The freezing process of a supercooled water droplet freely falling through air is a remarkably dynamic and eventful process. During freezing from the outside in, the volume increase of liquid water upon solidification leads to a pressure rise inside the droplet. The pressure is released in various ways, e.g. by cracking or by complete fragmentation of the ice shell. These processes may be the source of secondary ice particles that are emitted during droplet freezing. In this study, the surface temperature of freezing drizzle-sized water droplets was measured with a high-resolution infrared thermography system, while recording the changes in shape and structure of the droplet by a high-speed video camera. The droplets were levitated in an electrodynamic trap under controlled conditions with respect to temperature, humidity and airflow velocity. Measurement of the surface temperature during freezing allowed for determination of the absolute pressure inside the liquid core. During the freezing of a droplet the pressure rise is interrupted many times by rapid pressure release events, each being a possible source of secondary ice. Pressure release events were three times more frequent for droplets freezing under free-fall conditions compared to droplets freezing in stagnant air. Naturally occurring sea salt content (< 100 mg/L) does not inhibit the pressure buildup inside freezing drizzle droplets.

&lt;p&gt;The freezing of a supercooled water drop freely falling through a mixed-phase cloud is an ubiquitous natural process fundamental for the formation of precipitation in clouds. The freezing is known to proceed in two stages: first, a network of ice dendrites spreads across the volume of a supercooled droplet resulting in ultrafast release of latent heat and warming of the droplet up to the melting point of ice; during the second stage a solid ice shell grows from the outside into the droplet, leading to a pressure increase inside the liquid core. Once the pressure gets too high, either the shell cracks open or the droplet explodes. The resulting secondary ice fragments start growing in the water-saturated environment or cause the freezing of neighbouring droplets. This secondary ice production mechanism is important for the rapid glaciation of mixed-phase clouds, however, the details of the underlying mechanisms are poorly understood. To quantify this process of ice multiplication, the evolution of the droplet&amp;#8217;s surface temperature during the second freezing stage was investigated with a high-resolution infrared thermography system (INFRATEC). Drops of about 300 &amp;#181;m diameter were levitated in an electrodynamic trap under controlled conditions with respect to temperature, humidity and ventilation. The surface temperature of the droplet was measured with the IR system while the freezing process and shattering of the freezing droplet was recorded by a high-speed video camera. Combining experimental results and comprehensive process modelling, we explore the thermodynamic conditions beneficial for secondary ice production upon freezing of freely falling drizzle droplets.&lt;/p&gt;

&lt;p&gt;During the freezing of supercooled drizzle droplets, the ice shell forms at the droplet surface and propagates inwards, causing a pressure rise in the droplet core. If the pressure exceeds the mechanical stability of the ice shell, the shell can crack open and eject secondary ice particles or cause the full disintegration of the ice shell leading to droplet shattering. Recent in-cloud observations and modeling studies have suggested the importance of secondary ice production upon shattering of freezing drizzle droplets. The details of this process are poorly understood and the number of secondary ice particles produced during freezing remains to be quantified.&lt;/p&gt;&lt;p&gt;Here we present insight into experiments with freezing drizzle droplets levitated in electrodynamic balance under controlled conditions with respect to temperature, humidity and airflow velocity. Individual droplets are exposed to a flow of cold air from below, simulating free fall conditions. The freezing process is observed with high-speed video microscopy and a high-resolution infrared thermal measuring system. We show the observed frequencies for various events associated with the production of secondary ice particles during freezing for pure water droplets and aqueous solution of analogue sea salt droplets (300 &amp;#181;m in diameter) and report a strong enhancement of the shattering probability as compared to our previous study (Lauber et al., 2018) conducted in stagnant air. Analysis of pressure release events recorded by high-resolution infrared thermography suggest that pressure release events associated with the possible ejection of secondary ice particles occur far more frequent than previously quantified with observations by high speed video microscopy only.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Lauber, A., A. Kiselev, T. Pander, P. Handmann, and T Leisner (2018). &amp;#8220;Secondary Ice Formation during Freezing of Levitated Droplets&amp;#8221;, Journal of the Atmospheric Sciences 75, pp. 2815&amp;#8211;2826. &lt;/p&gt;

&lt;p&gt;Supercooled drizzle droplets may produce multiple ice particles upon freezing. This mechanism could potentially explain the high ice number concentrations outside of temperature range where the well-known Hallett-Mossop mechanism of ice multiplication would take place. Limited experimental methods in the past prevented direct observations of the shattering droplets, resulting in a wide range of experimental results, unsuitable for the development of a sophisticated cloud model parameterization. Recently, we have revived experiments on secondary ice production by levitating individual drizzle droplets in electrodynamic balance (EDB) and observing the freeze-shattering with high-speed video microscopy and high-resolution infrared thermal measuring system. In this way we have been able to identify three additional SIP mechanisms (cracking, jetting and bubble bursts) associated with the freezing of drizzle droplets (Lauber et al., 2018). &lt;br&gt;Additionally, we have extended the range of experimental conditions to mimick the freezing of continental (pure water) and maritime (aqueous solution of analogue sea salt) drizzle droplets suspended in the updraft of cold moist air. We report a strong enhancement of shattering probability as compared to the previous studies conducted under stagnant air conditions. The high-definition video records of shattering events reveal the coupling between various microphysical processes caused by ice propagation inside the freezing drop and reveal striking difference between freezing of pure water and SSA solution droplets. Application of high-resolution infrared microscopy allowed us to record the evolution of the droplet temperature under realistic flow conditions and thus constrain the thermodynamic parameters controlling the pressure build-up inside the droplet. Based on these new observation data and theoretical model of freezing droplet, we discuss the physical mechanism behind the shattering of drizzle droplets and its implication for mixed-phase cloud modeling.&lt;/p&gt;&lt;p&gt;Lauber, A., A. Kiselev, T. Pander, P. Handmann, and T Leisner (2018). &amp;#8220;Secondary Ice Formation during Freezing of Levitated Droplets&amp;#8221;, Journal of the Atmospheric Sciences 75, pp. 2815&amp;#8211;2826.&lt;/p&gt;