Warm Fog Dissipation Experiment by Hygroscopic Seeding

In South Korea, to seek a way to dissipate warm fog for operational use at airports, ground-based experimental attempts to modify the microstructure of warm fogs have been conducted on the 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) in the stable fog condition. Most of experiments showed broadening of the FSSP-measured fog droplet size distribution, and an increase of the number of raindrops was detected in some experiments by the optical disdrometer. In most of experiments, it is shown that the CaCl2 nuclei observed by PM10 or OPC remain for a long time (more than 1 hour) around the target area due to the weak surface wind speed, and that the FSSP-measured fog droplet size distribution broadened into the larger size region after seeding. Also, the evolution from fog to rain droplets from the first seeding position has been observed by the 35 GHz Ka-band array radar. 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 may be more advantageous when a rapid response such as an emergency landing is needed. On average, two methods show the 1.24-times improvement of visibility during about 24 minutes after the seeding. Most of experimental results are in good agreement with Trabert's approximation theory (1901), though slightly smaller. Further simulation approaches are needed to better understand the effects of different seeding paths.

Acknowledgments. This work was supported by the NIMR/KMA project “Advanced research on industrial meteorology”, “Development of meteorological resources for green growth”, “Hydrometeorological Research in the Test-bed Region, and by National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2009-0085533).