Preliminary Results of Warm Fog Dissipation by Hygroscopic Particle Seeding

In South Korea, warm fog occurring at temperature above 0^oC is more common than cold fog. Warm fog events happen more than 80%. Therefore, we need the warm fog dissipation techniques. We have taken attempts to modify the microstructure of warm advection fogs during 2005-2013 summer and autumn seasons. A series of ground-based experiments to study the effects of hygroscopic particle seeding for clearing warm fogs has been conducted at Daegwallyeong observatory. The final goal of this study is to figure out the best way of modifying warm fog in operational use at airports and highways. We define and calculate two variables related to visibility improvement from time-series graphs for the visibility during the experiment period to evaluate the seeding effects. The visibility improvement ratio is determined by the visibility improvement degree comparing before and after the seeding. And the visibility improved period is determinded by the time maintaining the visibility improved. During the 8 years, among 76 experiments, 48 experiments are confirmed improvement of visibility. On average, successful experiments show a 1.45-times visibility improvement ratio and a 29-minute visibility improved period after the seeding. Also, most of experiments that verified the seeding effects show the broadening of a fog droplet size distribution measured by the optical particle counter. The increments of the number of raindrops are detected in some experiments by the optical disdrometer. Those results above imply the physical evidence of a growth of fog droplets to raindrops by the collision-coalescence process. Further numerical simulation approaches are necessary to find out the effects of hygroscopic particle seeding more detail.

Acknowledgements: This work was supported by the ²Advanced Research on Applied Meteorology² of National Institute of Meteorological Research (NIMR) funded by the Korea Meteorological Administration (KMA).