Deriving a frozen area fraction from Metop ASCAT backscatter based on Sentinel-1

• Main Authors: Bergstedt, H. (Helena), Bartsch, A. (Annett), Neureiter, A. (Anton), Höfler, A. (Angelika), Widhalm, B. (Barbara), Pepin, N. (Nicholas), Hjort, J. (Jan)
• Format: Article in Journal/Newspaper
• Language: English
• Published: Institute of Electrical and Electronics Engineers 2020
• Subjects: Advanced Scatterometer (ASCAT); Sentinel-1; freeze-thaw; permafrost; surface state; The Sentinel; Northern Finland
• Online Access: http://urn.fi/urn:nbn:fi-fe2020101684231

Abstract Surface state data derived from spaceborne microwave sensors with suitable temporal sampling are to date only available in low spatial resolution (25—50 km). Current approaches do not adequately resolve spatial heterogeneity in landscape-scale freeze–thaw processes. We propose to derive a frozen fraction instead of binary freeze–thaw information. This introduces the possibility to monitor the gradual freezing and thawing of complex landscapes. Frozen fractions were retrieved from Advanced Scatterometer (ASCAT, C-band) backscatter on a 12.5-km grid for three sites in noncontinuous permafrost areas in northern Finland and the Austrian Alps. To calibrate the retrieval approach, frozen fractions based on Sentinel-1 synthetic aperture radar (SAR, C-band) were derived for all sites and compared to ASCAT backscatter. We found strong relationships for ASCAT backscatter with Sentinel-1 derived frozen fractions (Pearson correlations of −0.85 to −0.96) for the sites in northern Finland and less strong relationships for the Alpine site (Pearson correlations −0.579 and −0.611, including and excluding forested areas). Applying the derived linear relationships, predicted frozen fractions using ASCAT backscatter values showed root mean square error (RMSE) values between 7.26% and 16.87% when compared with Sentinel-1 frozen fractions. The validation of the Sentinel-1 derived freeze–thaw classifications showed high accuracy when compared to in situ near-surface soil temperature (84.7%–94%). Results are discussed with regard to landscape type, differences between spring and autumn, and gridding. This article serves as a proof of concept, showcasing the possibility to derive frozen fraction from coarse spatial resolution scatterometer time series to improve the representation of spatial heterogeneity in landscape-scale surface state.