Firn-Line Monitoring over the Juneau Icefield in Alaska Using SAR Data

The equilibrium-line altitude (ELA) is a key indicator of the climatic limit for glacier sustainability. Monitoring ELA is therefore essential for understanding and projecting glacier change. In practice, however, direct ELA measurements from field surveys (the glaciological method) are available for only a limited number of glaciers because sustained observations require substantial time and labor.Here, we observe multi-decadal variations in firn-line altitude (FLA) over an Alaskan icefield located near a well-monitored reference glacier with a long-term in situ ELA record, using a time series of L-band SAR imagery from JERS-1, ALOS, ALOS-2, and ALOS-4. We validate the SAR-derived FLA against the in situ ELA record and find that FLA variations closely track the observed ELA changes. This agreement indicates that L-band SAR-based FLA retrieval can serve as a proxy for long-term ELA trends.To clarify how winter observations differ between frequencies, we compare winter backscatter behavior at C-band and L-band. Winter C-band backscatter is strongly influenced by scattering from seasonal snow cover. By contrast, wintertime dry snow is largely transparent at L-band, and winter L-band SAR therefore primarily reflects surface conditions established at the end of the preceding summer. These results suggest that FLA can be monitored with a single winter L-band SAR acquisition.