Detection of soil and canopy freeze/thaw state in the boreal region with L and C Band Synthetic Aperture Radar

• Published in: Remote sensing of environment 2024-05, Vol.305, p.114102, Article 114102
• Main Authors: Cohen, Juval, Lemmetyinen, Juha, Jorge Ruiz, Jorge, Rautiainen, Kimmo, Ikonen, Jaakko, Kontu, Anna, Pulliainen, Jouni
• Format: Article
• Language: English
• Subjects: Backscatter model; Boreal forest; Canopy freezing; Freeze/thaw detection; SAR
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/j.rse.2024.114102

Satellite instruments operating at microwave frequencies enable the monitoring of soil freeze/thaw (F/T) state of large areas. In this study, seasonal variations in space-borne SAR backscatter at L (1.2 GHz) and C (5.4 GHz) bands are investigated in the context of F/T detection in the boreal region of Finland. The focus is on the influence of canopy freezing during the winter and backscatter trends during the thaw season. A new algorithm for soil F/T monitoring in the boreal region with L and C band SAR is demonstrated and assessed. A wet snow flag is included, enabling the monitoring also during the melting season. The soil F/T state is derived by first fitting a zeroth-order forest model to backscatter observations, leading to the retrieval of ground and canopy backscatter components. Then, a classification algorithm is applied on these backscatter components and the corresponding references representing F/T conditions. The results demonstrate the opposite backscatter response at L band compared to C band to canopy freezing. We also show a decreasing backscatter trend between spring and autumn thaw conditions at both frequencies. The use of autumn thaw references for the freezing season and spring thaw references for the end of the winter is therefore suggested. The overall similarity of the presented L and C band F/T detections with air temperatures reached 90–95% and with soil measurements at 5 cm depth 67–75%. Both frequencies were thus sensitive to F/T variations in the top-soil layer, detecting the onset of freezing before it is indicated by the soil sensors. C band wet snow detections corresponded well with fractional snow cover maps derived from optical satellite data, while L band was less affected by the wet snow. •Freeze/thaw (F/T) cycles in boreal forest are analyzed with spaceborne L-C band SAR.•Canopy freezing increases backscatter at C band while decreases it at L band.•Ground backscatter in thaw conditions decreased from spring to autumn.•Monitoring of soil F/T state in boreal forests is demonstrated at C and L bands.•Adaptation to outside of Finland will require examination of global forest datasets.