  We examine ice crystal growth from water vapor at temperatures near the
melting point, when surface premelting creates a quasiliquid layer at the
solid/vapor interface. Recent ice growth measurements as a function of vapor
supersaturation have demonstrated a substantial nucleation barrier on the basal
surface at these temperatures, from which a molecular step energy can be
extracted using classical nucleation theory. Additional ice growth measurements
from liquid water as a function of supercooling exhibit a similar nucleation
barrier on the basal surface, yielding about the same molecular step energy.
These data suggest that ice growth from water vapor and from liquid water are
both well described by essentially the same underlying nucleation phenomenon
over a substantial temperature range. A physical picture is emerging in which
molecular step energies at the solid/liquid, solid/quasiliquid, and solid/vapor
interfaces create nucleation barriers that dominate the growth behavior of ice
over a broad range of conditions. Since the step energy is an equilibrium
quantity, just as surface melting is an equilibrium phenomenon, there exists a
considerable opportunity to use many-body simulations of the ice surface
structure and energetics at equilibrium to better understand many dynamical
aspects of ice crystal growth.
