Secondary ice processes upon heterogeneous freezing of cloud droplets

Sometimes, the number of ice crystals observed in clouds exceeds the concentration of ice nuclei by several orders of magnitude. In order to explain this discrepancy, secondary ice processes are usually invoked, most prominently the splintering of cloud droplets during the process of riming. Other mechanisms have been discussed, but have generally not been able to account for the observed ice concentrations. We report on laboratory experiments on secondary ice processes accompanying the contact- or immersion freezing of cloud droplets. The freezing of individual, electrodynamically levitated cloud droplets was initiated by contacting them with ice nuclei or by immersed ice nuclei. The freezing process itself and secondary ice formation by either splintering of the freezing droplet or the ejection of gas bubble membranes has been observed and analyzed by high speed light microscopy. In our contribution, we classify these processes and quantify their temperature dependent probability as a function of the mode of freezing and the presence of immersed particles.