Radiation Fog. Part I: Observations of Stability and Drop Size Distributions

Data from several cases of radiation fog occurring at the Met Office field site at Cardington, Bedfordshire, UK have been analyzed with a view to elucidating the typical evolution in its static stability from formation to dissipation. Typically the early stages of radiation fog are characterized by a stable thermal profile and a relatively shallow depth. However, when the fog reached approximately 100 m depth it was observed to become optically thick (to longwave radiation), with a subsequent change over several hours to a saturated adiabatic stability profile. At this time turbulence levels were seen to increase significantly. The mechanisms involved appear to be radiative cooling from fog top and a positive heat flux to the atmosphere from the soil. The importance of this change in stability for numerical modelling of fog episodes is discussed. Several case studies are made to gain some insight into how common this transition is. Droplet spectra were measured at 2-m height for many of the cases considered, and their evolution is discussed. It is found that distributions fall into an initial phase with small drop sizes (approximately ≤ 10 μm diameter) and concentration, and a mature phase with the appearance of much larger drop sizes with a mean diameter of approximately 15−20 μm. It is found that the appearance of the mature phase does not coincide with the change in stability from stable to saturated adiabatic, but there is some evidence that once a saturated adiabatic profile is established, the droplet spectra variations are significantly less than for the stable period. The observed evolution of these spectra brings into question the suitability of microphysical schemes that assume constant spectral shape, drop diameter, and number density.