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gain should be less than heat needed the raise the surface layer temperature to the melting
point. If a lake forms, the summer melt and winter ice growth become equal when:
2
s
t þ
k
Q 0 þ
Q T
L f
ð
T a t
Þ þ
b 2
b
¼
ð
6
:
16
Þ
q
q
L f
where T a - represents the winter mean air temperature, and t - + t + ¼
1 year. The heat taken
out to cool the ice compensates for the heat added for warming the ice in the annual heat
balance. These factors are small, since the energy needed to raise the temperature of ice
from
inequality in
Eq. ( 6.16 ), we have seasonal or perennial lakes, respectively. The balance between
summer and winter to determine the survival of the lake over summer is delicate. But
since ice thickness is proportional to the square root of the winter air temperature, how
summer climate changes in future is more critical to assess whether these presently
seasonal lakes can change to perennial.
15 to 0
°
C is only 10 % of the energy to melt the ice. With
>
or
<
-
100 W m 2 . The summer
melt is 2.55 m, and the winter growth is less, 1.63 m, resulting in a perennial, growing
lake. Decreasing the temperature to
Example 6.5
. Take T a - * -
10
°
C, t + ¼
90 days and Q 0 þ
Q T *
C and length of summer to 60 days, the summer
melt becomes 1.71 m and the winter growth is (in theory) more, 2.13 m, turning the lake
into a seasonal lake.
If the lake survives one summer, there will be a liquid pocket as the initial condition in
the following summer, and therefore perennial lake is not stable but growing year by year.
Changing of a supraglacial lake from seasonal to perennial accelerates local melting, leads
to lake growth and eventually to formation of fractures taking the water in and ending in a
major change in the local glacial environment.
15
°
-
6.3.4 Case Study
A case study of summertime thermodynamics of Lake Suvivesi, western Dronning Maud
Land, was presented by Lepp
eld
measurements, is shown in Fig. 6.7 . The net radiation as well as the radiation penetrating
into the lake (i.e. sunlight) varied within 50
ä
ranta et al. (2013b). The radiation balance, based on
150 W m 2 , while the surface balance was
-
50 and +20 W m 2 . Solar radiation is relatively high in the study region during
summer since the sky is mostly clear. In January 3
between
-
20 the surface balance was slightly
positive for a longer time, indicating the thinning of the surface ice layer. Turbulent
-
fl
uxes
were not continuously measured. The sensible heat
fl
flux was low in summer in the study
region but evaporation was up to 1
2 mm/day in windy conditions, corresponding to
-
60 W m 2 . These losses are transient but re
latent heat loss of 30
ected in the thickness of
the surface ice layer. The loss of ice by sublimation accounts to the total of 2
fl
-
4 cm during
-
summer.
 
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