基于陆基L波段辐射计的青藏高原地表冻融反演研究
Based on a Relative Frost Factor Algorithm Retrieving Landscape Freeze/Thaw State by Ground-Based L-Band Radiometer

DOI: 10.12677/AG.2020.105039, PP. 437-446

Keywords: 青藏高原，冻融，L波段，陆基辐射计，极化指数
Tibetan Grassland, Freeze/Thaw, L-Band, Ground-Based, Polarization Index

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Abstract:

青藏高原作为中低纬度地区面积最大的季节性冻土分布区，其地表冻融在气候变化及反馈中扮演了关键性角色。L波段被动微波辐射计观测作为最具潜力的全球尺度冻融反演技术还未被评估其在青藏高原的适用性，因此，本文基于高频时次的陆基辐射计观测配合详尽的站点土壤监测，尝试以相对冻结因子阈值判断算法为基础算法，并详细探讨了应用多种微波极化指数在青藏高原典型季节性冻土区的适用性。结果表明：针对青藏高原季节性冻土，最适采用垂直极化指数作为相对冻结因子阈值判断算法的输入指数，其总精度能够在60？入射观测角度时达到0.92，而极化差异指数NPR作为最广泛应用的判别指数，并不适合青藏高原地区。本研究为青藏高原地区开展L波段星载微波辐射计反演土壤冻融提供了客观的极化指数选取依据。
Soil freezing dramatically alters the soil hydraulic and thermal properties that in turn affect surface energy and water budgets, net ecosystem exchange of carbon and the surface runoff over grassland. Passive microwave observations at L-band (1 - 2 GHz) benefits from a relatively high contrast in the dielectric constant between free water and ice with lower scattering by surface vegetation. Thus, this study uses a relative frost fac-tor algorithm with four polarization ratios to detect temporal changes of soil freeze/thaw (F/T) state based on a soil temperature and freeze/thaw synthesis experiments conducted over the north-eastern part of the Tibetan Plateau. The L-band brightness temperature and the in-situ soil temperature are measured simultaneously for the purpose of evaluating performances of the relative frost factor algorithm on different incident angles. Results of estimated soil F/T state indicate that the algorithm corresponding to Vertical Polarization index (Vpol) achieves the highest preci-sion (0.92) with incident angle of 60？. NPR index, as wide and successful applications for frost factor algorithm, is still needed to be optimized to decrease its degree of confusion in discriminating F/T state. The evaluation of the performances in this study is useful for selecting optimum polarization index for spaceborne L-band radiometers retrieving soil freeze/thaw state in the seasonally frozen Tibetan grassland.

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