A calibrated mass balance model for Vatnajökull, Iceland

Vatnajökull (Iceland) is the largest ice cap in the world where the energy and mass balance have been studied with good spatial and temporal resolution.In this paper we use these data to analyze the energy balance and to construct a calibrated and spatially distributed mass balance model.The incoming longwave radiation is best modeled as a function of meteorological variables in the free atmosphere just above the relatively thin katabatic layer, instead of those at the 2 m level.The ratio of changes in the 2 m temperature to changes in the free atmospheric temperature (the climate sensitivity) is smaller than 1.Therefore, when the bulk method is used to compute the turbulent fluxes, the 2 m temperature must be explicitly calculated.Otherwise the sensitivity of Vatnajökull to climatic change would be overestimated.When the model is forced with data from a permanent weather station not on the ice cap, it reproduces the observed mass balance reasonably well.Horizontal precipitation gradients over Vatnajökull are large, which results in a strongly varying sensitivity to external temperature changes over the ice cap.The mass balance and its sensitivity is thus highly dependent on local climatic conditions.For a temperature increase of 1 K and a simultaneous precipitation increase of 5.3%, the mean specific mass balance of Vatnajökull decreases by 0.56 m w.e.