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

ABSTRACT

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° 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.

Acknowledgments

The authors are grateful to the editor Timothy Warner and the other anonymous reviewers for their constructive remarks. The station-based air temperature data are archived at the NOAA National Climatic Data Center. The global land temperature anomaly data are available on the NOAA GHCN Gridded Products website. Research product (AMSR-E L1S) is provided by the GCOM-W Research Product Distribution Service, Japan Aerospace Exploration Agency (JAXA). AMSR2 data was provided by the GCOM-W Data Providing service, JAXA.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was jointly supported by the National Key Basic Research Program of China under Grant [2015CB953701]; the National Natural Science Foundation of China under Grant [41671355]; Chinese Academy of Sciences Strategic Priority Research Program [XDA19070204], “Light of West China” Program and Youth Innovation Promotion Association under Grant No. [2016061].