Jökull - 01.01.2009, Side 17

Langjökull, energy balance and degree-day models
leysingar og lofthita eru breytileg eftir veðurfari, árs-
tíðum og breytingum á yfirborðseiginleikum jökuls-
ins. Með gögnum um orkustrauma, veðurþætti og
leysingu sem aflað hefur verið á Langjökli er unnt
að meta þetta. Einnig er unnt að kanna hvort lofthiti
mældur utan jökla eða ofan við bráðnandi jökluyfir-
borðið lýsi betur jökulleysingu.
Niðurstöðurnar sýna að gráðudagalíkön spá allvel
meðalleysingu yfir nokkra daga, þegar breytileiki jök-
ulleysingar orsakast af varmastraumum frá hlýju lofti
og vegna þéttingar loftraka yfir jöklinum. Undantekn-
ingar frá þessu eru þegar sveiflur í vindhraða frem-
ur en lofthita breyta varmastraum. Almennt reynd-
ust gráðudagalíkön lýsa betur jökulleysingu með hita
mældum utan jökuls en á honum. Skýringin er sú
að lofthiti lækkar fljótt yfir bráðnandi jökli og lýs-
ir verr sveiflum í sólgeislun og varmastraumum sem
berast að jöklinum heldur en hiti mældur utan jökuls-
ins. Annað áhyggjuefni sem varðar gráðudagalíkön,
sem eru stillt af við núverandi veðurfarsaðstæður, er
hvort þau gildi við hlýnandi loftslag. Okkar niður-
stöður benda til að gráðudagalíkön séu nothæf til að
spá fyrir um breytingu á meðaltali jökulleysingar ef
hlýnun er ekki umfram 3 !C frá þeim hita sem gráðu-
dagalíkanið var bestað fyrir.
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