Interdecadal variability of degree-day factors on Vestari Hagafellsjökull (Langjökull, Iceland) and the importance of threshold air temperatures

Abstract

The skill of degree-day glacier melt models is highly dependent on the choice of degree-day factor (DDF), which is often assumed to remain constant in time. Here we explore the validity of this assumption in a changing climate for two locations on Vestari Hagafellsjökull (1979-2012) using a surface energy-balance (SEB) approach that isolates the effect of changes in the prevailing weather on the DDF. At lower elevation, we observe stable DDF during the period of study; however, at higher elevation, DDF is noted to be more variable and a statistically-significant downward trend is observed. This is found to result from an inappropriate threshold air temperature (Tcrit) from which to initiate the positive-degree-day sum, and is removed by setting Tcrit to -1.83°C, rather than the usual value of 0°C used in degree-day melt models. The stationarity of DDF once Tcrit is adjusted contradicts previous research and lends support to the use of constant DDF for projecting future glacier melt. Optimizing Tcrit also improves the skill of melt simulations at our study sites. This research thus highlights the importance of Tcrit for both melt model performance and the evaluation of DDF stationarity in a changing climate.