Laboratory Experiments on the Droplet Shattering Secondary Ice Production Mechanism

Supercooled drizzle droplets may produce multiple ice particles upon freezing. This mechanism could potentially explain the high ice number concentrations outside of temperature range where the well-known Hallett-Mossop mechanism of ice multiplication would take place. Limited experimental methods in the past prevented direct observations of the shattering droplets, resulting in a wide range of experimental results, unsuitable for the development of a sophisticated cloud model parameterization. Recently, we have revived experiments on secondary ice production by levitating individual drizzle droplets in electrodynamic balance (EDB) and observing the freeze-shattering with high-speed video microscopy and high-resolution infrared thermal measuring system. In this way we have been able to identify three additional SIP mechanisms (cracking, jetting and bubble bursts) associated with the freezing of drizzle droplets (Lauber et al., 2018).
Additionally, we have extended the range of experimental conditions to mimick the freezing of continental (pure water) and maritime (aqueous solution of analogue sea salt) drizzle droplets suspended in the updraft of cold moist air. We report a strong enhancement of shattering probability as compared to the previous studies conducted under stagnant air conditions. The high-definition video records of shattering events reveal the coupling between various microphysical processes caused by ice propagation inside the freezing drop and reveal striking difference between freezing of pure water and SSA solution droplets. Application of high-resolution infrared microscopy allowed us to record the evolution of the droplet temperature under realistic flow conditions and thus constrain the thermodynamic parameters controlling the pressure build-up inside the droplet. Based on these new observation data and theoretical model of freezing droplet, we discuss the physical mechanism behind the shattering of drizzle droplets and its implication for mixed-phase cloud modeling.

Lauber, A., A. Kiselev, T. Pander, P. Handmann, and T Leisner (2018). “Secondary Ice Formation during Freezing of Levitated Droplets”, Journal of the Atmospheric Sciences 75, pp. 2815–2826.