Glacier melt detection at different sites of Greenland ice sheet using dual-polarized Sentinel-1 images

Synectic Aperture Radar (SAR) backscatter coefficient is sensitive to glacier surface physical characteristic changes, including the states of melting and refreezing, but it is also sensitive to incidence angle variation. This study explores the capability of monitoring Greenland Ice Sheet (GrIS) melting status with Sentinel-1 dual-polarized images by referring to Automatic Weather Station (AWS) records. Sentinel-1 SAR images at five coastal regions of the GrIS are obtained from 2017 to 2021. The backscatter coefficients are normalized to an incidence angle of 30 & DEG; with an empirical model. Time series of five backscatter coefficients profiles covering AWS illustrates different patterns of the ice surface dielectric constant dynamics in different elevations. The wet snow radar zone shows clear backscatter coefficients decreasing during the melting seasons, but the bare ice radar zone behaves more complexly during the melting seasons. The numbers of melting days at different elevations are also derived for each profile based on -3 dB backscatter coefficient decrease of HH and/or HV polarization, showing the heterogeneous ablation processes over the GrIS. The daily maximum 2 m air temperature on two consecutive days (before and on the SAR acquisition day) exceeds 0 & DEG;C, and the daily average 2 m air temperature exceeds -0.5 & DEG;C on the SAR acquisition day that was recorded by the AWS finds good agreements with the -3 dB decrease of the backscatter coefficients, suggesting the GrIS surface melting can be well captured by dual-polarized Sentinel-1 C-band SAR images. The overall agreement and Kappa coefficients are mostly better than 0.85 and 0.70, respectively, for HH images and 0.80 and 0.60, respectively, for HV images, suggesting a better performance of the co-polarized image. High temporal resolution and wide-swath SAR sequence imagery provide suitable data sources for monitoring glacier surface melting-refreezing stats; further analysis is requested to quantitatively link the volume of melting with backscatter coefficient and other SAR data sources.