Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 201

7.1 Ice Formation

7.1.1 Cooling Process

The annual cycle of cold region lakes is in uenced by the ice formation. Seasonal ice cover

is typical in the boreal zone, while a perennial lake ice cover occurs in some high polar or
high mountain lakes. The ice cover buffers the surface water temperature to the freezing
point and largely decouples the water body from the atmosphere. The circulation changes
from wind-driven to thermohaline (Kirillin et al. 2012b). Processes beneath the ice are
slowed down, and lake memory effects extend over the winter. In the ice season, the in

fl

fl ow

and out ow also normally decrease. The heat storage of the bottom sediments becomes an

important source (Falkenmark 1973), and heat leakage is only by molecular conduction
through the ice cover. Oxygen is consumed especially in the bottom waters of eutrophic
lakes, and the primary production becomes light-limited in snow-covered frozen lakes.
The

fl

first phase of the winter season in seasonally freezing lakes can be taken as the
period when the lake is open and the surface temperature T 0 is beneath the temperature of
maximum density, T 0 ≤ by Kirillin et al. (2012),

can be long in freshwater and brackish lakes but the lake has then the potential to freeze
rapidly. It needs only a short, calm and cold event to establish a stable strati

T m . This period, called as ' pre-winter '

cation in a
thin surface layer resulting in ice formation. Example for a 1-m water layer, cooling by
50 W m − 2

lowers the temperature by 1 ° C in 1 day, easily reached by net terrestrial

radiation loss.
The quality of the cooling process in seasonally freezing lakes can be classi

ed in
terms of the water salinity (Table 7.1 ). In brackish and saline lakes, the seawater formulae
(see Eqs. 7.2

7.7 ) can be used for the temperatures of the freezing point and maximum
density, but in hypersaline lakes the exact chemical constitution of the liquid water
becomes signi

-

cant and these temperatures need to be determined individually for each
lake. In saline and hypersaline lakes, free convection to the halocline continues until ice
formation, and the pre-winter does not exist in the sense it does in freshwater and brackish
lakes. In brackish lakes, the strength of the strati cation at temperatures T < T m decreases

with salinity (Fig. 2.5 ).
During the pre-winter, strati

cation and mixing events come and go until freezing.
Since the thermal expansion coef

uence of
temperature on density is small. Then small differences in salinity may overcome the
temperature effects on the water density (see Example 2.4).

cient drops to zero at T m , near T m the in fl

Table 7.1 Characteristics of cooling process in seasonally freezing lakes

Salinity S ( ‰ ) Freezing point Tf° Tf ° (C) Cooling Turnover

≈ 0

Fresh water S < 0.5 T Tf < Tf m Full

Brackish 0.5 ≤ S < 24.7 0 to − 1.3 T Tf < Tf m To halocline

Saline 24.7 ≤ S <40 − 1.3 to − 2.5 T Tf ≥ T m To halocline

Hypersaline S ≥ 40 < − 2.5 T Tf ≥ T m To halocline