was measured as well during the observational period. Quasi-periodic oscillations were observed before and during the fog event with a time period of about 30 minutes. These oscillations were detected in liquid water content, temperature, wind direction, relative humidity, and turbulent kinetic energy. A significant phase correlation between wind direction and visibility was found where a rapid decrease in visibility always following a rapid change of wind direction.