Abstract
The soil freeze/thaw (F/T) cycles play an important role in the climate system and human activities. However, the harsh environment in the Qinghai-Tibet Plateau (QTP) poses great challenges for both in-situ observation and remote-sensing monitoring of the soil F/T process. In this article, the time series of scattering and coherence of the high-resolution Sentinel-1 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C-</i> band synthetic aperture radar (SAR) is analyzed to identify the soil F/T state. The time series of scattering, including intensity and decomposition parameters, and coherence, are analyzed based on three typical landcover types (i.e., desert, grassland, and meadow) in the QTP. They are given the mathematical description by second-order and fourth-order Fourier functions, respectively. Based on Fourier functions, the initial F/T time points of the soil are detected in each pixel to draw the F/T map of the entire study area. The experiment results are cross-validated with the initial F/T time points of the soil calculated from the MODIS land surface temperatures, showing that the differences in days are less than one revisit cycle of Sentinel-1 (i.e., 12 days). Furthermore, the possible impacts of environmental factors acquired from the Wudaoliang meteorological station, including air temperature, ground surface temperature, snow depth, and precipitation, on scattering and coherence are discussed. This study explores that Sentinel-1 has great potential for soil F/T monitoring in the QTP, which can indicate F/T states of the surface soil as well as F/T information of the deeper soil with a high spatial–temporal resolution.
Highlights
AS the largest plateau in the world, the Qinghai-Tibet Plateau (QTP) is known as the “Asian Roof” and “Third Pole” [1], directly affecting the surrounding climate and environment through atmospheric and hydrological processes [2]
This study aims to investigate the potential of Sentinel-1 for monitoring soil F/T cycles within permafrost areas on QTP by the time series of scattering and coherence
This part analyzes the correlation between meteorological factors and scattering features of synthetic aperture radar (SAR) images to better explain the scattering with the soil F/T process, including air temperature (TEM), ground surface temperature (GST), and snow depth (SD)
Summary
AS the largest plateau in the world, the Qinghai-Tibet Plateau (QTP) is known as the “Asian Roof” and “Third Pole” [1], directly affecting the surrounding climate and environment through atmospheric and hydrological processes [2]. As an important indicator of climate change [5], the total area of permafrost has decreased by approximately 410,000 km2 [6], while the rate of increase in the thickness of the active layer is the highest up to 7.5 cm per year [7]. The soil F/T cycles have an impact on the climate system, surface energy balance, land hydrological cycle, ecosystem cycles such as vegetation growth and soil respiration, ground-air energy exchange, and human activities [10]–[13]. It is of great significance to monitor the spatiotemporal changes of soil F/T cycles in the permafrost region of the QTP
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