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Below the temperature of maximum density, further surface cooling is a stabilizing
factor. After the ice cover has formed, solar radiation provides the strongest external
forcing on the temperature strati
cation, especially in the ice decay phase after snowmelt.
As soon as the solar heating can penetrate the ice sheet, strati
cation weakens and turns to
convective mixing. The second factor is heating by the bottom sediments. Usually the
bottom water temperature is below the temperature of maximum density in the beginning
of winter, and sediment heat
flux drives density currents down to greater depths. But in the
case of strong geothermal heat
fl
flux, convective overturning may be triggered. The relative
importance of the solar and sediment heat
fl
fl
fluxes varies in the course of the winter.
field data, e.g., by
Kenney (1996), Malm (1998), and Terzhevik et al. (2009). Small boreal lakes reveal high
inter-annual variability depending on snowfall and radiation conditions. The magnitude of
the sediment heat
The heat
fl
flux from bottom sediments has been examined from
10 W m
−
2
(Bengtsson 2011; Kirillin et al. 2012b), and it is a
major factor in lake thermodynamics when the surface is protected by the ice cover. For a
lake depth of 10 m,
fl
flux is 1
-
this heat
fl
flux would increase the mean water temperature by
C month
−
1
. As a result, normally the water temperature increases in the lower
layer during the ice season. Apart from strong geothermal heating, the surface temperature
of the sediments is close to the temperature of maximum density of the lake water that
limits the temperature level the heating may reach. For strong heating as observed in many
geothermal lakes (100 W m
−
2
or more), the lake would stay open all the winter.
The latent heat released in ice growth must be conducted to the atmosphere through the
ice and it does not in
0.06
0.6
°
-
uence on the temperature of the liquid water that remains. For a
growth rate of 1 cm day
−
1
, the heat transfer is 35 W m
−
2
. In addition, there is an upward
heat
fl
flux from the water body to the ice depending on the temperature gradient and water
currents. This heat
fl
flux is conducted away through the ice or used for ice melting at the
bottom of the ice. It has been estimated as 1
fl
10 W m
−
2
(e.g., Jakkila et al. 2009; Yang
et al. 2012), i.e. the magnitude is the same as is usually the magnitude of the heat
-
fl
ux
from the sediments in boreal lakes. As a consequence, heat
fluxes at the upper and lower
surface of the water body largely compensate each other, and the temperature pro
fl
le of
the lake remains rather stable as long as solar radiation is absent or weak (see Fig.
7.8
).
Sunlight starts to heat the surface water layer (see Sect.
3.5
) when the snow cover is
thin or absent and the sun is above the horizon long enough. In fresh and brackish water
lakes this increases the water temperature in the upper layer and triggers convection
(Fig.
7.8
). However, in brackish waters, if ice is melted the salinity of the surface water
decreases that adds, in turn, increases the stability of the strati
cation. The radiational
heating can be up to 100 W m
−
2
, and it is used both for heating of the water and melting of
the ice at the bottom surface. In freshwater lakes, convective mixing penetrates deeper
with time into the stably strati
fluid below reaching the bottom as T
→
T
b
beneath the
immediate viscous boundary layer under the ice. This process raises the water temperature
more in shallow areas, and circulation similar to the one forced by the sediment heat
ed
fl
ux
follows. It is the most energetic transport process in ice-covered lakes and dominates the
circulation in spring and summer. Because of the strength of the solar heating, the water
fl
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