Sensitivity of Numerical Prediction of Fog Events to WRF Model Physical Parameterization Schemes: A Study with MATERHORN Fog-X Observations

Cold air pool events develop in mountain basins during winter months due to topographic trapping of cool, dense air. This air can become significantly decoupled from the warmer, more turbulent air above the mountain barrier and can provide a preferential fog environment. During January 2015, the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Fog-X field campaign was held in the Salt Lake and Heber Valleys of northern Utah to investigate the processes of fog formation and evolution. The Weather Research and Forecasting (WRF) model was used to provide real-time forecasting during the campaign. Three fog events were observed: 8-9, 9-10, and 15-16 January. Early evaluation against observations shows that WRF-generated fog onset is earlier and the duration of this fog lasts longer than observed events in the Salt Lake Valley. This can be attributed to errors in model physics, initial and boundary conditions, resulting in colder temperatures and lower dew point depressions. In Heber, however, the WRF failed to generate all fog events. There was a warm temperature bias in this valley, resulting in lower relative humidities, but this could be an issue with the WRF resolving the thin radiative fog due to the vertical resolution.

The objective of this study is to conduct sensitivity experiments using the WRF model with different options of physical parameterization schemes and initial and boundary conditions to further examine the predictability of fog in both Salt Lake and Heber Valleys during MATERHORN Fog-X. The performance of the WRF model in forecasting fog and implications of the results on the processes that contribute to fog formation and dissipation will be investigated. Details will be discussed in this presentation.