Large Particles in Supercooled Regions of Northern Canadian Cumulus Clouds

Abstract

Measurements have been made of the concentration and phase of large particles (>70 ?m) within the supercooled regions of northern Canadian cumulus clouds. During June and July, for the years 1975 and 1976, a total of 58 cumulus clouds near Yellowknife, N.W.T., were examined with a specially equipped Twin Otter aircraft. The cumulus clouds studied were mainly 1?3 km deep with most of the 130 cloud penetrations being made within 300 m of cloud top, at temperature levels between ?1 and ?11°C. The median penetration average (Johnson-Williams) liquid water content was 0.3 g m?3. The median penetration average concentration of particles >70 ?m and >350 ?m was 0.9 l?1 and 0.015 l?1, respectively. The concentration of large particles was not well correlated with J-W liquid water content or temperature, and considering all the clouds, no consistent change in the concentration was observed in successive cloud penetrations. These large particles were predominantly water drops. Ice was only found in clouds with summit temperatures colder than ?8°C. Clouds containing ice had significantly higher concentrations of large particles than did all-water clouds. The data suggest that both cold and warm rain precipitation formation mechanisms were present in some of these clouds.