MODIS-derived albedo changes of Vatnajökull (Iceland) due to tephra deposition from the 2004 Grímsvötn eruption

Occasionally, the surface albedo of glaciers may be abruptly altered by deposition of light-absorbing aerosols, which consequently has a sustained impact on their energy- and mass balance. Volcanic eruptions may spread tephra deposits over regional-scale glacierized areas. In November 2004, an explosive, phreatomagmatic eruption of the subglacial Grímsvötn volcano, located in the centre of the Icelandic ice cap Vatnajökull, produced ash fall covering an area of ∼1280 km2 in the northwestern part of the ice cap. This event affected the surface albedo of the glacier over several years after the eruption. We use MODIS surface-albedo data and an ash-dispersal dataset obtained from in situ measurements on the ice cap to develop a novel, empirically based modelling approach to describe the albedo decrease across the glacier surface caused by the deposited tephra. We present analyses of the temporal and spatial variability of the albedo pattern over the post-eruption period from November 2004 to December 2008. The tephra-induced albedo changes were largest and most widely distributed over the glacier surface during the summer season 2005. The observed albedo decrease reached 0.35 when compared to modelled, undisturbed conditions. In the low-lying ablation area, where strong surface melting takes place, the tephra influence on albedo diminished with time and completely faded out within four years after the eruption. In contrast, at the rim of the Grímsvötn caldera surrounding the eruption site the tephra influences on albedo considerably increased with time. Throughout the rest of the high-lying accumulation area, the influences were scattered in both space and time.