Detection of soil freeze/thaw states in the Arctic region based on combined SMAP and AMSR-2 radio brightness observations

In this study, a new approach to identify the freeze/thaw states of tundra topsoil was developed based on the polarization ratio index, which was calculated from the reflectivity values of soil. Reflectivity was estimated from radiometric measurements of the SMAP satellite using the values of vertical polarization brightness temperature measured by the AMSR-2 radiometer at 6.9 GHz; this value was used to characterize the effective temperature of the soil. The proposed approach was tested using weather station data on soil surface temperatures for six test sites located in the North Slope of Alaska and the Yamal Peninsula collected from April 2015 to June 2018. The modified polarization ratio index, calculated from values of reflectivity rather than from brightness temperatures, significantly improved the possibility of determining the reference values of the index in the winter and in the summer. During testing, the modified index showed a good correlation between the dates of transition through the threshold level and soil temperature transition through 0oC, as recorded at meteorological stations.