Glaciological
Measurementdoc
How
would you go about measuring the net mass balance of a 10,000 km2 Arctic ice
cap which terminates in the sea? 10,000 km2 is at the very top of the size
range of Arctic ice caps. Detailed measurement programs of mass balance are
relatively rare for Arctic glaciers, and those that are available mostly relate
to smaller glaciers (around 10km 2) terminating on land, and not to larger ice
caps above 100km2 (Dowdeswell & Hagen, 2004). Observation of such an ice
cap would therefore be quite scientifically valuable. Precedents include the
Hans Tausen Ice Cap in Greenland (4000 km2) and the Austfonna Ice Cap (8000
km2) in Svalbard (Dowdeswell & Hagen, 2004).
The
equation for calculating total mass balance is V / a = Ma Mm Mc ± Mb (Hagen
& Reeh, 2004). There are two main approaches for measuring the mass balance
of ice masses, which quantify the left and right sides of this equation
respectively: (1) the cartographic method, directly measuring the change in
volume by monitoring the changes in surface elevation through remote sensing or
(2) the direct glaciological method, seperately measuring each element of
accumulation and ablation through local measurements (Reeh, 2006; Hagen &
Reeh, 2004).
The
direct glaciological method is more resource-intensive than the cartographic
method, but is potentially more accurate (Hagen & Reeh, 2004). Since a
10,000 km 2 ice sheet is sufficiently small for direct glaciological methods to
be feasible but big enough that remote sensing may be preferable, both methods
will be considered here.
Cartographic
method The basis of the cartographic method is that the change in volume can be
estimated by comparing the topographic changes in an ice mass between different
years. Topographic changes can be measured using photography, airborne laser
altimetry, or satellite radar altimetry (Hagen & Reeh, 2004), however the
accuracy of current satellite radar altimetry is not sufficient for an ice cap
of this size, so air-based methods would be required (Dowdeswell & Hagen,
2004). Of the two methods, aerial photography has been more commonly used in
the past, but laser altimetry is far more accurate (down to ± 10 cm compared
with ± 1-2 m for photography) (Bamber &
Kwok, 2004). Either could potentially be used here.
Kwok, 2004). Either could potentially be used here.
Hagen
& Reeh (2004) describe the process of measuring mass balance using this
method. An aerial survey using photography or laser altimetry is made at the
same point late in the ablation cycle in different years, and the data is used
to create digital elevation models (DEMs) showing the change in topography over
the period, which can be converted to the volume change in water equivalent to
yield the net mass balance figure. Cartographic
estimation of mass balance based on remote sensing is often used
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