The potential of InSAR for assessing meltwater lake dynamics on Antarctic ice shelves

. Surface meltwater drains on several Antarctic ice shelves, resulting in surface and sub-surface lakes that are potentially critical for the ice shelf collapse. Yet (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) Despite (cid:58)(cid:58)(cid:58)(cid:58)(cid:58) these (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) phenomena, our understanding and assessment of the drainage refreezing of these lakes is limited, due to lack of ﬁeld observations and to the limitations of optical satellite during polar night and in cloudy conditions. Therefore, this paper explores the potential of backscatter intensity and of interferometric coherence and phase from C-band synthetic aperture radar (SAR) imagery as an alternative to assess the dy- namics of meltwater lakes. In two case studies four study over Amery and Roi Baudouin ice shelves, we analyse the spatial and the temporal variations of East Antarctica, phase-based (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) detection (cid:58)(cid:58)(cid:58)(cid:58) and (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) monitoring of drainage events. Additionally, the results provide signiﬁcant evidence on the potential of the interferogram fringe patterns to detect and characterise instant events, such as lake drainage eventsover ice shelves. The potential of this technique to monitor these meltwater change events ishowever , however, (cid:58) strongly determined by the satellite revisit interval and potential changes in 20 scattering properties due to snowfall or melt events. high be related to a line-of-sight ice Four regions ice analysed on C-band Sentinel-1 parts (cid:58)(cid:58)(cid:58) of (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) Antarctica, this study shows a promising possibility to monitor the local dynamics of speciﬁc water features on the ice shelves by using InSAR, potentially paving the way towards (cid:58)(cid:58)(cid:58) that (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) InSAR (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) provides promising potential for monitoring meltwater (cid:58)(cid:58)(cid:58)(cid:58) lake (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) dynamics (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58) beyond (cid:58)(cid:58)(cid:58) that (cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)(cid:58)


Study areas
Two ice shelves in East Antarctica with well known :::::::::: well-known : meltwater dynamics (Kingslake et al., 2017) have been :: are : used as case studiesfor this study. The first case study is on the Roi Baudouin Ice Shelf (RBIS), where in situ research has been ::: was : conducted and the exact locations of several lakes were mapped during a field campaign in Jan. /Feb. 2016 105 (Lenaerts et al., 2016), which were revisited in Nov. 2017 when their lake collapse has been observed (Dunmire et al., 2020).
For Amery a and Amery d this increase results in a temporary overshoot until reaching the winter NRCS level again.

Coherence analysis
The coherence time series show a completely different behaviour than the NRCS :: σ 0 time series (Fig. 3). On the Amery ice shelf :: Ice ::::: Shelf, for example, snow, ice and lakes all have low or null coherence in summer, because of the altering scattering properties due to melt water content. For the ice and snow zones, the coherence rises abruptly when the surface refreezes in 210 spring, while the coherence over the lakes rises only gradually until winter, when the lakes reach coherence values that are similar to snow and ice. During winter, the coherence levels from snow, ice and lakes show a similar behaviour with large temporal variations when the coherence suddenly drops (i.e. fluctuating between 0.2 and 0.8 on 6 day time spans). These sudden drops are probably due to weather-induced changes in scattering properties (e.g. after a snowfall event). : , :: as :::::: shown :: in :::: panel :: a) ::: of ::: Fig. ::: 5). These drops are however sparse as the 6-day revisit cycle allows to get good overall coherence.
Local volume changes caused by gradual refreezing may be hard to quantify because the refreezing process may change the ice-water interface constantly, affecting the quality of the interferogram. However, the ::: For ::::::: example, :::: the deformation due to instant :::: rapid : meltwater events, such as drainage and collapse, may be captured, if the fringe pattern in the lake area appears highly distinct to the surroundings affected by tidal displacement ::: and ::::::::: horizontal :::::: motion. Within this context, we identified :::::: identify : two advantages of the phase fringes over the NRCS :: σ 0 : and coherence alone: i) an easier detection of stable ice or refrozen ::: and :::: lake ::::::::: refreezing : than coherence and backscatter intensity and ii) the detection of relative motion related due to uplift and subsidence events as a result of lake drainage or lake filling. The first advantage is clear in Figs. 7-: -10, where the phase patterns allow additional interpretation of the refreezing patterns which cannot be revealed by the coherence or backscatter intensity alone. The second advantage is in Fig. 9, where we could estimate the presence :: of a uplift event due to 335 lake drainage.
The paper also points at the limitations that due to the 12-days :::::: Despite ::::: noted ::::::::: limitations :: to ::::::: current Sentinel-1 repeat cycle over the RBIS, is it hardly sufficient to provide an inter-annual observation and comparison. A revisit larger than 6 days may greatly reduce the coherence and compromise the quality of the (complicated) unwrapping step over the ice shelf. Several 380 InSAR products completely impaired by weather events, in winter, most likely attributable to precipitations and wind, and by fast surface changes in summer, were found as a result.
Code availability. The DORIS software used to process Sentinel-1 SLC data is available at http://doris.tudelft.nl.
Data availability. The TanDEM-X data used for geo-coding the InSAR SLC products on the RBIS are available at https://doi.org/10.1594/ pangaea.868109.