DEVELOPMENT OF ICE AND PRECIPITATION IN NEW MEXICAN SUMMERTIME CUMULUS CLOUDS

An experiment, involving the National Center for Atmospheric Research's King Air aeroplane, was conducted in order to measure the microphysical properties of New Mexican summertime cumulus clouds. Since the clouds formed and developed essentially in place, over the mountains, it was possible to make multiple penetrations through a single cloud, thereby observing a significant fraction of the cloud's life cycle. In this paper, the questions of primary- and secondary-ice production, and the development of precipitation particles, are addressed. Primary-ice nucleation was found to occur when the temperature within the cloud reached a value of between -10 and -12-degrees-C irrespective of whether this was in the updraught or downdraught. Drops with diameters of about 0.5 mm were often observed in concentrations of about 10 L-1 before the formation of ice, which suggests a nucleation mechanism involving large drops, The maximum concentrations of ice particles observed in these clouds (up to about 1300 L-1) are much greater than typical concentrations of ice particles that can be attributed to primary-ice nucleation. Evidence suggests that the most likely explanation is the Hallett-Mossop process of secondary-ice-crystal production. Ice particles generally were first observed in the downdraughts. The development of precipitation is often thought to occur via downdraught transport, followed by sedimentation or mixing of ice particles into fresh, liquid-laden turrets. The multi-thermal nature of the clouds is considered to be central to this process.