Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 187

The size of supraglacial lakes ranges from small ponds to medium-size shallow lakes of
several square kilometres in area. They are mainly seasonal. In the Dronning Maud Land,
Antarctica they are recurrent summer lakes with the depth scale of 1 m (Winther et al.
1996; Lepp ranta et al. 2013b). In Greenland they are much more developed and have a

major impact on the ice mass balance (Hoffman et al. 2011; Liang et al. 2012). Since the
main water source is the glacial ice, the water quality depends on the state of the surface
layer.
The depth of seasonal supraglacial lakes is of the order of 1 m and their surface area
can be up to several square kilometres. The thermodynamics of their formation is gov-
erned by the summer radiation balance and conduction of heat: penetration of solar
radiation provides heat for subsurface melting of ice, and the low thermal conductivity of
ice prevents fast heat leakage. This is one kind of a greenhouse phenomenon. The surface
must be essentially snow-free to allow the penetration of sunlight.

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Example 6.3. There are two vertical length scales associated with supraglacial lakes. The
light attenuation distance

D p , where

κ − 1

1 m and heat conduction distance d ¼

*

0.10 m 2 day − 1 is the heat diffusion coef D cient. If the consequent heat gain and loss at

the depth z are equal, the temperature difference

*

Δ T between the surface and depth z is

e j z e j z Q T

q

¼ 1 a

ð Þ c Q s

D T z ¼ z

q

cD cD

100 W m − 2 , we have C. If the temperature

difference is larger, absorbed solar heat is leaked out and vice versa.
The life cycle of supraglacial lakes is a reverse process to the freezing cycle of sea-
sonally ice-covered lakes. Their formation begins well beneath the surface, and when the
ice layer on top becomes thinner, albedo decreases providing a positive feedback to the
lake growth. A supraglacial lake has an ice cover, if the surface heat balance is negative.
In fall, supraglacial lakes start to freeze down from the upper surface, as lakes usually do
(e.g., Lepp

For z 1 m and Q T * Δ T 10 °

* *

ranta 2009a), and up from the bottom. In principle, it is possible that pockets
of liquid water could survive over the winter resulting in multiyear supraglacial lakes.
Lake water masses may also transport latent heat deeper into the ice sheet by con-
vection across the lake water body and penetration of lake water into fractures (Chikita
et al. 1999; Joughin et al. 2008; Hoffman et al. 2011). In particular, multiyear supraglacial
lakes are not stable but grow year-by-year. This will accelerate local melting and even-
tually lead to catastrophic formation of fractures that ends up in a major change in the local
glacial environment. Fracturing of the ice may even empty a whole lake in a short time.
Meltwater convection is therefore a very important mechanism in the warming and
deterioration of ice sheets and glaciers.

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