Thermal imaging reveals details of ice multiplication upon freezing of drizzle droplets

Abstract:

The freezing of drizzle drops is an ubiquitous process fundamental for ice formation and precipitation in the mixed-phase clouds. The freezing is known to proceed in two steps: first, upon nucleation, the ice dendrites spread across the volume of a supercooled droplet, releasing latent heat and warming the droplet to the melting point of ice; during the second step, a solid ice shell forms on the surface of the droplet, leading to a pressure rise inside the liquid core. Once pressure exceeds the mechanical strength of ice, the shell cracks open, ejecting ice fragments or leading to the droplet shattering. This mechanism is potentially important for the rapid glaciation of mixed-phase clouds, but the underlying details are poorly understood.

The growth rate of ice is limited by the rate of latent heat release to the environment across the surface of the droplet by heat diffusion and forced convection. Therefore, the droplet surface maintains the temperature of melting point until all liquid water is converted to ice. The rise of internal pressure, however, leads to the depression of melting point according to the Clapeyron equation with approximately 1 K per 135 bar, and thus the surface temperature is sensitive to the pressure. We make use of this relationship to measure the time evolution of the internal pressure by recording the surface temperature of a freezing droplet with a high-resolution infrared thermography system. In this study, the drops of 300 μm diameter were levitated in an electrodynamic trap under controlled temperature, humidity and ventilation. Alongside with the IR measurements, the freezing process was recorded with a high-speed video camera. We find that depending on temperature and air flow conditions, a single freezing drop can experience multiple pressure release events (PREs) visible as downward spikes on the surface temperature curve. As every PRE must be associated with a crack formation in the ice shell, the number of secondary ice particles ejected by a single freezing droplet could be by far higher than our previous estimation based on the high-speed video records. We also note, that the droplets freezing under free fall conditions exhibit more PREs than the droplets in stagnant air, supporting our previous finding of ice multiplication enhancement caused by ventilation of a freezing drizzle droplet.