Molecular simulations provide evidence in support of ice nucleation upon collision or breakup of supercooled cloud droplets
Elise
Rosky
University of Michigan
Talk
The tendency of supercooled water to freeze when mechanically agitated has long caused speculation that supercooled droplet collisions or breakup could produce ice in clouds. If active in clouds to a significant extent, this process could be a potential cloud glaciation mechanism, or act as a secondary ice production mechanism involving large supercooled droplets. However, gaps in our understanding of ice formation in supercooled water can be traced all the way down to the molecular scale, and the physical mechanism of freezing during mechanical agitation remains unresolved.
Motivated by laboratory experiments which suggest a role for pressure fluctuations in the onset of freezing during droplet agitation, molecular simulations of water are used to quantify the increase in ice nucleation rate as a function of negative pressure (i.e. tension) within supercooled water. The increase in freezing temperatures with negative pressure is approximately linear within the atmospherically relevant range of 1 to −1000 atm, and a physical equation describing the slope provides a starting point for experimental predictions or parameterization. Results indicate that negative pressures of −500 atm, which can result from nanometer-scale agitation of the water droplet surface, lead to a roughly 4 K increase in heterogeneous-freezing temperatures. In mixed-phase clouds, this would result in a notable increase in active INP concentrations.
The findings presented here indicate that any process leading to negative pressure (i.e. tension) within supercooled water may play a role in ice formation, pointing towards the potential for dynamic processes such as contact nucleation and droplet collision or breakup to increase ice nucleation rates through pressure perturbations. Dynamic processes such as droplet collision or breakup may warrant further investigation as potential ice nucleation mechanisms.
Funding provided by NASA (80NSSC20M0124), Michigan Space Grant Consortium (MSGC), NSF grants AGS-1541998 and AGS-2019649, and award CBET-2053330.
Motivated by laboratory experiments which suggest a role for pressure fluctuations in the onset of freezing during droplet agitation, molecular simulations of water are used to quantify the increase in ice nucleation rate as a function of negative pressure (i.e. tension) within supercooled water. The increase in freezing temperatures with negative pressure is approximately linear within the atmospherically relevant range of 1 to −1000 atm, and a physical equation describing the slope provides a starting point for experimental predictions or parameterization. Results indicate that negative pressures of −500 atm, which can result from nanometer-scale agitation of the water droplet surface, lead to a roughly 4 K increase in heterogeneous-freezing temperatures. In mixed-phase clouds, this would result in a notable increase in active INP concentrations.
The findings presented here indicate that any process leading to negative pressure (i.e. tension) within supercooled water may play a role in ice formation, pointing towards the potential for dynamic processes such as contact nucleation and droplet collision or breakup to increase ice nucleation rates through pressure perturbations. Dynamic processes such as droplet collision or breakup may warrant further investigation as potential ice nucleation mechanisms.
Funding provided by NASA (80NSSC20M0124), Michigan Space Grant Consortium (MSGC), NSF grants AGS-1541998 and AGS-2019649, and award CBET-2053330.
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