Inter-decadal variability of degree-day factors on Vestari HagafellsjΓΆkull (LangjΓΆkull, Iceland) and the importance of threshold air temperatures

2016-01-13T10:44:07Z (GMT) by Tom Matthews Richard Hodgkins
The skill of degree-day glacier melt models is highly dependent on the choice of degree-dayfactor (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 HagafellsjΓΆkull (1979-2012) using a Surface Energy Balance (SEB) approach that isolates the effect of changes in theprevailing weather on the 𝐷𝐷𝐹. At lower-elevation, we observe stable 𝐷𝐷𝐹 during the period 10 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 (π‘‡π‘π‘Ÿπ‘–π‘‘ 12 ) from which to initiate the positive-degree-day sum, and is removed by setting π‘‡π‘π‘Ÿπ‘–π‘‘ to -1.83Β°C, rather than the usual value of 0Β°C used in degree-day melt models. The stationarity of 𝐷𝐷𝐹 once π‘‡π‘π‘Ÿπ‘–π‘‘ is adjusted contradicts previous research and lends support to the use of constant 𝐷𝐷𝐹 for projecting future glacier melt. Optimizing π‘‡π‘π‘Ÿπ‘–π‘‘ also improves the skill of melt simulations at our study sites. This research thus highlights the importance of π‘‡π‘π‘Ÿπ‘–π‘‘ for both melt model performance and the evaluation of 𝐷𝐷𝐹 stationarity in a changing climate.