Key Parameters for the Life Cycle of Nocturnal Radiation Fog: Results from a Comprehensive Large-Eddy Simulation Study

We employ high-resolution large-eddy simulations (LES) with grid spacings down ot 0.5 m in order to investigate the effect of turbulence as well as the interaction between atmosphere and soil on the life cycle of nocturnal radiation fog. Special emphasis is put on the question which parameters are most decisive for the development of radiation fog.

In this paper, we present results from a reference LES for a deep radiation fog case as observed in Cabauw (Netherlands), accompanied by a comparison against observational data from the Cabauw super-site. Differences found in the fog life cycle, its depth, and liquid water content between LES and observations can be ascribed to the presence of local advection processes and possible uncertainties in the measurement data that was used to initialize the model. Also, the pre-defined droplet number concentration turned out to be a critical parameter that determines the liquid water content of the fog layer.

Unlike former LES studies on radiation fog, the study is complemented by comprehensive sensitivity studies to elucidate the effects of turbulent mixing and the initial state of the soil on the development of radiation fog. The results show that turbulent mixing has a strong impact on the fog formation time, which is complicated by the interaction with both radiative cooling and water vapor removal by dew deposition. Furthermore, it is found that the near-surface soil temperature plays a key role for the exact time of fog formation, while near-surface soil moisture is a key parameter for fog lifting and dissipation time after sunrise.