Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 227

Fig. 7.11 Evolution of isotherms at two measurement points in Lake P ää j ä rvi, Finland, in March
2010. The short-term oscillations of isotherms associated with internal, basin-scale waves persist in
the nearshore record only, demonstrating the Kelvin-type wave shape (Kirillin et al. 2012b)

inertial gravity waves traveling around the lake — analogs of Kelvin and Poincar é waves

(Gill 1982) for enclosed basins. Such a de ection of the internal basin-scale waves implies

important consequences to redistribution of the mixing energy. In contrast to planar
seiches, Kelvin waves have their maximum amplitudes and current speeds near the lake
shores (Fig. 7.11 ) affecting thus the water

fl

sediment mass exchange in the littoral zone
(Kirillin et al. 2009). Internal, basin-scale waves are known to be a major factor producing
mixing in open-water lakes (W

-

ü est and Lorke 2003); their contribution into mixing under

ice is still unclear.

7.3.3 Melting Period

The melting period differs much from the mid-winter. It is dynamic, there is intensive
ice

water interaction, and the decay of ice possesses a positive feedback via the evolution of
albedo. Lake ice melts fast compared with its growth, the melt rate is 1

-

3 cm day − 1 . A heat
gain of 100 Wm − 2 , which is a high level, would melt 2.8 cm ice in 1 day. Ice is fresher than
the lake water (e.g., Lepp

-

ranta et al. 2003b), and therefore the water salinity decreases
beneath melting ice with a consequent stabilizing effect on the surface layer strati

ä

cation.