Post-collapse ice cliff backwasting at the Marmolada Glacier observed by terrestrial radar interferometry

The unexpected collapse of the Marmolada Glacier in the Italian Alps on 3 July 2022, caused 11 fatalities and emphasized the need for a deeper understanding of unstable glaciers. In response, we initiated a radar monitoring campaign to assess the stability of the remaining ice cliff and obtained a near-continuous data set of displacement at sub-hourly resolution and sub-millimeter accuracy. The survey spanned July 4–October 2, covering most of the ablation season and allowing insights into the post-collapse evolution of the potentially unstable scar. Rather than observing downslope motion, our measurements revealed progressive ice cliff backwasting driven by melt, with overall displacements up to 1.2 m on the sun-exposed rim and about three times smaller on the shaded wall. The observed pattern suggests that the wall retreated more slowly than the rim, consistent with typical radiation-driven ice cliff morphologies. Daily backwasting peaked at 53 (Formula presented.) 1 mm (Formula presented.) on the hottest day and fluctuated with air temperature. Sub-freezing temperatures halted the melt, revealing minor displacements toward the sensor, consistent with ice flow of up to 6 (Formula presented.) 1 mm (Formula presented.). Correlation analyses confirmed a strong link between daily backwasting rates and air temperature (Spearman coefficient (Formula presented.) at the rim). Independent estimates of backwasting from topographic data were consistent with our measurements, demonstrating the potential of terrestrial radar interferometry for studying ice cliff morphodynamics.