Surface energy balance in the ablation zone of Midtdalsbreen, a glacier in southern Norway: Interannual variability and the effect of clouds

We present a record of almost six years of data (2000-2006) from an automatic weather station (AWS) in the ablation zone of Midtdalsbreen, a glacier in southern Norway. Measured incoming longwave radiation is used to estimate cloudiness, revealing that high cloud fractions occur almost 50% of the time in all seasons. Measured wind speeds and humidity are higher for cloudy conditions, especially in winter. Net solar radiation dominates the surface energy balance in summer, contributing on average 75% of the melt energy. The turbulent fluxes supply 35% of the melt energy while net longwave radiation and the subsurface heat flux are energy sinks of 8% and 2%, respectively. Although the melt rate is generally larger under clear skies, almost 60% of the melt occurs under cloudy skies, a consequence of the prevailing cloudy conditions. Interannual variability in the total melt is found to be equally determined by variations in the date of ice reappearance and differences in the meteorological conditions during melt. Comparing the results for Midtdalsbreen with measurements from an AWS on Morteratschgletscher, Switzerland reveals that the larger ice ablation on Morteratschgletscher primarily results from an earlier start of the melt season and larger net solar radiation. The energy balance model used in this study is found to be more sensitive to changes in the stability correction than to an order-of-magnitude change in the roughness length for momentum.