Numerical modelling assessment of glacier surge impact on observed elevation change signals

Glacier surges periodically move ice masses to lower elevations and hence produce dynamic patterns of substantial thinning and thickening, but the net mass change over a typical time period of elevation change assessment of a few years to decades is not obvious.  Surging glaciers may therefore affect regional scale elevation change assessments as acquired from differencing of remotely sensed elevations, as for example for the observed Karakoram mass gain anomaly.

In this study I synthetically model glacier surges for a range of glacier sizes (slopes, thicknesses) and investigate the impact on the surface elevation change and total mass change for a typical range of surge durations, intensities and periods.

When keeping the climate forcing constant I find that the mean glacier elevation (or volume) is almost symmetric around the surge phase. Hence, when sampling elevation change over a large population of glaciers with randomly occurring surges there is little impact on the detected average elevation changes over all glaciers. The exceptions are steep glaciers which produce very short advance phases and much more extended phases of mass recovery. When sampling elevation change over a couple of years to decades, it is therefore much more likely to detect a thickening and therefore the population mean is biased to positive elevation change values.

When assessing mean elevation change on a regional scale, usually one fixed glacier outline is chosen for masking the data. However, for surging glaciers the extent can undergo large fluctuations. I therefore further assess the mean elevation change for glaciers extent masks that are varying between the maximum and minimum values of a surge. Despite a constant climate, the mean elevation change turns out to be increasingly biased towards detecting a thickening signal the further upstream the glacier extent is taken. This implies that for minimizing this thickening bias from glacier surges in assessing regional elevation change, glacier outline masks from their most extensive extents should be used.

Further modelling experiment showed that, the results are still valid when prescribing a variable climate forcing, but the surging effect is slightly subdued.