Abstract
Transition of freeze/thaw (F/T) affects land-atmospheric interactions and other biospheric dynamics. Global F/T statuses are normally monitored using microwave remote sensing, but at coarse resolutions (e.g., 25 km). Integration of coarse microwave remote sensing data with finer satellite products represents an opportunity to further enhance our ability to map F/T statuses regionally and globally. Here, we implemented and tested an approach to generate daily F/T status maps at a 5-km spatial resolution through the fusion of passive microwave data from AMSR2 and land surface temperature products from MODIS, using China as our study area for the year 2013 and 2014. Moreover, possible influences from elevation, vegetation, seasonality, etc., were also analyzed, as such analysis provides a direction to improve the approach. Overall, our freeze/thaw maps agreed well with ground reference observations, with an accuracy of ~86.6%. The new F/T maps helped to identify regions subject to frequent F/T transitions through the year, such as the Qinghai-Tibetan Plateau, Xinjiang, Gansu, Heilongjiang, Jilin, and Liaoning Province. This study indicates that the combination of AMSR2 and MODIS observations provides an effective method to obtain finer F/T maps (5-km or lower) for extensive regions. The finer F/T maps improve our knowledge of the F/T state detected by satellite remote sensing, and have a wide range of applications in regional studies considering land surface heterogeneity and models (e.g., community land models).
Highlights
About 55% of the exposed land surface in the northern hemisphere experiences freeze/thaw (F/T) transitions annually [1]
This study indicates that the combination of Advanced Microwave Scanning Radiometer-2 (AMSR2) and MODerate-resolution Imaging Spectroradiometer (MODIS) observations provides an effective method to obtain finer F/T maps (5-km or lower) for extensive regions
The foundation of detecting the F/T status by microwave remote sensing is the distinct contrast between dielectric properties that occurs when water transitions between solid and liquid [10], which means the phase of soil water during the F/T transitions will severely affect the sensitivity of radiometer brightness temperature (Tb) and radar backscatter signatures
Summary
About 55% of the exposed land surface in the northern hemisphere experiences freeze/thaw (F/T) transitions annually [1]. By controlling water and heat flux exchange at the land-atmosphere interface, F/T status imposes a significant constraint on ecosystem processes [2,3] and vegetation growth [4,5], resulting in important impacts on surface water, and the nitrogen [6] and carbon cycle [7]. To evaluate these ecological or hydrological impacts accurately, data of the F/T status with high spatial and temporal resolutions are essential [8]. The foundation of detecting the F/T status by microwave remote sensing is the distinct contrast between dielectric properties that occurs when water transitions between solid and liquid [10], which means the phase of soil water during the F/T transitions will severely affect the sensitivity of radiometer brightness temperature (Tb) and radar backscatter signatures
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