Aircraft Observations of Cumulus Microphysics Ranging from the Tropics to Midlatitudes: Implications for a “New” Secondary Ice Process

In situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud base temperature, drop size distribution and the development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the Southeast United States, and the High Plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the Southeast United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the High Plains do not generate supercooled large drops, and rarely are drops > 70 microns observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process, which is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the High Plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to -35.5 °C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds. In the case of hygroscopic seeding, it is speculated that candidate clouds with a combination of cloud base temperature and drop size distribution that would not naturally form supercooled large drops can be stimulated to do so. Once supercooled large drops are formed the natural secondary ice process will enhance glaciation and rainfall.