Investigating seasonal surface elevation changes of Wolverine Glacier, Alaska, using high-resolution geodetic surveys, ground-penetrating radar, and firn modeling

Abstract:

Repeat high-resolution digital elevation models (DEMs) offer a promising approach for measuring the mass balance of mountain glaciers from airborne or satellite platforms. A single glacier-wide density can be used to equate the observed volume change to mass change when the elapsed time between surveys is on a multi-annual to decadal scale. However, surface mass balance, firn densification, and ice dynamics (emergence velocities) have variable effects on the surface elevation of glaciers on shorter (seasonal to annual) time frames, complicating the calculation of mass change. Here we use repeat fall/winter lidar and Structure-from-Motion derived DEMs, end-of-winter snow depths derived from ground-penetrating radar surveys, and a distributed firn densification model to examine the distribution and magnitude of winter surface elevation changes on Wolverine Glacier over three accumulation seasons (2015-2016, 2016-2017, 2019-2020). Emergence/submergence velocities were calculated by differencing the distributed snow depths and observed surface elevation change, while accounting for firn densification. The results show a realistic distribution and magnitude of ice emergence, with negative velocities (submergence) in the accumulation zone and positive velocities (emergence) in the ablation zone. Repeat GPS surveys of mass balance stakes show good agreement with the combined rates of firn densification and emergence, ranging from -6 m/yr in the accumulation zone to +5 m/yr in the ablation zone. The surface lowering attributable to firn densification calculated from the firn model showed significant variation from year to year (±1 m/yr in the upper accumulation zone). Constraining the proportion of annual firn densification which occurs during the winter season introduces uncertainty into our calculation of emergence velocities. The results highlight the importance of distinguishing between surface mass balance, firn densification, and emergence velocity when converting between volume and mass changes on seasonal to annual scales. Parametrization of these processes will facilitate future seasonal mass balance measurements over larger areas using geodetic methods.