Estimation of high-resolution near-surface freeze/thaw state by the integration of microwave and thermal infrared remote sensing data on the Tibetan Plateau

The objective of this study is to investigate how the complementarity between microwave and thermal infrared remote sensing can be exploited for a high-resolution near-surface freeze/thaw state estimation. The basic idea is to establish a feasible relationship between the microwave-derived freeze/thaw state and thermal infrared observations. A quantitative freeze/thaw index from microwave observations at 18.7 and 36.5 GHz is innovatively defined and is assumed to be linearly correlated with land surface temperature from thermal infrared observations. Thus, a linear regression method is proposed and verified to be effective over a multiscale network of Naqu of the Tibetan Plateau. In order to demonstrate the potentiality of the proposed method, it is implemented in the entire Tibetan Plateau. It is found that the linear relationship is quite reliable for most areas and can obtain a high-resolution near-surface soil freeze/thaw state with integrated information from microwave and thermal infrared remote sensing. The validation of the high-resolution freeze/thaw state against soil temperature measured at active layer monitoring sites along the Qinghai-Tibet Highway illustrates a moderate accuracy over a decade scale. This study provides new insights for high-resolution freeze/thaw mapping beyond the Soil Moisture Active Passive mission.