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Abstract:
Permafrost ground surface undergoes seasonal subsidence and uplift during freeze-thaw cycles of the active layer on top of permafrost. GPS interferometric reflectometry (GPS-IR), a technique that uses the signal-to-noise ratio (SNR) to infer the ground surface properties, has been applied to retrieve frozen ground movements in the thaw and freezing seasons. Over permafrost areas, the complete ground freezing process is rarely observed by GPS-IR due to the normal autumn snow accumulation on the ground. In this study, we combine GPS-IR measurements and temperature records to predict the freezing-season ground uplift under snow cover. First, we apply GPS-IR to the SNR data collected by GPS site SG27, located in Utqiaġvik Alaska, to retrieve daily elevation changes of the ground surface from 2018 to 2019. Second, the timings of snow accumulation and disappearance are estimated by GPS-IR measurements, and the onset of freezing and thaw seasons are determined with the thermal indices calculated from temperature records. Third, we fit the modified Stefan model to the GPS-IR measurements of ground uplift and thermal indices from freezing onset to snow-in day. The fitting results show good agreements with R2 of 0.65 and 0.80 for 2018 and 2019 respectively. Last, extending the fitted model to the completely frozen day, we predict daily snow-covered ground uplift of up to 1.7 cm and 1.8 cm in 2018 and 2019 respectively. Our study is the first attempt to estimate the daily ground uplift under snow accumulation with GPS-IR, offering insight to understand the permafrost dynamics in freeze-thaw cycles.
