A laboratory and model investigation of secondary ice production during to supercooled drop collisions with ice surfaces&#160;
Authors: Connolly Paul , James Rachel , Phillips Vaughan 
Published: March 2021

&lt;p&gt;This work presents new laboratory data investigating collisions between supercooled drops and ice particles as a source of secondary ice particles in natural clouds. Furthermore we present numerical model simulations to put the laboratory measurements into context.&lt;/p&gt;&lt;p&gt;Secondary ice particles form during the breakup of freezing drops due to so-called &amp;#8220;spherical freezing&amp;#8221; (or Mode 1),&amp;#160;where an ice shell forms around the freezing drop. This process has been studied and observed for drops in free-fall&amp;#160;in laboratory experiments since the 1960s, and also&amp;#160;more recently by Lauber et al. (2018) with a high-speed camera. Aircraft field measurements (Lawson et al. 2015)&amp;#160;and lab data (Kolomeychuk et al. 1975) suggest that such a process is dependent on the size of drops, with larger drops being more effective at producing secondary ice. &amp;#160;Collision induced break-up of rain drops has been well studied with pioneering investigations in the mid-1980s, and numerous modelling studies showing that it is responsible for observed trimodal rain drop size distributions in the atmosphere, which can be well approximated by an exponential distribution.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In mixed-phase clouds we know that rain-drops can collide with more massive&amp;#160;ice particles. This, depending on the type of collision, may lead to the break-up of the supercooled drop (e.g. as hinted by Latham and Warwicker, 1980), potentially stimulating secondary ice formation (Phillips et al. 2018 - non-spherical, Mode 2).&amp;#160; There is a dearth of laboratory data investigating this mechanism.&amp;#160; This mechanism is the focus of the presentation.&lt;/p&gt;&lt;p&gt;Here we present the results of recent experiments where we make use of the University of Manchester (UoM) cold room facility. The UoM cold room facility consists of 3 stacked cold rooms that can be cooled to temperatures below -55 degC. A new facility has been built to study secondary ice production via Mode 2 fragmentation. We generate supercooled drops at the top of the cold rooms and allow them to interact with different ice surfaces near the bottom. This interaction is filmed with a new&amp;#160;camera setup.&lt;/p&gt;&lt;p&gt;Our latest results will be presented at the conference.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Kolomeychuk, R. J., D. C. McKay, and J. V. Iribarne. 1975. &amp;#8220;The Fragmentation and Electrification of Freezing Drops.&amp;#8221;&amp;#160;&lt;em&gt;Journal of the Atmospheric Sciences&lt;/em&gt;&amp;#160;32 (5): 974&amp;#8211;79. https://doi.org/10.1175/1520-0469(1975)032&lt;0974&gt;2.0.CO;2.&lt;/p&gt;&lt;p&gt;Latham, J., and R. Warwicker. 1980. &amp;#8220;Charge Transfer Accompanying the Splashing of Supercooled Raindrops on Hailstones.&amp;#8221; Quarterly Journal of the Royal Meteorological Society 106 (449): 559&amp;#8211;68. https://doi.org/10.1002/qj.49710644912.&lt;/p&gt;&lt;p&gt;Lauber, Annika, Alexei Kiselev, Thomas Pander, Patricia Handmann, and Thomas Leisner. 2018. &amp;#8220;Secondary Ice Formation during Freezing of Levitated Droplets.&amp;#8221; Journal of the Atmospheric Sciences 75 (8): 2815&amp;#8211;26. https://doi.org/10.1175/JAS-D-18-0052.1.&lt;/p&gt;&lt;p&gt;Lawson, R. Paul, Sarah Woods, and Hugh Morrison. 2015. &amp;#8220;The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds.&amp;#8221; Journal of the Atmospheric Sciences 72 (6): 2429&amp;#8211;45. https://doi.org/10.1175/JAS-D-14-0274.1.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;