Future of Frozen Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 273

and the evaluation of climate change consequences becomes more reliable than with semi-
analytic models. Slush layers are well reproduced and the physical representation of
melting ice is more realistic.
In addition, mechanical models are used to examine the breakage of ice, a factor, which
is a critically important to the character of lake ice seasons. Mechanical models have not
been much discussed in the lake ice literature over the last 25 years. Ice drift models have
been constructed for marine basins, down to the size of 100 km, but in lakes this work has
been quite limited. The main area of lake ice drift modelling has been the Great Lakes of
North America, where shipping has needed more such information. A major characteristic
of lake ice drift is that it consists of short-term displacements between immobile states that
require a very close approximation to plastic behaviour in the modelling. The occurrence
of ice displacements in a given basin depends on the ice thickness, and therefore climate
warming may in uence the quality of ice cover.

In the lake water body, physical phenomena and processes are very different under ice
cover from the open water conditions. The ice cover cuts the transfer of momentum from
the wind to the water body that damps turbulence and mixing. The surface water tem-
perature is at the freezing point, and there is very little vertical transfer of heat, apart from
geothermal lakes. In all, the temperature structure and circulation are quite stable.
In fully ice-covered lakes the water

fl

fl flow under ice can be in laminar or laminar-

turbulent transition state that makes the ice cult problem for

observation techniques and modelling. Even though the water velocities are small, they
are stable and result

water body interaction a dif

-

cant long-term transport of water with its impurities.
However, in very large lakes, the ice sheet may experience episodic movements and
disturb the water body. In spring, solar radiation provides a strong downward

in signi

fl flux of heat,

which constitutes the strongest heat flux into fully ice-covered lakes, and the ice melt

water with its impurities is released into the water column.
Research on winter biology of lakes has largely increased (Salonen et al. 2009; Shuter
et al. 2012). Questions of life in extreme conditions have brought in more research of
lakes with perennial ice and pro-glacial lakes (Vincent and Laybourn-Parry 2008;
Keskitalo et al. 2013), especially in the Antarctic continent (Fig. 1.3 ). The ecosystem of
subglacial lakes has been largely unknown (Christner et al. 2006), but in austral summer
2012

fl

first water samples ever were taken from these lakes showing signs of
bacterial life beneath the ice sheet (Shtarkman et al. 2013).
Lake ice belongs to the part of cryosphere, which closely interacts with human living
conditions. The ice cover has caused problems but people have learnt to live with them
and also to utilize the ice. Especially, in the present time this is true for on-ice traf

2013 the

-

c and

recreation activities. Ice fishing has become a widely enjoyed winter hobby. Apart from

fishing, winter sports such as skiing, skating and ice sailing are now popular winter
activities on frozen lakes. In the past, lake ice was used to store perishable products, when
it was removed during the winter and stored for the summer. On the negative side, the
freezing of lakes makes boating impossible and limits their use for commercial and
recreational transport. The bearing capacity of ice is a major safety question. It increases