Application of Contact-Freezing Nuclei in Convective Cloud Seeding

N Fukuta

Abstract


Continuing ice crystal generation and larger and longer seeded area development as well as the tendency to form desirable rimed crystals and graupel, previously reported with silver iodide (Agl) seeded convective clouds were examined in comparison with dry ice seeding cases. The slow contact-freezing nucleation by Agl particles was identified as the main reason for this. It was pointed out that the contact-freezing mechanism avoids over-generation of ice crystals by deactivating when cloud droplets are exhausted. In order for the mechanism to occur effectively in clouds, use of a nucleant whose preceding fast ice nucleation mechanisms (deposition nucleation in particular) are not highly active is advantageous to avoid early nuclei loss. From their relatively low deposition nucleation activity and slower contact-freezing rate observed in the laboratory, the application of organic ice nucleants to convective cloud seeding was suggested, and based on the Brownian coagulation formula, the desirable nucleus size range was estimated for this process.

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