Jökull - 01.12.1972, Page 46

Bægisárjökull, North-Iceland
Results of glaciological investigations 1967—1968
Part II. Tlie energy halance
HELGI BJÖRNSSON,
SCIENCE INSTITUTE, UNIVERSITY OF iCELAND
ABSTRACT
The energy budget ierms for a melting glacier
were estimaíed for periods in July and August
in 1967 and 1968. The turbulent energy fluxes
were computed by application of the Monin
and Obukhov theory. Evidence showed that this
method gave better results than assuming loga-
rithmic profiles of wind, temperature and
specific humidity. The relative importance of
net radiation, sensible heat, and latent heat in
supplying energy at the glacier surface was
found to be about 51%, 53% and 16% re-
spectively during the observation period in
1968. For the observation periocl in 1967 the
net radiation was found to be responsible for
about 57% of the ablation.
For a period of 36 days midsumrner in 1968
calculated and measured ablation were found
to be in good accordance. By assuming the first
one to be representative for the latter, averag-
ed over shorter time intervals, tlie glacier systern
response to input of meltwater and rain could
be illustrated. Further it is demonstrated that
the discrepancy between the results obtainecl
using the hydrological method and the glacio-
logical method for mass balance measurements
reported by tíjörnsson (1971) might partly be
caused by the release of delayed water melled
earlier in the summer and was parlly due to
an underestimate of the rain volurnes.
INTRODUCTION
A general description o£ meteorology on Bæg-
isárjökull for the observation periods during
the summers of 1967 and 1968 has been given
by Björnsson (1971). It appeared that net radia-
44 JÖKULL 22. ÁR
tion, sensible heat and latent lreat were all
important sources of ablation. The purpose of
the work presented by this paper was to esti-
mate these components in the energy budget
and cletermine the quantitative importance of
each process.
Recorded global radiation was usecl to find
an areal estimate for the incoming short-wave
racliation for the whole glacier. The long-wave
radiation balance was estimated empirically
with the aid of cloud observations. The turbu-
lent energy fluxes were estimated at the micro-
meteorological site at 1100 m a. s. 1. These point
values were used as representative mean values
for the glacier. The results were compared
with measured ablation on the glacier and with
the run-off in the glacier river.
Fig. 1, 2 and 3 give a view of the glacier.
THEENERGYBUDGET
The equation of the energy budget at the
air-snow interface was considered in the simple
form
L-a = Q1.+ Hd + H1 (1)
where
Qr is the net radiation balance
Hd is the vertical eddy ílux of sensible heat
Hj is the vertical eddy flux of latent heat
a is the ablation of the ice phase in the
weathering crust
L is the specific latent heat of melting ice
All terms are expressed in cal/cm2 min (lang-
ley/min). Only periods with melting were con-
sidered and the glacier was assumed to be in