A Numerical Study of the Formation and the Dissipation of Radiation Fogs

Abstract

The evolution of radiation fog has been investigated by numerical simulations. A onedimensional boundary layer model was adopted for the atmosphere. No moisture transport in the soil layer was assumed; the dew and falling droplets are assumed to be stored at the ground surface. The P3-approximation method was employed to incorporate the effect of radiative transfer into the boundary layer model.
With the development of radiation fog, the radiative cooling rate increases greatly and temperature decreases rapidly just below the fog top. This effect accelerates the condensation of water vapor near the fog top and, consequently, increases the height and liquid water content of the fog. The sudden increase of downward radiative flux from the fog layer enhances the rise of surface temperature, and the temperature profile in the fog layer turns from stable into unstable. With the increase of instability of temperature stratification in the fog layer, eddy diffusivities also increase rapidly. This effect enhances the transport of water vapor from the surface to the fog layer, and the development of the fog is again accelerated. In contrast to that within the fog layer, the temperature profile above the fog layer remains stable; consequently a sharp temperature inversion appears near the fog top. The height of this inversion rises gradually as the height of the fog top increases. When the moisture stored on the ground surface is exhausted and the transport of water vapor from the surface to the fog layer vanishes, the surface temperature rises suddenly and fog begins to disappear from the lowest layer near the ground.