EUMeTrain: Prolonged Fog Episode in the UK December 2006

Summary

This case illustrates a typical, but more long-lasting than a usual fog event in wintertime conditions over land. The conclusions that we can make based on our study are:

• Fog forming within a large-scale anticyclone
• Calm or near calm conditions
• Large-scale subsidence , very dry mid-troposphere
• Moist boundary layer
• Development of a strong surface-based temeprature inversion
• No atmospheric fronts nearby -> foggy conditions persist
• Occasionial improvement of visibility very difficult to forecast/nowcast: not associated with diurnal solar radiation
• Eventually advection of another low cloud deck onto the foggy area finally lifts the fog, allowing the surface layer to mix.

For forecasting or nowcasting the fog, this case is a perfect example of how difficult even the short range forecasting may be. Clearly it can be seen that even a global model with a reasonable resolution can nowadays be used as a guidance for evaluating the likelihood of fog, but for the detailed and more precise analysis the satellite and conventional surface based observations are crucial.

Geostationary data and advanced RGB methods allow us to monitor the fog areas even at night with great accuracy. For this case their use was absolutely necessary: with IR images alone the task to monitor the advection of low level clouds would be virtually impossible.