Advancements for Snowmelt Monitoring by Means of Sentinel-1 SAR

Abstract

Abstract:The Sentinel satellite constellation series, developed and operated by the European Space Agency, represents the dedicated space component of the European Copernicus program, committed to long-term operational services in environment, climate and security. We developed, tested and evaluated an algorithm for generating maps of snowmelt area from C-band synthetic aperture radar (SAR) data of the Sentinel-1 mission. For snowmelt classification, a change detection method is applied, using multitemporal dual-polarized SAR data acquired in Interferometric Wide swath (IW) mode, the basic operation mode over land surfaces. Of particular benefit for wet snow retrievals are the high instrument stability, the high spatial resolution across the 250 km wide swath, and the short revisit time. In order to study the impact of polarization, we generated maps of melting snow using data of the VV-polarized channel, the VH-polarized channel and a combined VV-and VH-based channel using a weighting function that accounts for effects of the local incidence angle. Comparisons are performed with snow maps derived from Landsat images over study areas in the Alps and in Iceland. The pixel-by-pixel comparisons show good agreement between the snow products of the two sensors, with the best performance for retrievals based on the combined (VV and VH) channel and a minor decline for the VH-based product. The VV-based snowmelt extent product shows a drop-off in quality over areas with steep terrain because of the decreasing backscatter contrast of snow-covered versus snow-free surfaces on fore-slopes. The investigations demonstrate the excellent capability of the Sentinel-1 mission for operational monitoring of snowmelt areas.

Mapping Radar Glacier Zones and Dry Snow Line in the Antarctic Peninsula Using Sentinel-1 Images

“…The Sentinel-1 SAR instrument operates in four exclusive imaging modes, namely, the Wave, Strip Map, Extra Wide (EW) Swath, and Interferometric Wide Swath modes, with different resolutions and coverage. Sentinel-1 has single polarization (VV or HH) for the Wave mode, and dual polarization (VV + VH or HH + HV) for other modes [44]. This study used the HH polarized Ground Range Detected (GRD) imagery product in EW mode to derive RGZs and DSL in the AP (see the image frames in Figure 1b).…”

TerraSAR-X Time Series Fill a Gap in Spaceborne Snowmelt Monitoring of Small Arctic Catchments—A Case Study on Qikiqtaruk (Herschel Island), Canada

“…This was also confirmed by our analysis of backscatter dynamics. Particularly, the X-band can be sensitive to refreezing snow layers because of its short wavelength and minimal penetration through the frozen layer [43,69]. In this context, the observed deviations between the 2015 as well as 2016 data from the 115 and 61 orbits in late April and early May most likely result from the difference in acquisition timing.…”

“…The observed variability indicates that both channels react to other surface features. Nagler et al [69] also reported lower accuracies of VV compared to the VH channel in C-Band, an effect likely connected to low local incidence angles. In our study, the strong overestimations in VV are also likely influenced by local topography as the areas of false snow detection are predominantly on the flat tundra uplands on the western part of the island.…”

“…The freezing snow layers can drastically lower the detection of wet SCE when using the wet snow detection threshold of −2 dB (see Figure 7: row 2) and, therefore, the derived FSC product if the acquisition time is during the minimum temperatures of the day (Figure 11). The cross polarized channels in C-Band show less angular dependence than the co-polarized channels and, therefore, perform more consistently in deriving snow cover [69].…”

“…The reduced backscatter signal of melting snow, as well as the increased backscatter of frozen ice layers, is the basis for a threshold segmentation that efficiently differentiates between wet snow or ice layers and dry snow or no snow. While this method was originally developed for C-Band of ERS-1 data and was further improved and adapted for ERS-2, RADARSAT-1, ENVISAT ASAR and Sentinel-1 data [40,68,69], it was also applied for X-Band data from TSX and COSMOSkyMed data [38,70,71]. The backscatter intensity of images acquired during the spring show notable differences between acquisitions caused by drifting sea ice and snowmelt that reduce the quality of co-registration when using cross-correlation.…”