Jökull - 01.01.2009, Side 16

Guðmundsson et al.
It should be noted that these synthetic calculations
underestimate the absolute values of the temperature
change on seasonal ablation. If the regional tempera-
ture rose, it would extend the ablation season, reduce
bW (as long as winter precipitation did not also in-
crease), moving the exposure of the low-albedo sum-
mer surfaces from earlier point in time in the season
with the highest incident radiation falling on a sur-
face with already reduced albedo. The ablation season
would also be prolonged into the autumn when so-
lar radiation has been reduced and melting is mainly
driven by high temperatures and strong winds. The
former effect is difficult to take into account with a
degree-day model. Our data set include two years
with a warm September month with high melting
rates, which allow for testing of the degree-day mod-
els for warm and windy autumn months with low so-
lar radiation. The degree-day models did satisfacto-
rily predict the accumulative melting during those pe-
riods but were not reliable on a daily basis at the lower
weather station.
CONCLUSIONS
The observed daily melt rates on the glacier were
successfully simulated by energy balance calculations
based on meteorological observations on the glacier.
As a rule, net radiation was the main contribution
to melting, although it was occasionally equalled by
eddy fluxes. Sporadically, radiation contributed to
melting even when eddy fluxes were negative. Net ra-
diation typically peaked in the ablation area in May to
June and in August around the ELA when the melting
reached the previous year’s summer surfaces. Turbu-
lent fluxes increased during the summer, reaching a
maximum in August-September. Every energy com-
ponent increased downglacier: radiation due to the
lower albedo and turbulent fluxes owing to higher
temperatures and the persistent down slope glacier
wind.
Degree-day models successfully described sea-
sonal variations in melting, but were less success-
ful for simulating daily values. The most success-
ful degree-day predictions were obtained by applying
temperature observations away from the glacier and
a constant adiabatic lapse rate with elevation, rather
than temperatures observed on the glacier itself. Air
temperatures in the low-albedo surroundings of the
glacier represent daily variations in the global radi-
ation flux better than the damped boundary layer tem-
peratures above the melting glacier. Given no ex-
treme changes in albedo the derived empirical degree-
day models may provide reasonable predictions of in-
creased ablation in response to a regional temperature
change of less than 3 !C.
Acknowledgements
This work was supported by the National Power Com-
pany of Iceland, the University of Iceland Research
Fund, the EU project Icemass (ENV4-CT97-0490),
and the Nordic projects Climate and Energy, and Cli-
mate, Water and Energy. SPOT5 images were made
available by OASIS (Optimising Access to Spot In-
frastructure for Science) projects number 36 and 94.
We are indebted to Sigmundur Freysteinsson and Óli
Grétar Blöndal for their part in planning the field-
work, Guðfinna Aðalgeirsdóttir for constructive com-
ments on the manuscript, and the reviewers Tómas
Jóhannesson and one anonymous for improving the
manuscript.
Samanburður líkana til mats á sumarleysingu á
Langjökli; orkuþættir og gráðudagar
Unnið hefur verið að því að kanna tengsl veðurfars
og jökulleysingar á Langjökli á hverju sumri síðan
2001. Reknar hafa verið sjálfvirkar veðurstöðvar á
Hagafellsjökli vestari (í um 500 m y. s. við jaðar jök-
ulsins og í 1100 m y. s. nálægt snælínu í meðalári) til
þess aðmeta orkustrauma sem berast að yfirborði jök-
ulsins og valda leysingu. Orkuþættirnir eru sólgeisl-
un, langbylgjugeislun, varmastraumur frá hlýju lofti
og varmastraumur vegna þéttingar loftraka yfir jökl-
inum. Með þessum gögnum hafa verið sett fram og
prófuð reiknilíkön sem tengja jökulleysingu við orku-
strauma og einstaka veðurþætti.
Nákvæm orkubúskaparlíkön krefjast viðamikilla
mælinga á jöklum og því er reynt að finna einföld töl-
fræðileg líkön sem lýsa leysingu í hlutfalli við mæld-
an lofthita, svonefnd gráðudagalíkön. Annmarkar
reynslubundinna gráðudagalíkana hafa hins vegar ver-
ið lítið kannaðir. Þar er einkum áhyggjuefni hve tengsl
16 JÖKULL No. 59