Using Airborne Observations to Characterize Primary Nucleation and Secondary Ice Formation Processes in Cumulus Congestus Clouds

The pathways for ice formation in mixed-phase clouds (MPCs) include both heterogeneous nucleation and secondary ice production (SIP). Heterogeneous nucleation is characterized by the inclusion of an ice nucleating particle (INP) that catalyzes the freezing event. SIP may follow heterogeneous nucleation by initiating new ice particle formation, either during the primary freezing event or through the subsequent interactions between existing droplets and ice crystals, resulting in ice crystal number concentrations (N_ice) that exceed the number of INPs. Using in situ observations from the Secondary Production of Ice in Cumulus Experiment (SPICULE) campaign, this study attempted to elucidate the pathways of ice formation and the conditions that favor the onset of SIP during the early stages of congestus clouds. Airborne INP measurements collected in air feeding the clouds, as well as inside of congestus clouds, were compared to in-situ observations of N_ice to deduce whether SIP was active between temperatures of -10 ºC and -20 ºC. Findings indicated that the most important SIP mechanism during the early stages of congestus cloud growth was fragmentation of freezing droplets (FFD), and that vertical velocity and cloud base temperature strongly controlled the onset of FFD. Finally, simple numerical simulations of a congestus cloud, designed to resemble the clouds sampled during SPICULE, were used to evaluate the impacts of including SIP mechanisms besides the Hallet-Mossop (HM) rime splintering mechanism. Predicted N_ice were in the same range as observations from SPICULE when SIP was inferred to be active, but only when the FFD mechanism was included in the simulations. The simulations provided insight into the sequence of ice formation mechanisms in the early stages of congestus clouds, pointing to the importance of the FFD mechanism in initiating ice formation that proceeded via HM and other routes later in the cloud development.