Secondary ice production - No evidence of a productive rime-splintering mechanisms during dry and wet growth
Authors: Hartmann Susan , Seidel Johanna , Keinert Alice , Kiselev Alexei , Leisner Thomas , Stratmann Frank 
Published: February 2023

Mixed-phase clouds are essential elements in Earth&#8217;s weather and climate system. Atmospheric observation of mixed-phase clouds occasionally demonstrated a strong discrepancy between the ice particle and ice nucleating particle number concentration of several orders of magnitude at modest supercooling [1, 4, 6]. Various secondary ice production (SIP) mechanisms have been hypothesized which can increase the ice particle number concentration by multiplication of primary ice particles [2, 3].In this study, we focus on SIP as a result of droplet-ice collisions, commonly known as rime-splintering or Hallett-Mossop (HM) process. During riming supercooled droplets collide with an ice particle and freeze upon impact and lead to the formation of secondary ice particles. Our main objectives are to quantify the number of secondary ice particles and to learn more about the underlying physics. Therefore, we conducted laboratory experiments at IDEFIX (Ice Droplets splintEring on FreezIng eXperiment) in which small droplets collide with a fixed ice particle of 1 mm in diameter. IDEFIX is designed to simulate atmospheric relevant conditions regarding temperature, humidity, impact velocities and collision rates. The riming process was observed with high-speed video microscopy and infrared thermography to visualize the growing rimer structures and the surface temperature of the riming ice particle, respectively. Further, the secondary ice particles were counted via inertial impaction on a supercooled sugar solution in the ice counting device (cut off diameter of 2&#160;&#181;m) developed at IMK-AAF, KIT.The following parameters were investigated: the air temperature was varied between -4&#176;C and -10&#176;C, the ice-droplet impact velocities were set either to 1&#160;ms-1 or 3&#160;ms-1, and the lognormal droplet size distribution was adjusted to have the mode diameter between 18&#160;&#181;m and 30&#160;&#181;m with the standard deviation between 1.6&#160;&#181;m and 8.4&#160;&#181;m. Under these conditions, the collisions rates between droplets and rimer were between 102 and 103&#160;mm-1s-1 , as determined from the video records and with a rimer heat balance model [5] using measured surface temperature as input data. Thus, the simulated riming process is typical for convective clouds; both dry and wet growth could be realized in IDEFIX. We found no efficient and reproducible secondary ice production during riming within the range of the investigated parameters. The amount of secondary ice particles produced in all our experiments was well below the values expected from the HM mechanism [3, 7], where several hundreds of secondary ice particles per mg rime were found at optimal conditions. Six potential SIP cases (out of 31) could be identified where ice was detected in the ice counting device. Four of them could be attributed to rime spicules break-off due to sublimation.[1] Crosier, J., et al. 2011, DOI: 10.5194/acp-11-257-2011.[2] Field, P.R., et al. 2016, DOI: 10.1175/amsmonographs-d-16-0014.1.[3] Korolev, A. and T. Leisner 2020, DOI: 10.5194/acp-20-11767-2020.[4] Luke, E.P., et al. 2021, DOI: 10.1073/pnas.2021387118.[5] Pruppacher, H.R. and J.D. Klett, Microphysics of Clouds and Precipitation. 2010, Springer Dordrecht.[6] Taylor, J.W., et al. 2016, DOI: 10.5194/acp-16-799-2016.