Turbulence Effects on the Formation of Cold Fog over Complex Terrain: Observations from CFACT 2022 Field Campaign and Large-Eddy Simulations
Abstract
Fog is considered a type of low-lying cloud near the Earth's surface. It forms as a result of a variety of processes such as nucleation, radiative cooling, advection, and turbulent mixing and is difficult to forecast. Cold fog (fog that forms when the ambient temperature is below 0 oC) in mountainous regions is particularly poorly understood and difficult to predict. The Cold Fog Amongst Complex Terrain (CFACT) is an NSF-funded field campaign and science project that devotes to better understanding the life cycle and characteristics of cold ice fogs across a range of scales over complex terrain. CFACT was conducted from 7 January through 23 February 2022 in Heber Valley, Utah, USA, and was supported by Lower Atmospheric Observing Facilities managed and operated by NCAR/EOL. The deployment included nine satellite sites with modified 3-m Integrated Surface Flux System towers, two supersites with 32-m turbulence and radiation profiling towers, eight low-cost weather stations, an aerosol measurement site, and a microphysics measurement site. 9 Intensive Observation Periods (IOPs) were conducted.
This study emphasizes turbulence effects on the formation of cold fog using CFATC field campaign observations and large-eddy simulations. First, we use the CFACT campaign observations to characterize the boundary layer conditions and derive the turbulence properties during fog formation and evolution for selected IOPs. Then, a large eddy simulation (LES) embedded in the mesoscale community Weather Research and Forecasting model is used to examine the effects of turbulence on the cold fog formation. The simulations are compared with observations. An early case study using the LES indicates that large-scale turbulent eddies prevail and dominate the mixing effects in the planetary boundary layer (PBL). The combination of turbulence mixing effects and the ultra-cold land surface significantly reduces the near-surface air temperature, thus leading to the formation of fog. The omission of these turbulent eddies in the PBL parameterization scheme of the cloud-permitting scale simulation has resulted in a weak mixing effect in PBL, thus failing to reproduce the fog in the simulation. The findings from LES will be further investigated with the CFACT observations.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.A46C..05P