Microphysical properties of supercooled cumulus clouds in which an ice particle multiplication process operated

Abstract

AbstractMicrophysical measurements made in small wintertime cumuli on five days in June 1981 are described. On all days an ice crystal multiplication process was active. the main conditions for the operation of the Hallett‐Mossop multiplication process were fulfilled—namely, the presence of both graupel particles and drops ≃ 24m̈m in diameter in the cloud zone between the ‐3 and ‐8°C levels. Shortcomings of the available theoretical models prevented any worth‐while comparisons between their predictions and the observed cloud behaviour. In these clouds, large drops (⩾300 m̈m diameter) grew by coalescence and appeared in a concentration of ≃0.1 per litre at about the same time as ice particles (graupel) of similar size. the drops reached a concentration of ≃5 per litre and then disappeared while the graupel continued to increase to concentrations of ≃10 per litre. There is evidence to indicate that the concentration of ice particles increased exponentially with time, being multiplied by a factor of 10 every 10 min between the values of 1 and 100 per litre. It is likely that the original ice nuclei activated were more numerous than the generally accepted average concentration of about 10m−3 at ‐10°C.