HR: 1340h
AN: A23C-0970    [Abstracts]
TI: Contact Nucleation Linked to `Evaporation Freezing'
AU: Shaw, R A
EM: rashaw@mtu.edu
AF: Department of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 United States
AU: * Durant, A
EM: ajdurant@mtu.edu
AF: Department of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 United States
AU: * Durant, A
EM: ajdurant@mtu.edu
AF: Geological Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 United States
AB: Ice formation in atmospheric clouds plays a fundamental role in precipitation and cloud radiative properties. Contact nucleation, an important but poorly understood heterogeneous nucleation pathway, defines the freezing of a supercooled cloud drop on contact with an ice-forming nucleus (IN), traditionally from the outside of the drop. In recent work it was shown that heterogeneous ice nucleation rates are enhanced by a form of `surface crystallization'. Here we describe additional experiments and consider the implications for contact nucleation and its relevance to ice nucleation in atmospheric clouds. Our observations suggest that the notion of contact nucleation should be generalized to include surface crystallization from particles contacting a supercooled drop from the inside out, as well as from the outside in. Specifically, (1) we challenge the existing hypothesized mechanisms for contact nucleation in light of the laboratory observations; (2) we present laboratory evidence for enhanced ice nucleation during drop evaporation; and (3) we hypothesize that this more general picture of contact nucleation can result in `evaporation freezing' in atmospheric clouds. Our observations are not consistent with the three leading hypotheses for contact nucleation which include partial solubility of the IN, incomplete adsorption upon initial contact with water, and mechanical disturbance of the water-air interface upon contact: All of these mechanisms are related in some way to the transient nature of contact between a dry IN and a supercooled water drop, which is not present in our experiments. The generalized view of contact nucleation has implications for atmospheric ice formation. For example, there are abundant observations of enhanced ice formation in regions where cloud droplets are evaporating in cumuliform, stratiform, and wave clouds, and it has been speculated that contact nucleation may be responsible for evaporation freezing. Our results lead to the hypothesis that the freezing temperature of an evaporating drop will suddenly increase once the drop surface contacts an immersed IN. This mechanism for evaporation freezing is therefore a plausible explanation for observations of high ice concentrations associated with cloud dilution and droplet evaporation.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 4801 Aerosols (0305, 4906)
DE: 4906 Aerosols (0305, 4801)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005