Identifying Soil Freeze/Thaw States Using Scattering and Coherence Time Series of High-Resolution C-Band Synthetic Aperture Radar in the Qinghai-Tibet Plateau

The soil freeze/thaw (F/T) cycles play an important role in the climate system and human activities. However, the harsh environment in the Qinghai-Tibet Plateau (QTP) poses great challenges for both in-situ observation and remote-sensing monitoring of the soil F/T process. In this article, the time series of scattering and coherence of the high-resolution Sentinel-1 C-band synthetic aperture radar (SAR) is analyzed to identify the soil F/T state. The time series of scattering, including intensity and decomposition parameters, and coherence, are analyzed based on three typical landcover types (i.e., desert, grassland, and meadow) in the QTP. They are given the mathematical description by second-order and fourth-order Fourier functions, respectively. Based on Fourier functions, the initial F/T time points of the soil are detected in each pixel to draw the F/T map of the entire study area. The experiment results are cross-validated with the initial F/T time points of the soil calculated from the MODIS land surface temperatures, showing that the differences in days are less than one revisit cycle of Sentinel-1 (i.e., 12 days). Furthermore, the possible impacts of environmental factors acquired from the Wudaoliang meteorological station, including air temperature, ground surface temperature, snow depth, and precipitation, on scattering and coherence are discussed. This study explores that Sentinel-1 has great potential for soil F/T monitoring in the QTP, which can indicate F/T states of the surface soil as well as F/T information of the deeper soil with a high spatial-temporal resolution.