An Overview of the MATERHORN Fog Project: Observations and Predictability

A field campaign design to study fog processes in complex terrain was a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. The experiment was conducted in the Wasatch Mountains during January 5–February 15, 2015. Fog and in particular, Ice fog (IF), defined as fog composed of only ice crystals, was studied during a part of the campaign, and this component of the program was dubbed MATERHORN-Fog. Ice fog often occurs in mountainous regions due do rapid cooling, such as radiative cooling, advective cooling, and cooling associated with mountain circulations (e.g., slope and valley winds). A variety of major instrument platforms were deployed, which included meteorological towers, a SODAR, a LiDAR, ceilometers, and a tethersonde profiler. In addition, in situ measurements took place at several locations surrounding Salt Lake City and Heber City. During the campaign, ice fog occurred at temperatures below −5 °C down to −13 °C and lasted for several hours until radiative heating became significant. The visibility (Vis) during ice fog events ranged from 100 m up to 10 km. At the Heber City site an array of sensors for measuring microphysical, radiative, and dynamical characteristics of IF events were deployed. Some local effects such as upslope advection were observed to affect the IF conditions. As expected during these events, ice water content (IWC) varied from 0.01 up to 0.2 g m⁻³, with radiative cooling fluxes as strong as 200 W m⁻²; turbulent heat and moisture fluxes were significantly lower during fog events than those of fog dissipation. At times, the measured ice crystal number concentration was as high as 100 cm⁻³ during periods of saturation with respect to ice. N_i was not a constant as usually assumed in forecasting simulations, but rather changed with increasing IWC. Measurement based statistics suggested that the occurrence of IF events in the region was up to 30 % during the study period in the winter of 2015. Temperature profiles suggested that an inversion layer contributed significantly to IF formation at Heber. Ice fog forecasts via Weather Research and Forecasting (WRF) model indicated the limitations of IF predictability. Results suggest that IF predictions need to be improved based on ice microphysical parameterizations and ice nucleation processes.