Key parameters for the life cycle of nocturnal radiation fog: a comprehensive large-eddy simulation study

High-resolution large-eddy simulations (LESs) are used to investigate the effect of turbulence as
well as the interaction between atmosphere and soil on the life cycle of nocturnal radiation fog. The first part of the article focuses on the validation of the LES model system for simulating radiation fog against measurement data from the meteorological super-site in Cabauw (Netherlands). Differences found in the fog life cycle, depth, and liquid water content between LES and observations can, as in former studies, be largely ascribed to the presence of local advection processes in the observational data and uncertainty in the measurement data used to initialise the model. Moreover it is found that the choice of the droplet number concentration within the cloud microphysical representation has a high impact on the liquid water content within the fog layer, but a rather small effect on its life cycle. In the second part of the article, a set of LES runs is analysed with idealised initial conditions in order to study the effect of turbulent mixing as well as the initial state of the soil on the development of radiation fog. As expected, the results show that turbulent mixing has a strong impact on the time of fog formation, which is complicated by the interaction with both radiative cooling and water vapour 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, whereas near-surface soil moisture is decisive for the lifting and dissipation time of the fog layer.