Structure and Properties of Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 63

congelation ice, and superimposed ice (Fig. 3.5 ). The primary ice layer is often very thin
and dif

cult to identify in vertical ice samples.
Primary ice forms the first layer of a lake ice sheet. Its texture depends on the pre-

vailing weather conditions (Gow 1986). In calm or laminar

flow conditions, the case
called quiet freezing (Gow and Govoni 1983), a thin supercooled water layer forms at the
surface and ice crystals appear when the nucleation temperature is reached. Supercooling
is then 0.5

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first on the surface with their main axis
perpendicular to the c-axis, and the number of crystals depends on the number of crys-
tallization seeds and the rate of cooling. The size of crystals varies largely depending on
the concentration of nucleation seeds and the rate of the heat loss from the water. Ice skim
grows horizontally in a supercooled surface layer. Smaller temperature gradient means
slow crystallization, and the needles expand into plate shape aligned with the surface and
with c-axis perpendicular to the surface. In the case of larger temperature gradient, the
crystals grow fast and their number is larger. Then the plates do not

1.5 ° C. Needle-shaped crystals form

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tally, and the orientation of the c-axes becomes random.
In quiet freezing the primary ice layer is very thin (less than 1 mm), and it may be
easily lost by melting or sublimation. This ice forms typically in a cold clear night with
large thermal radiation losses from the water body. The freeze-up starts from the shoreline,
where the lake is shallow and cools fast. The near-shore ice is called border ice. The ice
cover extends further offshore on the surface if the conditions remain calm or laminar.
Turbulent flow can break thin border ice to restart then in next calm phase. The crystal

structure of primary ice depends on the rate of growth, and it will in

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uence the crystal
structure of the secondary ice, which will grow underneath (Gow 1986).
If freezing is initiated by ice or snow crystals falling onto water, the ice crystals remain
small and randomly oriented. The solid precipitation may also come from the water body
itself by frozen splashes or by evaporation

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deposition mechanism. During heavy snow-
fall, the primary ice layer forms of congealed snow slush and can also be much thicker
than what results from quiet freezing.
In turbulent conditions, frazil ice crystals form. These are circular disks, and they
agglomerate into porous

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flocs. The c-axes are randomly oriented in three dimensions. The
buoyancy of ice damps turbulence, and when the frazil volume is large enough,

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frazil aggregates appear, which will then freeze into solid layers of ice. Frazil ocs are

effective in capturing impurities from the water column. Frazil ice formation can be fast
since the surface temperature is kept at the freezing point and heat losses from the water
body can be very large. Frazil ice formation is a common phenomenon in rivers, where the
surface can be open in rapid stream sections throughout the winter (see, e.g., Ashton
1986). The thickness of the frazil layer can be several centimeters. Frazil crystals may join
in the surface into plate aggregates, which collide with each other and become rounded.
This is called pancake ice due to its visual appearance, the size of the pancakes being up to
a few meters, often observed in large lakes (e.g., Nghiem and Leshkevich 2007).
Secondary ice grows down from the ice bottom into the water body as congelation ice
(Shumskii 1956; Sokol

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' nikov 1957; Michel and Ramseier 1971; Gow and Govoni 1983;