Supercooled fog as a natural laboratory for studying ice formation in clouds

The ice phase in clouds contributes largely to uncertainties in global climate models partly due to a lack of atmospheric observations. At moderate supercooling ice nucleating particles (INP) and ice particles (IP) are present in small concentrations and a large volume of air is necessary for observation. Here, we report on initial observations of IP in supercooled fog with a new setup. We use a 0.3 m wide, vertical curtain of light and a camera pointing perpendicularly at it to record light scattered by IP formed in radiation fog near the ground at temperatures between -3 {\deg}C and -9 {\deg}C. Deposition rates of IP were several times larger than expected from number concentrations of INP found on $PM_{10}$-filters at a nearby air quality monitoring station. The discrepancy might be explained by secondary ice formation through the fragmentation of freezing water droplets, the entrainment of INP from above the fog layer through settling, the loss or deactivation of INP on $PM_{10}$-filters prior to analysis, or radiative cooling of INP below the temperature of the surrounding air. In summary, radiation fog constitutes an easily accessible form of a supercooled cloud, in which observations can be made for long enough to quantify the deposition rate of rare IP produced in a natural environment.