Abstract
Abstract. Icebergs account for half of all ice loss from Antarctica and, once released, present a hazard to maritime operations. Their melting leads to a redistribution of cold fresh water around the Southern Ocean which, in turn, influences water circulation, promotes sea ice formation, and fosters primary production. In this study, we combine CryoSat-2 satellite altimetry with MODIS and Sentinel-1 satellite imagery and meteorological data to track changes in the area, freeboard, thickness, and volume of the B30 tabular iceberg between 2012 and 2018. We track the iceberg elevation when it was attached to Thwaites Glacier and on a further 106 occasions after it calved using Level 1b CryoSat data, which ensures that measurements recorded in different acquisition modes and within different geographical zones are consistently processed. From these data, we map the iceberg's freeboard and estimate its thickness taking snowfall and changes in snow and ice density into account. We compute changes in freeboard and thickness relative to the initial average for each overpass and compare these to estimates from precisely located tracks using the satellite imagery. This comparison shows good agreement (correlation coefficient 0.87) and suggests that colocation reduces the freeboard uncertainty by 1.6 m. We also demonstrate that the snow layer has a significant impact on iceberg thickness change. Changes in the iceberg area are measured by tracing its perimeter, and we show that alternative estimates based on arc lengths recorded in satellite altimetry profiles and on measurements of the semi-major and semi-minor axes also capture the trend, though with a 48 % overestimate and a 15 % underestimate, respectively. Since it calved, the area of B30 has decreased from 1500±60 to 426±27 km2, its mean freeboard has fallen from 49.0±4.6 to 38.8±2.2 m, and its mean thickness has reduced from 315±36 to 198±14 m. The combined loss amounts to an 80 %±16 % reduction in volume, two thirds (69 %±14 %) of which is due to fragmentation and the remainder (31 %±11 %) of which is due to basal melting.
Highlights
Iceberg calving accounts for roughly half of all ice loss from Antarctica (Depoorter et al, 2013; Rignot et al, 2013)
We first assess changes in the B30 iceberg area using boundaries mapped from satellite imagery, and we compare the observed trend to less accurate estimates derived from arc lengths and semi-major axes
Because deriving iceberg outlines requires a high degree of time-consuming manual interaction, we evaluate the efficacy of two alternative methods based on measurements of their orthogonal axes by the National Ice Center (NIC) and on arc lengths recorded in satellite altimetry which are considerably less laborious
Summary
Iceberg calving accounts for roughly half of all ice loss from Antarctica (Depoorter et al, 2013; Rignot et al, 2013). Scambos et al (2008) installed a range of measurement tools including a GPS receiver, a pre-marked accumulation mast, and buried bamboo poles observed with a camera on a large Antarctic iceberg to monitor melting They differentiate between three kinds of mass loss: rift calving, edge wasting, and rapid disintegration. A range of studies have employed laser and radar altimetry to study freeboard change in large tabular icebergs: Jansen et al (2007) studied the A-38B iceberg in the Weddell and Scotia seas with a combination of laser and radar altimetry, and Scambos et al (2008) included three Ice, Cloud, and land Elevation Satellite (ICESat) overpasses over the A22A iceberg to derive its thickness change Both studies make use of satellite imagery to colocate the altimetry tracks and to compare similar areas in terms of freeboard change.
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