Jökull - 01.01.2001, Síða 15

Jökulsárlón at Breiðamerkursandur
Figure 13. Observed and predicted position of the
calving front of Breiðamerkurjökull at Jökulsárlón.
– Mæld og reiknuð staða sporðs Breiðamerkurjökuls
við Jökulsárlón.
Figure 14. Estimated thermal energy contribution to
ice melt in Jökulsárlón. a) Total energy flux into the
lake (
). b) Various energy components absorbed
by Jökulsárlón. The average seawater discharge into
the lake was taken as 50 m  s  and its temperature
as 7  C. c) Difference between the energy required to
melt the observed ice flux of calving ice and an esti-
mate of available energy (

 ). – Varmaorka
sem bræðir ís í Jökulsárlóni.
ENERGY BALANCE IN THE LAKE
Jökulsárlón has grown because ice flow toward it has
not compensated for calving and melting at the glacier
terminus. The power required to melt all the ice calv-
ing into the lake ( 
) is:


 
(9)
where

is the density of ice and

  !
J kg " is the latent heat of fusion of water (Figure
14a).
increases with calving and reaches a to-
tal of about 2500 MW. The lake surface (of area #%$ )
receives this energy as radiation, through warm and
moist air, and from the inflowing sea. The energy in-
put ( & ) per unit area and per unit time is the sum of
total solar radiation &(') * ,+.- , where + is the albedo,
long–wave atmospheric radiation and radiation from
Earth’s surface ( &($ ), heat from warm ( &0/ ) and moist
( &(1 ) air and heat from seawater flowing into the lake
at tidal floods (
32
). Integrating over the lake area
gives the power:
45
)6&7') 8 +.-:9 &7$ 9 &(/ 9 &(1 - #8$ 9; 2=<
(10)
Measurements of the individual energy components
are not available. However, rough estimates can
be made of the partitioning of the total energy
(2500 MW), required to melt the calving ice in the
lake. The lake is normally covered with ice from late
fall to the end of April (Fjölnir Torfason, personal
communication, 1999) so solar radiation supplies heat
to the lake only 6 months of the year. From the be-
ginning of May to the end of October the average
solar radiation is about 135 Wm ?> (Oerlemans et al.,
1999) and we can assume that the entire short wave
radiation energy is absorbed in the lake (i.e.
+
= 0).
Total radiation from the atmosphere, glacier and lake
surface (long–wave radiation) could be assumed close
to zero ( &($ = 0). If we assume that the lake receives
an average solar radiation of &(' = 67 Wm ?> during
the year, the total radiation amounts to 1000 MW over
the 15 km > area of the lake. Heat from warm and
moist air is likely be about a fifth of the solar radia-
tion, 200 MW. Finally, the heat from seawater can be
estimated as:
JÖKULL No. 50 15