Coalescence and Secondary Ice Development in Cumulus Congestus Clouds

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. Significance Statement Cumulus congestus clouds occur globally and account for a significant amount of precipitation in the tropics. The physics underlying the warm rain process and development of ice in cumulus congestus clouds are fundamental to a better understanding of precipitation formation. The collected data show that the strength of collision–coalescence is strongly influenced by cloud-base temperature, and that millimeter-diameter supercooled cloud drops will form in convective clouds with base temperatures warmer than 20°C. When supercooled large drops form, there is a secondary ice process that rapidly freezes the large majority of supercooled cloud water before updrafts reach the −16°C level. Incorporating results from the observations will improve cloud-resolving and climate prediction models.