Sea ice leads from Synthetic Aperture Radar (SAR)

Sea ice leads

Fig 1. Result: Lead density in the European Arctic (4 Jan 2016, 1km resolution)

Leads are linear-like areas with open water or thin ice within the sea ice cover. They are important for heat and gas exchange, sea ice growth. Therefore, leads are of interest for climate studies, weather forecasting and ship navigation in polar regions. Within this sub-project we classify leads from satellite images obtained within the microwave spectrum.

Synthetic Aperture Radar (SAR) satellites provide all-weather and season observations and the necessary high resolution to identify leads. The Copernicus Sentinel-1 mission comprises a constellation of two polar-orbiting satellites, performing C-band (5.4 GHz) SAR imaging, which enables them to acquire high resolution imagery (5-40 m). The Sentinel-1 extra wide swath mode with 400 km swath width and 40 m pixel size provides dual channel products which include measurements in co- and cross-polarized modes. Use of single co-polarized band might result in misclassification of leads under windy conditions, therefore a lead detection algorithm benefits from the use of Sentinel-1 SAR dual channel images.

Classification is done for each single data product taken with Sentinel-1 within one day. Than results of classification are compiled into a map of leads. Based on the map a lead density distribution in the European Arctic is calculated (Fig. 1).

Leads on optical and SAR images

Below two images of a marginal sea ice area are shown. Optical data (Fig. 4) is a combination of 3 channels: 490 nm (blue), 560 nm (green), 665 nm (red). Sea ice is white, water is dark. Small black structures within sea ice cover are leads. Left bottom corner of the image is covered with clouds. On the SAR image (Fig. 5) similar dark structures can be observed. Leads have lower backscatter intensity than sea ice (due to lower surface roughness). SAR imaging is not affected with clouds and can be performed at any season.

Fig. 2. Optical image (Sentinel-2)
Fig. 3. SAR image (Sentinel-1)