NATURAL ICE NUCLEUS MEASUREMENT UNDER HIGH SUPERSATURATION

To understand ice nucleation under high supersaturation in clouds, mea surements of natural ice nuclei for supersaturation with respect to wa ter (S-w) up to 10% from -20 to -24 degrees C were carried out using a horizontal gradient, continuous flow, ice thermal diffusion chamber a fter solving the spurious count problem due to ice crystal protrusion on the bottom plate with the addition of a small amount of ethylene gl ycol. The measurements were made with continental air masses in early summer of 1993 at Salt Lake City, Utah in the United States of America . The measured ice nucleus concentration under the condition of -20 de grees C and S-w 5% showed a daily variation. The ice nucleus concentra tion changed from about 10 l(-1) in a warm air mass to about 1 l(-1) i n a cold air mass, respectively, before and after the passage of a col d front in the synoptic scale. It was inferred that the instability of the warm air mass helps raise the particulates from or near the groun d to increase the ice nucleus concentration. In order to pick out the supersaturation dependence at the various temperatures, the ratio of t he ice nucleus concentration (N) to that at water saturation (N-o) was estimated. On average, S-w increases of 5 and 10% raised the N/N-o ra tio by as much as a factor of two or more, respectively. It was found that a general relationship exists between N/N-o and S-w, and the slop e further steepens above water saturation.