Ice Fog Microphysical Properties at High Elevations: MATERHORN Observations and Parameterizations

A major challenge of weather forecasting in mountainous terrain is related to the high moisture and fog events, occurrence of which poses serious limits to military and aviation operations. Unmanned Aerial Vehicles (UAVs) are particularly vulnerable, given the sensitivity of hardware to moisture. Virtually no detailed studies exist on fog formation over complex terrain. A comprehensive wintertime experiment (2014-2015), to understand the life cycle of fog events in either high-altitude alpine basins (in Heber Valley) or larger-scale urbanized valleys (Salt Lake City), was conducted in Utah as a part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. A suite of instruments was deployed at both sites, which included tethered balloons, radio soundings, several flux towers, sodars and lidars and different types of particle counters as well as visibility, fog spectra, and precipitation measurement instrumentation.

In this study, the MATERHORN-Fog dataset is used to better understand ice-fog physical characteristics and develop improved microphysical parameterizations. Analyses are mainly based on observations of ice particle and aerosol spectral characteristics, visibility (Vis) due to fog and precipitation, fog ceiling, wind and turbulence measurements, and radiative fluxes. Intense ice fog events occurred on January 7th and 13th 2015, as consequence of favorable weather conditions and significant radiative cooling. Details of these events are presented here and analyzed for extinction calculations. Possible ice nucleation (IN) processes are also described. Assuming that ice fog usually occurs when relative humidity with respect to water (RHw) is less than 100%, a relationship between RHi (RH with respect to ice) and Vis is obtained. Based on ice microphysical spectral observations, the influence of both ice water content (IWC) and ice crystal number concentration (Ni) on extinction is also investigated, and a new parameterization is proposed.

The forecasting performances of numerical weather models routinely used to predict ice fog episodes is strictly related to their ability to properly estimate IN number concentration that is related to Ni. If either IWC or Ni is ignored and only RHi is used for ice fog Vis prediction, uncertainty in prediction of ice fog Vis can be larger. This result has significant repercussions for aviation and marine applications.

This research was funded by Office of Naval Research Award # N00014-11-1-0709, Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program and Environment Canada.