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ABSTRACT. Supraglacial deposits are known for their influence on glacier ablation. The magnitude of this influence depends on the thickness and the type of the deposited material. The effects of thin layers of atmospheric black carbon and of thick moraine debris have been intensively studied. Studies related to regional-scale deposits of volcanic tephra with thicknesses varying between millimetres and metres and thus over several orders of magnitude are scarce. We present results of a field experiment in which we investigated the influence of supraglacial deposits of tephra from Grímsvötn volcano on bare-ice ablation at Svínafelsjökull, Iceland. We observed that the effective thickness at which ablation is maximized ranges from 1.0 to 2.0 mm. At ∼10 mm a critical thickness is reached where sub-tephra ablation equals bare-ice ablation. We calibrated two empirical ablation models and a semi-physicsbased ablation model that all account for varying tephra-layer thicknesses. A comparison of the three models indicates that for tephra deposits in the lower-millimetre scale the temperature/radiationindex model performs best, but that a semi-physics-based approach could be expected to yield superior results for tephra deposits of the order of decimetres.

Extensive glaciological field measurements were carried out on the ice cap Vestfonna as well as on the minor ice body De Geerfonna (Nordaustlandet, Svalbard) within the framework of IPY Kinnvika. Field campaigns were conducted during the period 2007-2010 in spring (April/May) and summer (August). In this study we compile and present snow cover information obtained from 22 snow pits that were dug on Vestfonna during this period. Locations are along two transects on the northwestern, land terminating slope of the ice cap, on its central summit, Ahlmann Summit, and at a set of several other locations in the eastern and northern part of the ice cap. Snow-cover information acquired from four snow pits on adjacent De Geerfonna is also incorporated in this study. Field data are analysed regarding snow stratigraphy, snow density, snow hardness and snow temperature. Results reveal mean snow densities of around 400 kg m -3 for the snowpack of Vestfonna with no apparent spatial or interannual variability. A distinctly higher value of more than 450 kg m -3 was obtained for De Geerfonna. A spatial comparison of snow water equivalents above the previous end-ofsummer surface serves for obtaining insights into the spatial distribution of snow accumulation across Vestfonna. Altitude was found to be the only significant spatial parameter for controlling snow accumulation across the ice cap. IntroductionIn the framework of the IPY (International Polar Year) Kinnvika project ) and its follow-on activities several field campaigns with focus on snow-cover related fieldwork were carried out during seven individual field campaigns on Nordaustlandet, Svalbard, in the period 2007-2010. Snow cover conditions were investigated on the ice cap Vestfonna as well as the neighbouring minor ice body De Geerfonna. Snow-cover related fieldwork on Vestfonna focused on two transects along the northwestern slope of the ice cap and on its main ridges, including the two highest points,

In April and May 2010 the Icelandic volcano Eyjafjallajökull experienced an explosive eruption that led to substantial ashfall across the central-southern parts of the island. The resulting ash deposits covered Eyjafjallajökull, Mýrdalsjökull and parts of Vatnajökull ice caps. In order to quantify the influence of these deposits on albedo, we analyzed albedo evolution across Eyjafjallajökull and Mýrdalsjökull ice caps over the period 2001-2016 using the MOD10A1 and MCD43A3 data products of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard the Terra and Aqua satellites. A geostatistical model with a daily temporal resolution was used to delineate areas on the ice caps that show distinct ash cover-related albedo reductions over the post-eruption period. Results suggest that despite an overall decrease of the ash cover-related albedo reductions with time, noticeable albedo reductions persist on both, Eyjafjallajökull and Mýrdalsjökull over the entire post-eruption period. These reductions show means of 0.19 ± 0.11 and 0.17 ± 0.10, respectively, and occur most prominently during the summer seasons. Persistent albedo reductions are in agreement with and limited to areas of higher ash deposition during the volcanic eruption such as the southern parts of Eyjafjallajökull and Mýrdalsjökull ice caps. In addition, redistribution of Eyjafjallajökull ash deposited on the lowlands in southern Iceland contributed to dust storm events in the years after the eruption and caused additional albedo reductions.