Impact of dust deposition on the albedo of Vatnajokull ice cap, Iceland

Deposition of small amounts of airborne dust on glaciers causes positive radiative forcing and enhanced melting due to the reduction of surface albedo. To study the effects of dust deposition on the mass balance of Bruarjokull, an outlet glacier of the largest ice cap in Iceland, Vatnajokull, a study of dust deposition events in the year 2012 was carried out. The dust-mobilisation module FLEXDUST was used to calculate spatio-temporally resolved dust emissions from Iceland and the dispersion model FLEXPART was used to simulate atmospheric dust dispersion and deposition. We used albedo measurements at two automatic weather stations on Bruarjokull to evaluate the dust impacts. Both stations are situated in the accumulation area of the glacier, but the lower station is close to the equilibrium line. For this site (similar to 1210 m a.s.l.), the dispersion model produced 10 major dust deposition events and a total annual deposition of 20.5 g m(-2). At the station located higher on the glacier(similar to 1525 m a.s.l.), the model produced nine dust events, with one single event causing similar to 5 gm(-2) of dust deposition and a total deposition of similar to 10 g m(-2) yr(-1). The main dust source was found to be the Dyngjusandur floodplain north of Vatnajokull; northerly winds prevailed 80% of the time at the lower station when dust events occurred. In all of the simulated dust events, a corresponding albedo drop was observed at the weather stations. The influence of the dust on the albedo was estimated using the regional climate model HIRHAM5 to simulate the albedo of a clean glacier surface without dust. By comparing the measured albedo to the modelled albedo, we determine the influence of dust events on the snow albedo and the surface energy balance. We estimate that the dust deposition caused an additional 1.1 m w.e. (water equivalent) of snowmelt (or 42% of the 2.8 m w.e. total melt) compared to a hypothetical clean glacier surface at the lower station, and 0.6 m w.e. more melt (or 38% of the 1.6 m w.e. melt in total) at the station located further up-glacier. Our findings show that dust has a strong influence on the mass balance of glaciers in Iceland.