2 Observational Data at Crane Glacier
To use Crane Glacier as a tool for evaluating ice cliff models, we must
do three things:
- Infer the glacier geometry, the floatation state, and the grounding
line position through time.
- Show that fracture processes governed Crane’s retreat behavior after
the LBIS collapse.
- Constrain the rates of retreat and their evolution through time.
All three of these objectives are observationally challenging. Brittle
processes play out on very short timescales (from minutes to days) not
captured by the recurrence interval of most satellite observations. In
addition, we show here that ice penetrating radar data collected over
Crane Glacier before the LBIS collapse failed to clearly capture the ice
bottom geometry, leaving a poorly constrained ice thickness profile,
grounding line position, and terminus height at the time of collapse.
But by revisiting the question of Crane Glacier’s response to the LBIS
collapse 20 years later, this study benefits from (a) new marine
geophysical measurements of the sea-floor morphology at the 2002
grounding line and (b) aircraft and satellite-measured surface elevation
and ice velocity data that capture the glacier’s retreat and advance in
the decades following collapse. These data allow us to better constrain
the floatation state of the glacier terminus through time, refine
estimates of retreat rates, and potentially narrow the range of
thresholds (defined by current process models) that limit cliff failure.
Surface Topography and Ice Velocity
Glacier behavior before and after
LBIS collapse is best characterized by altimetry and velocity
time-series data (Figs. 1, 2). Surface elevation data were collected
along the centerline of Crane Glacier by Pre-IceBridge and IceBridge
campaigns (Blair et al., 2018; Studinger, 2014; Thomas & Studinger,
2010); these form the primary basis for our analysis. Satellite radar
altimeters (including CryoSat-2) suffer from reduced vertical accuracy
in the high slope regions of the peninsula (Fang Wang et al., 2015), and
while satellite laser altimeters (including ICESat and ICESat-2) capture
surface elevation with high spatial precision, their sampling is limited
by cloud-cover, track orientation, and spacing in the region. Here, we
use ICESat-2 data to calibrate photogrammetric DEM’s produced from Maxar
stereoimagery to extend the altimetry record through 2021.
There is uncertainty in the effective ice thickness at Crane in 2002 due
to uncertainty in the firn air content (FAC) there (which reduces mass
in the ice column, leading to stresses comparable to a shorter ice cliff
than measured). The GSFC-FDMv1.1 model predicts ~15 m of
FAC near the Crane terminus and its upstream catchment in 2002 (Medley
et al., 2020; Figure S1), in contrast with the 0 m of FAC observed on
the LBIS immediately before its collapse (Holland et al., 2011). The
GSFC-FDMv1.1 does, however, agree with in-situ observations over the
Larsen C (Ashmore et al., 2017). We present both the conservative cliff
heights (observed height minus 15 m) and maximum possible cliff heights
(observed height assuming no FAC).
In addition to using the altimetry record, we use MEaSUREs (Mouginot et
al., 2017) and ITS_LIVE velocity data (Gardner et al., 2019) to
characterize the evolving flow field of Crane Glacier. Together with ice
terminal retreat rates, we use velocities to estimate Crane Glacier’s
calving rate post LBIS collapse.