Thermal imaging of a shattering freezing water droplet

Abstract

<p>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’s surface temperature during the second freezing stage was investigated with a high-resolution infrared thermography system (INFRATEC). Drops of about 300 µm 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.</p>

Continuous secondary-ice production initiated by updrafts through the melting layer in mountainous regions

“…This is observed for example in shallow stratocumulus (Mossop et al, 1972) or thin stratus clouds but often does not hold true in more convective clouds. A discrepancy between the INPC and the ICNC of up to 4 orders of magnitude has been observed in several studies (e.g., Koenig, 1963;Hobbs and Rangno, 1985;Crawford et al, 2012;Lasher-Trapp et al, 2016;Ladino et al, 2017;Beck et al, 2018). If surface-based processes like blowing snow (e.g., Beck et al, 2018) and the seeder-feeder process (hydrometeors which formed aloft precipitate into the cloud below; e.g., Bader and Roach, 1977;Ramelli et al, 2020a) can be excluded as a potential ice source, this discrepancy can only be explained by SIP.…”

“…The larger the droplet, the more likely it will fragment and the more splinters it will likely produce (Kolomeychuk et al, 1975;Lauber et al, 2018). Many field studies showed that large droplets are present in clouds before ICNCs exceed the INPCs by orders of magnitude (e.g., Koenig, 1963;Braham, 1964;Mossop et al, 1970;Hobbs and Rangno, 1990;Rangno, 2008;Lawson et al, 2017; and often explained these observations with SIP by droplet fragmentation. Even though large droplets are rare in clouds, they might start a cascading process of splinter production when produced splinters hit other droplets, which subsequently freeze and produce more splinters (Koenig, 1963;Chisnell and Latham, 1974;Lawson et al, 2015).…”

Continuous secondary ice production initiated by updrafts through the melting layer in mountainous regions

“…Apart from this, there is no physical basis for this correlation. There are so far no reliable measurements of the average number of fragments being produced per fragmentation and recent work by Kleinheins et al (2020) provides an indication that a majority of possible fragment ejections during freezing could not be observed by the applied measurement techniques. The application to the case study suggests that the number of splinters may be up to 5 times higher than assumed in eq.…”