Production of secondary ice particles and splintering of freezing droplets as a potential mechanism of ice multiplication Annika Lauber, Mona Schätzle, Patricia Handmann, Alexei Kiselev, Thomas Leisner

Three mechanisms potentially responsible for ice multiplication: Mechanical breakup Riming –Splintering Drizzle droplets shattering at freezing 80 µm 300 µm 0.5 m·s-1 to 3 m·s-1 d > 25 µm adapted from Lohmann, Lüönd, and Mahrt, An Introduction to Clouds, Cambridge University Press, 2016

Observation of the frozen drizzle droplets in a mixed phase cloud (AS-NS) HM 6000 m 5000 m 4000 m 3000 m Korolev et al., JAM 2004, 3d Canadian Freezing Drizzle Experiment, over Lake Ontario. Instruments: CPI, Nevzorov probe.

Experimental apparatus Metal rod cooled by liquid N2 High speed video camera Up to 200.000 fps Tiny ice crystals trickle into the trap Electrodynamic trap, connected to cryostat Levitated supercooled water drop, pure water or PSL suspension, d > 300 µm

Freezing of a water drop Freezing stage I: t ~ 2 ms 300 µm 258 K Initiation of freezing on contact  ice dendrite grows through the droplet  droplet warms up to the melting point  ice growth stops At the end of first freezing stage ~ 60% of the droplet is still liquid!

Freezing of a water drop Freezing stage II: 1 ms ~ 1 s Complete freezing, droplet deformation Pressure can be released via gas bubbles escaping through noses and spicules, bursting Pressure released via fracturing or fragmentation

The pressure inside a freezing droplet can reach 1000 bar D = 300 µm 1 kbar Pressure, kbar Fraction of freezing time t / tf Pressure release time compared to the time of complete droplet freezing tf Pressure rise with freezing time for different viscoelastic factor (hard shell n=0) King and Fletcher, J Phys D, 1973.

Previous results for 30 µm - 80 µm droplets (suspension of PSL particles, 10 to 1000 per drop) No shattering of pure water droplets T. J. Pander, PhD thesis, Heidelberg Univ., 2015, P. Handmann, diploma thesis 2015, Karslruhe

Complete and incomplete shattering with particle ejection Droplet size now > 300 µm Complete and incomplete shattering with particle ejection 400 µm 400 µm

Bubble breakup at -11°C and -13°C 2 300 µm

Jetting and particle ejection: drop shape does not change after ejection. Detection of secondary particles hardly possible!

Recoil-based detection of ejected particles: no need for visual search of events Droplet charge: ~14.7 pC Charge loss: 0.6 pC Particle velocity: 1 to 7 m/s Particle size: 5 – 30 µm ejection E_el = 1.4*1e-10 E_kin = 1.3*1e-9 Δq y x

Shattering and recoil frequencies for D = 300 µm Pure water droplets PSL suspension drops Max freq. D = 80µm Max freq. D = 80µm Shattering of pure water droplets (larger droplets – more inclusions?) 2x higher fragmentation probability for PSL-containing drops Average number of fragments per freezing droplet: Nf = 0 - 3.5 Shift towards higher temperature as compared to smaller drops (d = ~80 µm)

Big surprise… SSA drop Drizzle drop D = 300 µm Collision-coalescence Cloud drop d=10 µm 300 µm SSA, d=1µm 0.1 volume% sea salt solution (Instant Ocean) droplet freezing at 259K

Inclusion of salt in ice increases its viscoelasticity? 1 kbar Pressure, kbar Pressure rise with freezing time for different viscoelastic factor (hard shell n=0) King and Fletcher, J Phys D, 1973. Fraction of freezing time t / tf

Summary New recoil-based method of splintering detection in 300 µm drops Drop size matters, non-soluble inclusions enhance the mechanism Higher probability of shattering for larger droplets, up to 40% Frequency maximum shifted towards higher temperature Sea salt solution drop are too elastic to shatter

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