Warm fog dissipation and validation by the hygroscopic seeding

In South Korea, the experimental attempts to modify the microstructure of warm fogs have been conducted on Daegwallyeong Plateau for the 2005-2009 warm seasons. The attempts were to examine the effects of two different seeding paths: a hygroscopic material flare composed of mainly CaCl₂ circulating around the seeding target (the circle seeding method) and crossing the advection fog flow incoming into the target (the line seeding method). There were 13 line seeding and 11 circle seeding experiments. These experiments are seeking the best way to dissipate warm fog for operational use at airports and on highways. Most of experiments showed the broadening of the FSSP-measured fog droplet size distribution. The increment of the number of raindrops was detected in some experiments by the optical disdrometer. Also, the evolution of fog and rain droplets from the first seeding position was observed by the 35 GHz Ka-band radar. The PM10 measurements show that the CaCl₂ nuclei remain somewhat long time (more than 1 hour) around the target area, probably by the aerial drag. Comparison between two seeding experimental results showed that the line seeding method gives slower seeding impact time (defined as the time from the finish of seeding to the first peak of visibility improvement), but longer visibility improvement period and higher visibility improvement than those of the circle seeding. Generally, the line seeding method is more efficient than the circle one to modify the warm fog. But, the circle seeding method is more applicable where the rapid response such as the emergency landing is needed. On average, two methods show the 1.37-times improvement of visibility during about 22 minutes after the seeding. Most of experimental results give a good agreement with Trabert's approximation theory (1901), though slightly smaller. Further simulation approaches are needed to more understand the effects of different seeding paths.