地基微波遥感评估黄河源区草原下垫面土壤冻融过程研究

Study on evaluating soil freeze/thaw process of grassland underlying surface in source region of the Yellow River by using ground-based microwave remote sensing

土壤冻融过程对气候变化非常敏感,如何准确监测土壤冻融过程具有重要的科学意义。利用2017年6月至2018年6月中国科学院若尔盖高原湿地生态系统研究站玛曲观测场地基微波辐射计观测数据、浅层土壤温度和近地面气温数据,通过构建归一化极化比值冻结因子、极化差值冻结因子、组合水平极化差值冻结因子和组合垂直极化差值冻结因子等不同土壤冻结因子,评估了黄河源区草原下垫面土壤冻融过程。结果表明:L波段微波辐射计监测土壤冻融状态的结果与近地面气温和浅层土壤温度表征的土壤冻融过程基本一致。当入射角为50°时,归一化极化比值冻结因子和极化差值冻结因子与实测数据的一致性分别达到83.6%和82.8%。每种冻结因子具有明显的季节性变化,四种冻结因子在春季时的准确度低于夏、秋、冬三个季节。归一化后的相对冻结因子的标准差在秋季最大,可达0.3;在冬季和夏季最小,值小于0.2。在土壤发生冻结和融化转换时,垂直极化和水平极化下的亮温同时下降,其差值较完全冻结或者完全融化时的亮温差大。研究结果可为微波遥感监测土壤冻融过程提供技术参考。

Soil freeze/thaw process(F/T)is sensitive to climate change,accurately monitoring the soil F/T has important scientific significance.The brightness temperature data observed by ELBARA-III,as well as soil temperature and near-surface air temperature at the Maqu site of the of the Zoige Plateau Wetland Ecosystem Research Station,Chinese Academy of Sciences,from June 2017 to June 2018 are deployed in this study.Four soil frost factors(FF)of which are normalized polarization ratio,polarization difference,combined horizontal polarization difference,and combined vertical polarization difference were constructed with the ELBARA-III data.The characteristics of the freeze-thaw process in source region of the Yellow River were evaluated by these four frost factors.The results show:There are agreements between FFs and the F/T estimated from two reference data sets(in situ measurements of soil temperature and air temperature).Frost factor of the normalized polarization ratio and frost factor of polarization difference get the highest accuracy of 83.6%and 82.8%under the incident angle of 50°,respectively.All frost factors have seasonal changes,the four frost factors were higher in spring than in summer,autumn and winter.The standard deviation of the normalized relative frost factor is the largest in autumn,and the maximum can reach 0.3;it is the smallest in winter and summer,and the minimum is less than 0.2.The brightness temperature difference between H-polarization and V-polarization decreases in freeze/thaw transitions,and the difference in the polarization difference during the freeze/thaw transition period is more significant than that during complete frozen or complete thawed period.The results are potential to provide technical supports for monitoring soil F/T by using microwave remote sensing.