A continuous global record of near-surface soil freeze/thaw status from AMSR-E and AMSR2 data

Long term and consistent records of near-surface soil freeze/thaw (F/T) status are required for understanding hydrological, ecological, and biogeochemical responses of land surface to global warming. To create such a record, we compiled and inter-calibrated satellite observations from the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) and its successor, AMSR2, using linear regression models, and then applied a discriminant algorithm to the calibrated observations to map global F/T status from 2002 to 2018. The new global F/T dataset was rigorously assessed using in situ air and surface temperatures, and modelled soil temperature. Results show that agreement between remotely sensed F/T status and that determined by in situ or modelled temperature exceeds 85% and 79% for ascending and descending orbits, respectively. Moreover, consistency between the F/T datasets derived from two sensors is around 0.8 after calibration, in nonoverlapping time frames. With such an accuracy and consistency, we calculated frost days and frost trends using the F/T dataset. The mean annual number of frost days of high northern latitudes (>45 degrees N) is 279.2 +/- 44.1 days. Based on Mann-Kendall's tau-b test, 7.7% of global lands show a significant warming trend, and most of which are concentrated in the Western United States, Northern and Eastern Canada, Northern Europe and Western China. Such a spatial distribution was found to be consistent with the global land surface temperature anomalies trend from 2002 to 2018. Both the results of applications and favourable accuracy indicate the potential of this long, consistent F/T record to track global temperature change.