Nature research paper: Suppressed basal melting in the eastern Thwaites Glacier grounding zone
for salinity. The effect of tidal flow has been ignored in these calculations, as the tidal flow speeds are an order of magnitude weaker.An ApRES was established within 10 m of the borehole on 23 January 2020 and set to record a burst of 20 measurements once every 2 h. The data were recovered from the instrument on 27 December 2020.
ApRES uses frequency-modulated continuous-wave modulation, with a chirp that scans from 200 to 400 MHz over a 1-s period. The measurements in each burst were checked for quality and then averaged. Each averaged burst was processedBy using both amplitude and phase, ApRES can monitor the changing distance between the antennas and the ice-shelf base with millimetre-scale precision.
For tracking the ice base to obtain the total thinning rate, an assumption in ApRES data processing is that, over the period of the time series, the topography of the ice base local to the radar does not change at length scales at or longer than the wavelength of the radar waves in ice, in this case, at length scales greater than about 0.5 m. This requirement was not met at the borehole site nor at various periods of the time series from the upstream sites.
Vertical profiles of vertical ice velocities, averaged across the entire time series, were calculated by dividing the return into 4-m layers, cross-correlating as described above and calculating the mean vertical velocity for each layer. From these profiles a depth interval was selected from the lower part of the ice column from which to calculate the integrated non-melt contribution to the thinning rate.
The final melt-rate time series was calculated by subtracting the non-melt contribution from the thinning rate and then low-pass-filtering at a cutoff of 15 days.Icefin was equipped with a Neil Brown Ocean Sensors conductivity–temperature sensor and a Valeport ultraP pressure sensor. The stated manufacturer accuracies are ±0.001 S m, ±0.005 °C and 0.1 dbar for conductivity, temperature and pressure, respectively, which translate into uncertainties of ±0.
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