Advancements in Understanding Cold Fog over Complex Terrain: Results and Progress from the CFACT Project (Invited Presentation)

Cold fog forms via various thermodynamic, dynamic, and microphysical processes when the air temperature is less than or near 0°C. It occurs frequently during the cold season in the western United States, yet it is still challenging to detect using standard observations and very difficult to predict. The Cold Fog Amongst Complex Terrain (CFACT) project goals are to 1) investigate the lifecycle of cold-fog events over complex terrain with the latest instrument technology, 2) improve microphysical parameterizations and visibility algorithms used in numerical weather prediction (NWP) models, and 3) develop data assimilation and analysis methods for current and next-generation (e.g., sub-kilometer scale) NWP models. CFACT field campaign took place in Heber Valley, Utah, during January and February 2022.

Two years subsequent to the field campaign, significant advancements have been made across multiple research fronts, and they are: 1) Analysis of high-resolution field observations during Intensive Observation Periods (IOPs) have enabled the identification of near-surface and boundary layer conditions conducive to cold fog formation. Notably, atmospheric turbulence conditions near the surface were characterized for the first time in the context of mountainous cold fog. Additionally, the thermodynamic circulation governing moisture transfer and fog formation was investigated. 2) Utilizing a diverse array of instruments, the microphysical evolution of cold fog, particularly ice fog, was comprehensively characterized. This included studying the relationship between heat and moisture budgets during fog events observed, with a focus on particle (including droplets) spectra. 3) Air chemistry and aerosol measurements were employed to investigate "new particle formation" processes occurring within the fog environment. 4) Applying the rich field data facilitated unprecedented opportunities to validate high-resolution numerical weather prediction models. In this respect, large-eddy simulations of fog events and the sub-km scale prediction were conducted, highlighting the necessity of accurately predicting local ephemeral fog events using very high resolution simulations. 5) Identifying strategies for assimilating high-resolution field campaign data using advanced data assimilation methods.

This presentation will highlight the progress made in the mentioned research areas, presenting new findings, and sample results obtained through the analysis of CFACT project data.