Geoscience Reference
In-Depth Information
and because the surface temperature is held at the freezing point, the heat loss continues to
be large. In the presence of strong turbulence in shallow waters, frazil crystals may be
adhered into lake bottom to form anchor ice. Due to its buoyancy, anchor ice may rise to
surface to join the ice cover. This is often observed in rivers and shallow coastal sea areas.
Frazil ice is not largely discussed in this topic because of its likely unimportance in
freezing lakes. For more information, the reader is referred to topics and review papers in
river ice and sea ice (Michel 1978; Martin 1981; Ashton 1986; Wadhams 2000; Weeks
2010).
4.2.5 Ice Melting
Ice melting is a similar process for all ice types. The main differences are due to optical
properties of the ice. The melting season begins when the ice has warmed up to the
melting point and the heat balance continues to be positive. This is largely determined by
the net radiation balance. In winter the balance is negative, and the time it turns positive is
a function of the latitude (
˕
). Due to the penetration of sunlight into the ice and thermal
radiation losses at the surface, melting of ice begins in the ice interior some 10
-
20 cm
from the top surface or at the ice bottom.
Snow-covered dry ice has a high albedo, around 0.9. When the solar radiation
increases, at some time albedo begins to decrease that provides a positive feedback to the
net radiation:
[ 0
Þ Q s þ e 0 re a T a T 0
Q R ¼ 1 a
ð
ð
4
:
32
Þ
As seen this balance is highly sensitive to albedo:
ʴ
Q R =
- ʴʱ ·
Q R . The net heat loss
50 W m 2 , while solar radiation can be
500 W m 2 at
due to terrestrial radiation is
*
*
daily maximum. If there is dry snow on ice, the albedo is
*
0.9, and it is dif
cult to reach
positive radiation balance as is the case in the polar ice sheets (e.g., Lepp
ranta et al.
2013). To decrease the albedo, minor melting or recrystallization of snow layer at solar
noon are essential factors. In the case on snow-free ice, the radiation balance becomes
easily positive, and the transparency of the ice tells how the net solar energy is distributed
with depth in the ice and underlying water body.
In the
ä
first approximation, the daily incoming solar radiation is proportional to cosine
of the solar zenith angle at solar noon, which is obtained from the astronomical formulae
as (see Appendix A.1):
cos
Z 0 ¼
cos / þ d
ð
Þ
ð
4
:
33a
Þ
sin d ¼
sin e sin
ð
# # 0
Þ
ð
4
:
33b
Þ
 
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