Experimental evidence of ice multiplication initiated by freezing of drizzle droplets

The excess concentration of ice crystals as compared to the concentration of ice nucleating particles (INP) in marine mixed-phase clouds has been one of the longest debated issues in cloud physics. Several ice multiplication mechanisms have been proposed to explain this discrepancy, with the riming-splintering mechanism suggested by Hallett and Mossop (HM) being the most often sited of them. However, as the HM mechanism was found to be active in a narrow range of environmental conditions, an auxiliary ice multiplication mechanism is required to explain the anomalously high number of ice crystals observed, for example, in cold regions of marine cumuli. The shattering of drizzle droplets upon freezing could be such mechanism, but the quantification of its efficiency under realistic environmental conditions is still missing and a subject of ongoing research. The recent in-cloud observations and modeling studies (Korolev et al., 2019, Sullivan et al., 2018, Phillips et al., 2018) underline the potential importance of secondary ice production upon shattering of freezing drizzle droplets.
In this presentation, we will report the results of the experimental study aimed at clarifying the physics of this ice multiplication mechanism and its dependence on the environmental parameters. Applying the experimental technique reported in Lauber et al., (2018), we observe supercooled water droplets levitated in an electrodynamic balance (EDB) and observe the freezing process with a high-speed video camera. Presently, we extend the study into the range of realistic cloud conditions, mimicking continental (pure water) and maritime (0.1 wt% aqueous solution of see salt analog (SSA) drizzle droplets falling through cold humid air at terminal velocities. We observe a strong enhancement of shattering probability as compared to our previous study conducted under stagnant air conditions (Lauber et al., 2018). The high-definition video records of shattering events reveal the coupling between various microphysical processes caused by ice propagation inside the freezing drop and unravel striking differences between freezing of pure water and SSA solution droplets. Based on the hundreds of individual video records we explore the various effects leading to the production of secondary ice particles upon freezing of drizzle droplets. Additionally, using the high-resolution thermographic measurements of levitated droplets, we show that the shattering probability upon freezing positively correlates with the thermal non-equilibrium of supercooled droplet shortly before the initiation of the freezing process. Finally, we discuss the physical mechanisms behind the shattering of drizzle droplets and the potential implications for mixed-phase cloud modeling.

Korolev, A., Heckman, I., Wolde, M., Ackerman, A. S., Fridlind, A. M., Ladino, L., Lawson, P., Milbrandt, J., and Williams, E. (2019). "A new look at the environmental conditions favorable to secondary ice production", Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2019-611, in review.

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.

Sullivan, S. C., C. Hoose, A. Kiselev, T. Leisner and A. Nenes (2018). "Initiation of secondary ice production in clouds." Atmos. Chem. Phys. 18(3): 1593-1610.

Phillips, V. T. J., S. Patade, J. Gutierrez and A. Bansemer (2018). "Secondary Ice Production by Fragmentation of Freezing Drops: Formulation and Theory." Journal of the Atmospheric Sciences 75(9): 3031-3070.