Thermal Imaging of Freezing Drizzle Droplets: Pressure Release Events as a Source of Secondary Ice Particles

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 3 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-1) does not inhibit the pressure buildup inside freezing drizzle droplets.