Large-eddy simulation of radiation fog

In order to study the three-dimensional structure of radiation fog and to obtain a basic understanding of its generation mechanism, a numerical experiment is performed with a large-eddy simulation model and compared with the observation at Cabauw in the Netherlands. After confirming that the results are in satisfactory agreement with the observations, the structure of the fog and its generation mechanism are examined in more detail. Before the fog forms, the atmosphere is stable and an inversion layer exists almost adjacent to the ground surface. As the fog grows, however, the stratification is destabilized and a mixed layer develops gradually. The longwave radiative cooling near the fog top contributes to the destabilization more than the condensational heating does. The evolution of the fog can be classified into three stages according to the behaviour of turbulent kinetic energy (TKE): formation, development, and dissipation stages. The fog layer has different flow structures at each stage. During the formation stage, longitudinal rolls similar to streaks in channel flows appear near the ground surface. The development stage is characterized by an initiation of transverse bands due to Kelvin-Helmholtz instability and a sudden increase of TKE. During the dissipation stage, longitudinal rolls and polygonal cells due to convective instability are organized.