Coalescence and Secondary Ice Development in Cumulus Congestus Clouds

Abstract

Understanding ice development in cumulus congestus (CuCg) clouds, which are ubiquitous globally, is critical for improving our knowledge of cloud physics, precipitation and climate prediction models. Results presented here are representative of data collected in 1008 penetrations of moderate to strong updrafts in CuCg clouds by five research aircraft in six geographic locations. The results show that CuCg with warm (∼23°C) cloud-base temperatures, such as in tropical marine environments, experience a strong collision–coalescence process. Development of coalescence is also correlated with drop effective radius >∼12 to 14 μm in diameter. Increasing the cloud-base drop concentration with diameters from 15 to 35 μm and decreasing the drop concentration < 15 μm appears to enhance coalescence. While the boundary layer aerosol population is not a determinate factor in development of coalescence in most tropical marine environments, its impact on coalescence is not yet fully determined. Some supercooled large drops generated via coalescence fracture when freezing, producing a secondary ice process (SIP) with production of copious small ice particles that naturally seed the cloud. The SIP produces an avalanche effect, freezing the majority of supercooled liquid water before fresh updrafts reach the −16°C level. Conversely, CuCg with cloud-base temperatures ≤ ∼8°C develop significant concentrations of ice particles at colder temperatures, so that small supercooled water drops are lofted to higher elevations before freezing. Recirculation of ice in downdrafts at the edges of updrafts appears to be the primary mechanism for development of precipitation in CuCg with colder cloud-base temperatures.