Assimilation of Cosmic-Ray Neutron Counts for the Estimation of Soil Ice Content on the Eastern Tibetan Plateau
Assimilation of Cosmic-Ray Neutron Counts for the Estimation of Soil Ice Content on the Eastern Tibetan Plateau
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DOI：
10.1029/2019jd031529
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发表时间：
2020-02-16
影响因子：
4.4
通讯作者：
Su, Zhongbo
中科院分区：
文献类型：
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作者：
Mwangi, Samuel;Zeng, Yijian;Su, Zhongbo
在寒区水文研究中，土壤水分相形态的准确观测和模拟是至关重要的。因此，土壤冰含量的量化对于研究和理解该地区的冻融过程至关重要。本研究的重点是利用原位土壤水分（即液相）和宇宙射线中子测量（即包括液体和冰在内的总水）反演土壤冰含量，并进行观测系统模拟实验。为了从中子计数中获得土壤总水分，对宇宙射线中子探测器（CRNP）校准的不同加权方法（修正、常规和统一）进行了比较。比较表明，传统的非线性方法效果最好。在此基础上，采用STEMMUS-FT （Simultaneous Transfer of Energy， Mass and Momentum in非饱和土壤）模型作为物理过程模型，COSMIC （COSMIC -ray Soil Moisture Interaction Code）模型作为观测算子（即正向中子模拟器）来同化快中子。除了STEMMUS-FT中扰动初始化的背景输入外，模型不确定性被预定义为吸收快中子。我们观察到，在不确定性足够大的情况下，更新状态可以模拟CRNP观测。在所有设置中，同化CRNP测量值可以增强土壤总水分分析，从而改善土壤冰含量的检测，从而改善田间尺度上的冻结融化过程。
Accurate observations and simulations of soil moisture phasal forms are crucial in cold region hydrological studies. In the seasonally frozen ground of eastern Tibetan Plateau, water vapor, liquid, and ice coexist in the frost-susceptible silty-loam soil during winter. Quantification of soil ice content is thus vital in the investigation and understanding of the region's freezing-thawing processes. This study focuses on the retrieval of soil ice content utilizing the in situ soil moisture (i.e., liquid phase) and cosmic ray neutron measurements (i.e., total water including liquid and ice), with Observing System Simulation Experiments. To derive the total soil water from neutron counts, different weighting methods (revised, conventional, and uniform) for calibrating the cosmic-ray neutron probe (CRNP) were intercompared. The comparison showed that the conventional nonlinear method performed the best. Furthermore, to assimilate fast neutrons using the particle filter, the STEMMUS-FT (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) model was used as the physically based process model, and the COSMIC model (Cosmic-ray Soil Moisture Interaction Code) used as the observation operator (i.e., forward neutron simulator). Other than background inputs from disturbed initializations in the STEMMUS-FT, model uncertainties were predefined to assimilate fast neutrons. We observed that with enough spread of uncertainties, the updated states could mimic the CRNP observation. In all setups, assimilating CRNP measurements could enhance total soil water analyses, which consequently led to the improved detection of soil ice content and therefore the freezing thawing-process at the field scale.