Mechanisms of Temporary Improvement and Rapid Changes in Visibility in Fogs

Environmental situations associated with temporary improvement or rapid changes in visibility in the presence of fog were analyzed to understand the mechanisms of these changes in visibility at Rokkasho Village, Aomori Prefecture, northeast Japan. This region is frequently affected by sea fogs. The profiles of liquid water content and drop size distribution in fogs were obtained by a direct photographic method using a tethered hot-air balloon. On the basis of the results, the temporary improvement in visibility on July 10, 2006 was attributed to convection-driven downward transport of air parcels with lower total water content than air parcels near the ground surface. The rapid changes in visibility at the same location on July 24 seemed to be related to mixing by Kelvin-Helmholtz waves. Mixing with an upper layer containing less water initially caused an improvement in visibility but subsequently, it led to the formation of a dense fog just above the ground surface in an area of upward flow. Settlement of fog drops from the dense fog above degraded the visibility in this area.