Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 189

begins. In the closing period of the lake, the growth of ice is expected to be 1 2 cm/day,

based on experience of lake ice growth in general (see Sect. 4.3 ). This means that the lake
would be completely frozen by April

-

May. Thus, the inferred age of the lake is 4 6 months,

- -

1 4 months for close-up. Observations from two other lakes at

Plogen and Fossilryggen nunataks within 50 km from Aboa showed similar summer
evolution but the exact timing depends on the location. Further south, there is also a small,
seasonal supraglacial lake at Svea Research Station (74

2 months for growth and 3

- -

W, altitude 1,245 m).
The surface of a supraglacial lake may be liquid water or ice depending on the heat

° 35 ′ S, 11 ° 13 ′

fl fluxes and surface heat balance. The continuity of heat fl flux through the surface tells that in

equilibrium,

k T f

T 0

Q 0 T ðÞ ¼

¼

Q T ½ 1

ðj Þ ð : Þ

exp h 6 4

h

These two equations provide the solution for the surface temperature T 0 and ice
thickness h. If a physically realizable solution (T 0 ≤

T f , h > 0) does not exist; then, h ¼ 0

and the surface temperature is obtained from the balance between the first and third terms

in Eq. ( 6.4 ).
In Greenland there are open water lakes (Liang et al. 2012), but in the western
Dronning Maud Land the lakes have ice cover (Lepp ranta et al. 2013b). The surface ice

layer exists because the surface heat balance in this region is negative. Much of the
incoming solar radiation penetrates into the ice (and liquid water beneath), and the surface
absorption does not compensate for the thermal radiation loss. Only in the peak of the
summer, a few days in mid-January, melting surfaced in some spots and the ice bearing
capacity was below the level needed for on-ice walking or skiing. The thin ice cover
corresponds to the

ä

' cold skin phenomenon ' observed in lower latitude lake and sea

surfaces in daytime calm summer conditions.
The presence of hard and soft sub-layers in the lower layer is most likely due to the
variability of the absorption coef

cient of solar radiation with depth caused by sediments
and ice crystal structure. The absorption coef

cient of the sediment-rich ice is much larger
than that of the clear ice, and this enables more intensive heating deeper even though the
level of radiation is less. Example, if there is a strongly absorbing layer at (z, z + Δ z),

absorption of light in this layer relative to the previous Δ z thick clear layer is (see Sect. 3.5 ) :

R z þD z
z

dz 0

a exp ð az Þ

a exp a D ð z Þ

R z
z D z a exp

dz 0

ð 6 : 5 Þ

a 0

ð az Þ

where a 0 and a* are the absorption coef cients in the clear ice and sediment-rich ice,

1m − 1 , the ratio becomes 1.64. In summers
sediments have melted their way down. The presence of the concentrated sediment layer
below 200 cm re

respectively, If Δ z 10 cm and ½ a* a 0 *

* *

fl ects the maximum depth of the lake in recent history. The bottom of