Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 48

above the temperature of the maximum density (T m ), the lake water column is unstable in
cooling, thermocline deepens and heat is removed from the deep water. Due to
mechanically forced mixing, convection can continue at T < T m . Finally, inverse strati-

fication develops with surface layer close to the freezing point and a little warmer lower
layer. Heat transport from the warm deeper layers to the lake surface is reduced, the
surface cooling rate increases, and the surface temperature achieves the freezing point,
followed by ice formation. Hence, the timing of freezing is strongly dependent on synoptic
conditions over the lake. In deep lakes,

the convective phase can last long, even through the whole winter.
The heat balance during ice melting is fundamentally different from that during lake
cooling and ice growth (Jakkila et al. 2009). Then the stable strati

— passages of cold air masses and strong winds —

cation of atmospheric
surface layer strongly reduces the turbulent heat exchange at the lake

air interface. Solar
radiation is the dominant source of heat for snow and ice melting, partly absorbed by the
ice sheet and partly by the lake water below. This fact implies also that albedo and
transparency of the ice sheet have a strong in

-

uence on melting. The key role of the solar
radiation also explains the fact that ice breakup date is coherent at spatial scales of
hundreds of kilometres (Magnuson et al. 2000). Other factors able to ef

fl

ciently accelerate

ice melting are liquid precipitation and strong and warm winds.
The sequence of an ice season, from the

first freezing to the final break-up is not a

simple cycle but there may be melt refreeze events in between (Bernhardt et al. 2011;

Kirillin et al. 2012). Ice formation and melting are not symmetric processes but ice is self-
protecting. Once an ice cap has formed, it is dif

-

cult to melt it in since in winter solar
radiation level is low and albedo feedback keeps the absorption of solar radiation low.
Also the lake heat storage is largely isolated from the ice by weak mixing conditions. But
still it is possible that ice cover disappears and forms again within one ice season, even
several times. This is especially true in the vicinity of the climatological margin of
freezing lakes. Then the freezing date and breakup date are still de first and last

ones in the ice season, and the presence of ice-free periods is seen in the difference
between the length of ice season and the number of ice days. In the other extreme, in a
cold year at high-latitudes or high-altitudes, it is possible that some ice survives over
summer and becomes multi-year ice. The ice cover has then changed into perennial state.
Because of the heat budget, freeze-up of lakes depends on the lake depth and size,
while melting is more up to the growth of ice and snow accumulation during winter.
Therefore, ice breakup date follows closely the latitude, but individual lake characteristics
show up in the freezing date (Fig. 2.18 ).
After ice cover formation, ice thickness increases as long as the released latent heat
from freezing and the heat

ned as the

fl flux from the water body can be conducted through the ice to

the atmosphere (e.g., Lepp ä ranta 2009a). The main factors controlling the ice growth are

the heat fluxes at the upper and lower boundaries of the ice cover together with the thermal

properties of ice and snow. An early appearance of snow cover can play a crucial role in
decelerating the ice growth due to the very low heat conductivity of snow. The total
thickness of ice as well as the thicknesses of congelation ice and snow-ice layers is

fl