AGGREGATION (RIMIMG) of CLOUD DROPLETS(10 UM) to the EDGES of A HEXAGONAL ICE CRYSTAL(200UM) by COULOMB'S FORCE.

• The clumping phenomena of ice crystals was first pointed out by Magono (1968) in his keynote speech at the International Conference of Cloud Physics, as one of several problems remaining to be resolved or having newly arisen in the field of cloud physics. Furthermore, based on his field observation, Magano (1972) suggested that the aggregation of ice crystals may be caused by the Coulombs force between ice crystal of different signs of electric charge, and in a state of growth in which the charge separation could occur within at ice crystal, as pointed out by Cheng (1967). A recent report by Rydock and Williams (1991) confirmed that charge separation was closely associated with frost growth and may be applicable to phenomena that operate in thundercloud.

Microscopic observations of individual ice crystals sampled from seeding experiments and from natural snow storms, especially from storms with thunder and lightning revealed that the clumping phenomena of ice crystals could be classified into the following categories (1) Two crystals joined in perpendicular form. (2) Frozen cloud droplets at the center of a plate type crystal, (3) Many frozen cloud droplets concentrated at the edge of a plate crystal (figure), and (4) Many frozen cloud droplets on the surface of an ice pellet. Close examination of the growth patterns of an ice crystal show a massive vapor deposition on (1) both ends of a columnar crystal, (2) outer edge of a plate crystal, and (3) surface of a frozen drop. Heat transfer measurements indicated that higher temperature is always located at the growing area of an ice crystal (Zheng 1995).

Release of latent heat associated with crystal growth by diffusion of water vapor and by riming of cloud droplets results in a negative charge concentration near the warmer region of the ice crystal. The charge generation and distribution were determined by temperature gradient within the ice crystal in accordance with thermal electric effect (Workman .and Reynolds 1950).

Laboratory study and field observation revealed that charge generation and separation within an ice crystal definitely occur in the state of growth. Many recent reports strongly indicate that the riming splintering mechanism - charge transfer during ice-ice particles collision, is unlikely to be a major factor responsible for charged ice particle generation in thunderclouds. The sublimational breakup of negatively charged massive ice fragments from frosty surface of ice particles in a state of growth (Schaefer and Cheng 1971) may open up a new pathway for further understanding the microphysical properties of ice in thunderclouds.

REFERENCES:

• 1, Cheng, R J., Final Report, Yellowstone Field Research Expedition VII. 57-60, 1967.
• 2, Magon., C., Problems on physical understanding of snowfall phenomena.
• Proc. International Conference on Cloud Physics, Toronto, Canada, 243-248, 1968.
• 3, Magono, C., Aggregation phenomena of ice crystals,.
• J. Met. Soc. Japan, 50. 489-493.,1972.
• 4, Rydock, J. and E Williams-. Charge separation associated with frost growth,
• Q.J R Meterorol . Soc.. .117 409-419, 1991.
• 5, Schaefer, V. J. and R. J. Cheng, The production of ice crystal fragments by sublimation and electrification,
• J. Rech. Atmos., 5, 5-10, l971.
• 6, Workman, E. .J. and S. E. Reynolds, Electrical phenomena occurring during the freezing of dilute aqueous solution
• and their possible relationship to thunderstorm electricity.
• Phys. Rev., 78 254 259 ,I950.

Presented at: 10th INTERNATIONAL CONFERENCE on ATMOSPHERIC ELECTRICITY OSAKA, JAPAN. JUNE 10-14, 1996