A laboratory study of secondary ice production from collisions between supercooled raindrops and ice particles

As part of the CERTAINTY project, we present results from our laboratory study of secondary ice production (SIP) from collisions between supercooled raindrops and more massive ice particles, building on our previous proof-of-concept work (James et al., 2021) with a refined experimental setup for improved quantification.

In mixed phase clouds, ice formation can occur via two pathways: primary ice formation, via ice nucleating particles, or secondary ice production (SIP). Observations in both shallow and deep convective clouds often show ice concentrations that exceed those predicted by primary ice nucleation by several orders of magnitude. However, parameterisations of SIP mechanisms remain poorly constrained due to limited laboratory data.

One proposed SIP mechanism involves collisions between supercooled raindrops and more massive ice particles, where secondary drops may form during impact, with some freezing to create secondary ice. Our previous work (James et al., 2021) demonstrated the viability of this SIP mechanism, and showed that approximately 30 % of the secondary drops froze under a limited set of conditions.  Building on this, we have refined our experimental setup to reduce uncertainties in the freezing fraction of secondary drops by elevating the ice particle to allow the splashing to occur freely, without interference of a flat surface used in our previous experiments. We also explore a broader range of supercooled water drop diameters, ice particle sizes, impact velocities and temperatures to better reflect cloud conditions.

Finally, we incorporate our updated results into a parcel model with bin microphysics for idealised clouds, which previously used our earlier results (James et al. 2023), to demonstrate the impact this improved quantification in conjunction with other SIP mechanisms.

References
James, R. L., Phillips, V. T. J., and Connolly, P. J. (2021), Secondary ice production during the break-up of freezing water drops on impact with ice particles, Atmos. Chem. Phys., 21, 18519–18530, https://doi.org/10.5194/acp-21-18519-2021.
James, R. L., Crosier, J., and Connolly, P. J. (2023), A bin microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud, Atmos. Chem. Phys., 23, 9099–9121, https://doi.org/10.5194/acp-23-9099-2023.