Thermodynamics of Seasonal Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 122

Snow-ice formation can also take place due to melt-freeze cycles and liquid precipi-
tation. In both cases the question is that how much snow is available to form the slush. If a
slush layer is formed in the lower part of the snow layer, it will freeze into snow-ice
according to Eq. ( 4.49 ). Otherwise snow-ice formation is easier than by flooding since the

insulating effect of snow becomes reduced when snow is consumed in snow-ice forma-
tion. However, these cases do not produce much snow-ice as compared with

fl

fl flooding.

Example 4.6. A special case is a heavy snowfall before freezing and in early ice season.

The insulation effect of the snow cover is strong from the beginning, and the resulting ice
sheet is snow-ice with variable ice thickness. Such situation occurred in winter 2013

2014

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in Lake Kilpisj rvi, northern Lapland. Even though the winter was cold, there were even

weak spots in the ice cover.

ä

Example 4.7. It is easy to see that if lake water is pumped up to the snow layer, more ice

will be produced than by keeping the surface bare to aid congelation ice growth. This is
due to that by the pumping the insulation thickness of snow can be reduced and the ice
grows much faster than natural snow-ice. Although snow-ice has low mechanical strength,
the bearing capacity becomes large using the pumping method due to the large ice
thickness. This method was used in Lake Katumaj

ä rvi, which served as the starting area in

Finlandia Ski Marathon from H meenlinna to Lahti (Palosuo 1982). The bearing capacity

question was not critical for the skiers but it was so for the trucks, which collected the
skiers

ä

' bags to transport them to the goal area.

4.3.4 Frazil Ice

In open water conditions, frazil ice forms where the ice crystals move free in turbulent
eddies (see Sect. 3.2.2 ) . In this situation there is one important limitation in the surface
energy balance: the surface temperature is at the freezing point. Therefore, in low air
temperature conditions turbulent heat losses from the water surface can be quite large and,
consequently, frazil ice production is fast. The mathematical model of frazil ice accu-
mulation can be expressed as:

dh

dt ¼ k 0 þ k 1 T a T f

Q w

q L f ¼ q

ð 4 : 51 Þ

q L f

where h stands for the frazil ice volume per area. For frazil ice growth, we must have
q > 0. Clearly h = h 0 + qt, where h 0 is the initial value. In winter conditions,
k 0 * -

40 W m − 2 and k 1 * 15 W m − 2 C − 1 (Table 4.1 ), and thus the k 1 -term is strongly °

dominant when the air C.

Figure 4.10 shows a comparison between the snow-free congelation ice and frazil ice.
When the wind is strong, turbulent heat losses and frazil ice accumulation become much

surface temperature difference is more than 10 °

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