Effects of Near-Surface and Boundary Layer Conditions on the Formation of Cold Fog Over Complex Terrain: Observations From CFACT Field Campaign and Validation of WRF Numerical Forecasts at Sub-km and LES Scales

To better understand and predict winter-time fog, a recent field campaign, Cold Fog Amongst Complex Terrain (CFACT), was conducted in Heber Valley in Utah, during January and February of 2022. Using the CFACT field observations, the role of surface and boundary layer conditions leading to fog formation is studied. We first identify key characteristics in the surface and atmospheric boundary layer conditions of fog cases, including an established high-pressure over the Great Basin; clear skies; dry inverted conditions; calm winds for the duration of the diurnal cycle; warm (above freezing) air temperatures during the daytime; warm (above freezing) soil temperatures during the daytime; a period of elevated (above 0.5 m2s-2) turbulence, then calm conditions for the duration of fog; and finally supersaturation with respect to ice.

The plethora of observations validates the real-time numerical forecasts with the mesoscale community Weather Research and Forecasting (WRF) model at the sub-km scale during the CFACT field campaign. The WRF model’s performance shows good simulations of mesoscale conditions and evolution of atmospheric conditions, but surface conditions, especially related to complex terrain, require further improvement.

Additional numerical simulations were conducted with large-eddy simulations (LES; ~ 10 m horizontal resolution), and results indicated that LES could better resolve the turbulence processes in the near-surface boundary layer and successfully predict the fog event. Outcomes imply the importance of improving physical parameterizations within the NWP models for accurate fog prediction.