Jökull - 01.12.1972, Page 55

Assuming all profiles to be logarithmic in-
stead of log + linear the enthalpy fluxes for the
whole observation period of 1968 were calculated
to be 1.5 times as high as values given above.
Tlie difference in energy outcome from these
two methods of computation corresponds to a
melting of about 25 cm water equivalent. This
result compared with the good accordance be-
tween computed and measured ablation shown
in Table 1 is taken as a proof of the import-
ance of providing for the stability effect in the
present calculations.
DISCUSSION OF RESULTS
Calculation of the fluxes of sensible and
and latent heat indicated that these were
generally directed towards the glacier. During
the observation periods evaporation was quite
negligible compared with ablation providecl for
by condensation. Evaporation from a melting
surface took only place on clear days with dry
southerly winds. In 1967 it occurred for some
hours during July 11 and 12 and in 1968 dur-
ing the pronounced anticyclonic weather in
July 2 to 5. Wlien air temperature fell below
freezing point and the snow surface was warm-
ed up by the sun radiation the humidity
gradient some times indicated sublimation.
Some clear trends can be seen in Fig. 6. The
average decrease in incoming shortwave radia-
tion during the period appears to be compens-
ated for in decreased albedo and increased
cloudiness. The net radiation balance is there-
fore quite constant during the observation
period. The same trends can be seen for 1967
in Fig. 7. For the summer of 1968 the net
radiation balance was generally slightly higher
than the summer before. Tliis was mainly due
to tlie marked difference in mean albedo, being
0.54 for 1968 against 0.63 for the summer be-
fore. Both summers were rather sunny (Björns-
son 1971).
The turbulent energy fluxes on the other
hancl appeared to increase in magnitude dur-
ing tlie summer accompanied with increased
air temperature, vapor pressure and wind speed.
Details in the variation show especially a close
correlation with the wind speed e.g. in August
of 1968. Fig. 6 shows how the relative import-
300
5200
>100
0
NET RADIATION BALANCE
4JULY 11 21 1AUGUST 1967 11 18
Fig. 5. Daily radiation terms for Bægisárjökull
in 1967 (ly/day = 0.485 W/m2).
Mynd 5. Dagsgildi geislunarþátta á Bœgisár-
jökli, 1967.
I 0
""-100
<100
-SHORT-WAVE RAOIATION BALANCE(absorbed)
ESTIMATED LONG-WAVE RADIATION BALANCE
NET RADIATION BALANCE -
rhTfmfTKíMlWMimHThm
SENSIBLE HEAT
1AUGUST 1968 10
Fig. 6. Daily radiation and turbulent heat flux
terms on Bægisárjökull in 1968.
Mynd 6. Dagsgildi orkuþátta á Bœgisárjökli,
1968.
ance of the turbulent processes increased
throughout the observation period in 1968. For
the second part of the period it is evident that
the calculated ablation varied almost inversely
to the net radiation balance.
JÖKULL 22. ÁR 53