Jökull


Jökull - 01.06.2000, Page 51

Jökull - 01.06.2000, Page 51
Mass balance and precipitation on the summit plateau of Öræfajökull, SE-Iceland Magnús T. Guðmundsson Science Institute, University oflceland Hofsvallagata 53, 107 Reykjavík, Iceland Abstract — Mecisurements of mass balance were made in 1993-1996 and 1997-1998 at 1820 m elevation on the wide summit plateau of Örœfajökull, a large, partly-ice covered strato-volcano on the southeast coast of Iceland. The average winter balance over the period was about 6000 mm and the annual net balance as high as 7780 mm, about double the measured precipitation at the nearby meteorological station of Kvísker. The estimated annual precipitation on the summit plateau of Örœfajökull ranges from 7450 to 7800 mm in the period 1993-1998. Although sparse, the data suggest that in warm summers, ablation and rainfall may lead to several hundred millimeters difference between the net annual mass balance and precipitation. In cool summers the annual net balance is about equal to the annual precipitation. The estimated precipitation values are the highest sofar obtained in Iceland. INTRODU CTION Öræfajökull is a strato-volcano on the southeast coast of Iceland and its highest mountain. An ice cap co- vers the upper part of the mountain and merges to the north with the main part of the Vatnajökull ice cap. The volcano has a wide summit ice-plateau, cover- ing a caldera with a maximum ice thickness of 500 m (Björnsson, 1988). A few nunataks on the caldera rims rise above the summit ice plateau but the caldera rims to the south, west and the southeast are concealed by the ice cover. One of the nunataks is Hvannadals- hnúkur, the highest peak in Iceland. The flat summit plateau is 5 km long (N-S) and 3 km wide (E-W) and has an elevation of 1800-1850 m above sea level. Precipitation in Iceland reaches a maximum in the south and southeast parts (Eythorsson and Sigtryggs- son, 1971; Sigfúsdóttir, 1975) with the meteorological station at Kvísker (Figure 1), on the lowland under the eastern slopes of Öræfajökull, having the highest mea- sured precipitation. Thus, the setting of Öræfajökull and its meteorological conditions make it an interest- ing place to study extremes in precipitation. Moreo- ver, knowledge of the mass balance of its upper parts is important to studies of the behaviour of its outlet glaciers, most of which reach the lowlands. With these aims in mind, annual measurements of mass balance started in 1993, with the support of members of the Iceland Glaciological Society. The results of the work during 1993- 1998 are presented here. The 1993-1996 measurements were reported in Guðmundsson (1995, 1998a, 1998b) and Björnsson et al. (1998). METHODS The measurement site (63°59.9’N, 16°39.2’W, elevation 1820 m) is located in the centre of the flat summit ice-plateau covering the Öræfajökull caldera. The nunataks closest to the site are about 2 km away and the distance to the plateau edges is nowhere less than 1.5 km, minimizing potential bias due to snowdrift, as often seen close to mountain slopes. The site was visited once each summer, a snow core taken with an engine driven drill and the density of the core measured. The thickness of the annual layer was determined in a conventional way by defining the previous ye- ar’s summer surface from changes in grain size and texture of the core. In some cases the uncertainty in the location of the summer surface was 30-50 cm. JÖKULL No. 48 49

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Jökull

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