PLEASE POST
MMM SEMINAR NCAR
Ice formation by surface crystallization and
implications for evaporation freezing
Raymond Shaw
Department of
Physics
Ice formation in atmospheric clouds is crucial to our understanding of precipitation and cloud radiative properties, yet in many instances much more ice forms in clouds than can be accounted for with known nucleation mechanisms (Workshop on Ice Initiation 2004). Inexplicably high ice concentrations are sometimes associated with cloud dilution and droplet evaporation (Cooper 1986; Hobbs and Rangno 1985), motivating speculation of an evaporation nucleation mechanism (Beard 1992). For example, recent experiments in wave clouds, where Lagrangian trajectories of cloud particles are known, have provided compelling evidence for rapid ice formation where cloud droplets are evaporating (Cooper 1995; Cotton and Field 2002). With this in mind, we have carried out laboratory experiments aimed at understanding the mechanisms for ice formation by heterogeneous nucleation the possible role of droplet evaporation.
We have observed higher freezing temperatures when an
ice-forming nucleus is near the surface of an undercooled
water drop than when the nucleus is immersed in the drop. The nucleation rate at the water surface is a
factor of 
greater than in bulk
water, thereby providing evidence for a general form of surface
crystallization, apparently encompassing the familiar phenomenon of contact
nucleation. However, the data challenge
existing hypotheses for contact nucleation.
Instead, interpretation of the data via classical nucleation theory
suggests that the increase in nucleation rate is a result of changes in both
the thermodynamics and the kinetics of the ice formation process near the water
surface. Finally, we observe evidence
for ice nucleation occurring during drop evaporation as a result of surface
crystallization, and therefore we provide a plausible physical mechanism for
the phenomenon of evaporation freezing.
Thursday, 9 June 2005, 3:30
PM
Refreshments
NCAR-Foothills Laboratory
Bldg 2 Auditorium (Rm1022)