Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 214

Fig. 7.6 Lake ice environment in the ice season

Quantity Ice season compared to open water season

Wind stress Removed

Surface pressure No change

Solar radiation Reduced by more than 50 %

Air-lake heat exchange Reduced, realized mostly in phase changes

Water balance Atmospheric exchange removed

Air-lake exchange of matter Removed

Thus the ice cover largely isolates the lake with a degree of isolation depending on the
ice coverage, ice thickness, and snow thickness. In the heat conservation law (Eq. 7.4 ) the
main difference between ice-free and ice-covered lakes is in the surface boundary con-
ditions. In the case of a stable ice cover, the temperature is at the freezing point and the
current velocity is zero:

T ¼ T f ; u ¼ 0 ð 7 : 17 Þ

There is a weak loss of heat through the ice cover and gain from the bottom sediments.
Circulation under ice cover is thermohaline. Whether the

flow is laminar or turbulent is
indicated by the Reynolds number Re = UL/v, where U and L are the velocity and length
scales and v

fl

10 − 6 m 2 s − 1 ≈ 1.8 × is the kinematic viscosity of water. The laminar - tur-

10 3 10 4 . Therefore ice-free lakes are turbulent, but

bulent transition regime is at Re * -

10 − 3 ms − 1 , and then the transition is at L under compact ice cover U 10 m.

The winter of seasonally freezing lakes can be divided into four phases (Kirillin et al.
2012b). The pre-winter, discussed in Sect. 7.1 , forms the 1st phase. The 2nd phase is the

* * 1 -