CryoSat-2 delivers monthly and inter-annual surface elevation change for Arctic ice caps

Abstract

Abstract. We show that the CryoSat-2 radar altimeter can provide useful estimates of surface elevation change on a variety of Arctic ice caps, on both monthly and yearly timescales. Changing conditions, however, can lead to a varying bias between the elevation estimated from the radar altimeter and the physical surface due to changes in the ratio of subsurface to surface backscatter. Under melting conditions the radar returns are predominantly from the surface so that if surface melt is extensive across the ice cap estimates of summer elevation loss can be made with the frequent coverage provided by CryoSat-2. For example, the average summer elevation decreases on the Barnes Ice Cap, Baffin Island, Canada were 2.05 ± 0.36 m (2011), 2.55 ± 0.32 m (2012), 1.38 ± 0.40 m (2013) and 1.44 ± 0.37 m (2014), losses which were not balanced by the winter snow accumulation. As winter-to-winter conditions were similar, the net elevation losses were 1.0 ± 0.20 m (winter 2010/11 to winter 2011/12), 1.39 ± 0.20 m (2011/12 to 2012/13) and 0.36 ± 0.20 m (2012/13 to 2013/14); for a total surface elevation loss of 2.75 ± 0.20 m over this 3-year period. In contrast, the uncertainty in height change from Devon Ice Cap, Canada, and Austfonna, Svalbard, can be up to twice as large because of the presence of firn and the possibility of a varying bias between the true surface and the detected elevation due to changing year-to-year conditions. Nevertheless, the surface elevation change estimates from CryoSat for both ice caps are consistent with field and meteorological measurements.

Improved processing and calibration of the interferometric mode of the CryoSat radar altimeter allows height measurements of supraglacial lakes in west Greenland

“…Our methods in working with the L1b files, were described in Gray et al (2013) and Gray et al (2015). The current Matlab processing provides both POCA and swath mode results, and here we note any changes since the earlier work.…”

“…This is 20 essentially independent of the position of the POCA, hence our decision to estimate the POCA position based on the first significant leading edge in the waveform. Our approach (Gray et al 2015) uses the point of inflexion (maximum slope) on the first significant waveform increase, and is similar to that adopted by Nilsson et al (2016) and Smith et al (2016). Helm et al (2014 used a threshold level of the first significant leading edge for their work in Greenland and Antarctica, following the work of Davis (1997) who advocated a threshold retracker to minimize the dependency on varying microwave 25 penetration into, and backscattering from, various snow-firn-ice layers.…”

“…The low-pass filter uses a 3 dB width of ~ 4 samples and is designed to avoid introducing any bias in the 30 waveform phase. Averaging of SARIn waveform data is performed only in the range direction with a relatively small impact on the cross-track footprint size (Gray et al 2015), and none on the along-track resolution. It is not appropriate to average…”

“…Indeed, the L1b products have been used in several studies of change in Antarctica and Greenland (e.g. Helm et al, 2014, Nilsson et al, 2016, smaller Arctic ice caps (Gray et al 2015), and lake height (Kleinherenbrink et al 2014). In these studies, the authors claim improvements in the results over the standard level 2 product, due to the specialized processing.…”

The projected demise of Barnes Ice Cap: Evidence of an unusually warm 21st century Arctic

“…The narrow elevation range of its accumulation zone (700-1100 m above sea level (asl)) makes the ice cap more sensitive to climate change than any other glacier on Baffin Island [Gardner et al, 2012], with a slight increase in the equilibrium line altitude causing a marked reduction of the accumulation area. Barnes Ice Cap was probably close to equilibrium at the end of the 19th century but has experienced a negative mass balance trend since [Gilbert et al, 2016], with an acceleration in this trend since the mid-1990s [Abdalati et al, 2004;Sneed et al, 2008;Gardner et al, 2012;Gilbert et al, 2016] causing the disappearance of its accumulation area since about 2010 [Gray et al, 2015;Gilbert et al, 2016], and surface lowering at all elevations (see Figure S5 in the supporting information) on the south dome profile [Holdsworth, 1973;Hooke et al, 1987].…”