Large-eddy simulation study of the dissipation of radiation fog

Large-eddy simulations (LES) of radiation fog were performed to study the dissipation phase of fog in detail. These LES were carried out with the Meso-NH research model at a resolution of 4.5 m in the horizontal and 1 m in the vertical, over a domain of dimensions 4.5 x 1.5 km(2). The dissipation phase is characterized by a large scatter of the liquid water path and there is evidence that dry downdraughts at the top of the fog layer have a large impact on the liquid water content (LWC) variability inside the layer. These downdraughts promote an increase in solar radiation reaching the ground over small areas, leading to an increase of turbulence near the ground, through an increase of wind variance and an increase of the convective structure of the fog layer. However, downdraughts can also move strong LWC located at the top of the fog layer, transporting it to the ground. These coupled processes between the ground and the top of the fog layer can explain the spatial variability of fog during dissipation of the layer. Sensitivity studies proved that small-scale surface heterogeneities have no impact on the range of dissipation times, but do have an impact on the horizontal structure of fog at small scales. The wind intensity modifies the balance between warming of the surface and wind shear at the fog top, thus impacting the dissipation time of the fog, but it has little impact on the fog heterogeneity during the dissipation phase.