Landfast ice growth and displacement in the Mackenzie Delta observed by 3D time-series SAR speckle offset tracking

This study investigates the growth and displacement of landfast ice along the shoreline of the Mackenzie Delta by synthetic aperture radar (SAR) speckle offset tracking (SPO). Three-dimensional (3D) offsets were reconstructed from Sentinel-1 ascending and descending SAR images acquired on the same dates during the November 2017-April 2018 and October 2018-May 2019 annual cycles. The results showed horizontal and vertical displacements of floating landfast ice 10 caused by ice breakups and pressure ridges, which are mainly driven by drift sea ice motions and Mackenzie Delta discharges. Cumulative vertical offsets of approximately -1 to -2 m were observed from freshwater landfast ice, which is due to longer radar penetration into the ice-water interface with increasing landfast ice thickness. Numerical ice thickness model estimates confirmed that the cumulative vertical downward offsets indicate the growth of freshwater landfast ice thickness. Time-series analysis showed that significant growth and displacement of floating landfast ice in the Mackenzie Delta occur between 15 November and January.

The manuscript presents an interesting analysis of SAR speckle offset tracking applied to the measurements of horizontal ice motion and ice growth in landfast ice near the Mackenzie Delta. By combining results from imagery acquired from same-day ascending and descending orbits, the authors are able to estimate 3D motion of the surface from which radar energy is backscattered. In the case of the relatively low-salinity ice found in parts of the Mackenzie Delta, the scattering interface is assumed to be the ice-water interface and thus the observed vertical motion of the surface is interpreted as ice growth. This finding is supported by results from a 1D ice growth model which is in agreement with observed downward motion of the scattering surface. Elsewhere, positive vertical motion is assumed to indicate pressure ridging. Significant horizontal motion is also observed, which is attributed to wind, currents and river discharge.
The authors have chosen a complex region of the Arctic for their study, where atmospheric, cryospheric, marine and terrestrial processes interact. Unfortunately, the text suggests that the authors do not have a deep familiarity with the geophysics of these systems and as a result I fear they are misinterpreting their results. In particular, I am skeptical that the backscatter in the regions where the ice exhibits downward motion is coming from the bottom of the ice as the authors assume. Details are given in my major comments below, but in short, my reasons are: i. The backscatter from the ice bottom is unlikely to remain coherent over periods of 12 days or more during growth ii.
the authors present no direct observations of the ice salinity to support the assertion that the C-band radar is penetrating to the bottom of the ice iii.
the authors overlook a much more likely mechanism for downward motion of the ice surface near a large delta during winter.
3D measurements of small-scale ice motion could be of considerable value for understanding dynamical processes in the Arctic coastal zone and I do not wish to discourage the authors from continuing this line of research. However, if they must address my concerns below if they are going to continue to assert that their observations are related to thickening of the ice cover.

No explanation of why backscatter from ice bottom would remain coherent during growth
The speckle offset tracking technique requires that the scattering surface remains coherent between image acquisitions. However, in the case of scattering from the bottom of a growing floating ice cover, the radar is seeing an entirely new surface at each acquisition and I therefore see no reason why the speckle would remain consistent over timespans of 12 days. The authors need to provide more explanation of how the scattering characteristics of the underside of the sea ice (if indeed that is where the signal is coming from) would remain constant as new ice forms below each previously imaged surface.

Inadequately supported assumption that ice bottom is source of SAR backscatter
The authors assume that the SAR signal from the stabilized floating landfast ice is coming from the ice-water interface. The basis of this assumption is the low-backscatter signature of presumed bottomfast ice nearby and the presence of low-salinity ice in this region reported by MacDonald et al (1995). However, none of ice sampled by MacDonald et al was completely fresh and most contained a significant seawater fraction. Moreover, close inspection of the SAR intensity imagery (Fig 3d,e) shows linear spatial patterns typically associated with surface roughness features. It therefore seems likely that some fraction of the microwave energy returned from the ice is coming from its upper surface and volume. This has a significant bearing on the interpretation of the SPO results, but is not discussed in the manuscript.