Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 33

snowflakes, during snow flurry activities in the air, then
those crystals with their flat surfaces parallel to the sur-
face tend to grow faster than the others. Of course, this is
not the case when snowfall occurs and covers the water
surface. In those situations, the initial growth processes
are complex and flakes with their flat surfaces normal to
the water surface are in favorable conditions for growth
(for details on this topic, see section 4.3.3.4).
Knowledge of the early stages of nucleation of crystals
at the snow‐free surface of saline water and the initial
growth comes from studies conducted in the laboratories
[ Weeks and Assur, 1963; Weeks and Lofgren, 1967; Cox and
Weeks, 1975]. Under calm conditions, due to the favorable
orientation mentioned above, ice discs continue to grow
laterally up to 2-3 mm depending on the salinity of the
water [ Weeks and Ackley, 1982]. The freezing front
becomes wavy as the disc size increases. Initially circular
boundaries assume tree‐like or dendritic structures by
developing “arms” around their peripherals as shown
schematically in Figure 2.6. The term dendritic comes
from dendrology—the study of trees. This is true for both
freshwater and seawater ice but due to different mecha-
nisms. In freshwater ice, with low concentration of solutes,
formation of dendrites is primarily a thermally induced
phenomenon. The temperature gradient around the crys-
tal is asymmetric due to complications of heat flow. This is
mainly due to the instability at the growing front induced
by increase in size of the discs. A small perturbation at the
ice‐water interface ends up in even more supercooled liq-
uid so the interface becomes more unstable. In saline water,
on the other hand, initiation of dendritic arms is a salin-
ity‐related phenomenon. As a discoid continues to grow it
pushes the salt‐rich solute to its boundaries. Salt concen-
tration starts to build up around the disc with varying dis-
tribution leading to varying degrees of supercooled water.
This causes instabilities in the freezing such that any fur-
ther crystal growth takes place anisotropically (though still

in the horizontal plane). The growth direction follows
the path of supercooled water, i.e., the direction of least
resistance for growth. Details on this aspect of growth are
provided later in the section 2.3.2.
The initial discoidal crystal shape develops into den-
dritic or stellar form with very fragile arms. The average
diameter of these crystals may be around 2.5 mm [ Weeks,
1959]. The change from discoidal to stellar crystal shape is
accelerated under rapid cooling when the solute is rejected
at a rate fast enough to cause significant variation of the
salt distribution in the vicinity of the crystal in the plane
parallel to the major plane of the discoid. Moreover, the
arms of the stellar crystals may grow thicker because they
are surrounded by supercooled water and may not be able
to sustain their own weight and consequently break.
As the growth continues, the fragile arms of stellar
crystals, therefore, start to break off and form needle‐
shaped crystals as illustrated in Figure 2.6b. This is
triggered mainly by the wave actions that tilt and bend
the delicate discs to fracture into segments. Actually
this type of fragmentation of thin ice discs occur in
both freshwater and seawater. Early observations
[ Suzuki, 1955] showed that a fewer number of needles
are observed in case of freshwater ice. These needles are
confined to the thin supercooled layer at the surface
[ Hallet, 1960]. It is well known that frazil crystals are not
necessarily limited to thin layers at the water surface.
Turbulent wave action also causes inclined discs and
needles to sink into the thick supercooled mixed layer.
The needle‐shaped ice crystals may be geometrically
oriented or randomly oriented and usually carried away
by the ocean surface currents. The ocean wave may tend
to compress those needles and orient them with their long
axis parallel to the vertical plane. The spatial distributions
of the discoid, the broken discoid, and the needle‐shaped
crystals are usually sporadic and loose. The frazil needles
are typically less than 1 mm in thickness and a few mil-
limeters to a few tens of a millimeter in length. The aggre-
gation is known as frazil or grease ice [ Weeks and Ackley,
1982]. Reviews on frazil ice in rivers and ocean can be
found in Osterkamp [1978] and Martin [1981].
When frazil crystals cluster together over a large area
of a water surface, they dampen surface motion and give
the ocean surface a greasy or oily appearance. It is a soupy
layer on the ocean surface that does not reflect much
light, hence gives the surface a dark matt appearance. It is
known as qinu in Inuktitut, the language of the indige-
nous Inuit people in the Canadian Arctic and Greenland
as described in Chapter 1. In general, this is known as
“grease ice,” though it is not actually solid ice. Ocean
currents cause frazil crystals to herd into streaks or pile
up downwind against the edges of floating ice floes to
depths that can be on the order of 1 m [ Martin, 1981;
Sinha, 1986]. Figure 2.7 provides an example of frazil ice

(a) (b)

Figure 2.6 Stages of dendritic or stellar growth of a freshly nucle-
ated circular discoid in (a) saline water and (b) fragmentation of
the stellar crystal.