冻融交替季节黑河上游代表性地物类型的微波辐射时序特征

被动微波遥感是监测全球土壤冻融的有利手段,然而由于缺乏实验观测数据,对冻土的微波辐射特征和机理并不是很清楚.2008年3月利用地基微波辐射计(RPG-8CH-DP Radiometer)在黑河流域上游对典型地物18.7 GHz和36.5 GHz的微波辐射进行了连续观测,获取了3种典型土地类型的冻融过程中两个频率双极化的微波辐射特征,计算了土壤冻融过程中发射率和亮温梯度的时序特征并进行了分析.结果表明:土壤冻结会引起土壤发射率的变化,变化的大小主要受土壤表层很薄一层土壤的含水量影响,这是因为这两个波段对土壤的微波穿透深度很小,另外,尽管土壤冻结后亮温梯度会降低,但大多数情况下并没有观测剑冻土负的亮温梯度.     

复杂地表条件下冻融土的微波辐射特性模拟及判别分析

针对AMSR-E数据的新特点,在HUT积雪辐射模型的基础上,增加冻土介电模型计算冻融土的介电常数,并使用ATEM模型描述地表的散射特性,建立寒区地表微波辐射模型,对寒区6种典型环境的微波辐射进行了模拟.结果表明:使用36.5 V的亮温以及6.925 H、10.65 H、18.7 H与36.5 V的亮温比分别衡量地表温度和发射率的变化,最能体现地表土壤的冻融变化特征.通过Fishcr判别分析,建立了复杂地表条件下的冻融土判别算法,并使用微波辐射计观测数据进行修正.AMSRE星载数据的验证结果表明,该判别算法能够有效区分较长时间序列和人范围的地表冻融状态,是种可靠的判别模式.     

高寒冻土地区草甸草地生态系统的能量-水分平衡分析

为了揭示青藏高原高寒地区土壤冻融过程对地表植被大气三者之间能量水分循环的影响, 在青藏高原风火山左冒孔流域(长江源)开展了不同植被盖度条件下冻土活动层水热状态的野外观测(在30%、 60%、 90%的草甸盖度下观测分层土壤水分及温度)和相关试验. 选取考虑了积雪、植被覆盖及枯枝落叶层对土壤冻融影响的水热盐分耦合模型SHAW为动力学约束模型, 进行参数率定及其模拟计算. 结果表明: 青藏高原地气间的能量交换主要受冻土、植被生长和地表土壤含水量的影响, 并且呈明显的季节性变化;未退化高寒草甸草地对青藏高原冻土具有明显的隔热保温作用, 可以降低冻土对气候变化的响应. 在土壤活动层冻结过程期间, 土壤水分具有向表层和深层两向分流汇聚的特征, 植被覆盖变化对水分运移通量有明显影响.     

高温冻土物理力学特性研究现状

高温冻土又称近相变区冻土, 通常用来描述相对较高温度的冻土. 由于其温度区间处于冻土的剧烈相变区, 冻土中冰和未冻水的比例对温度的变化极其敏感, 因此, 高温冻土的物理力学性质具有强烈的不稳定性, 极易在温度变化的影响下发生实质性改变. 基于有关高温冻土物理力学性质研究的文献, 综述了高温冻土的定义及其温度界限, 同时论述了未冻水对高温冻土物理力学性质的巨大影响, 并提出了一种在冻结状态下高温冻土中未冻水孔隙水压力的测试方法. 重点从强度特征、变形特性以及本构模型三方面对高温冻土力学特性的研究现状进行了总结和分析, 并提出要进一步探究高温冻土变形机理及本构模型, 可以为高温不稳定多年冻土区各类工程的基础变形及稳定性预报、评价提供理论基础.     

人类活动对青藏高原冻土环境的影响

采用物探技术、植物样方调查对青藏公路沿线工程活动对冻土环境因素之间的影响进行了分析研究, 结果表明: 由于工程活动对原冻土区地貌、植被及表土层结构等的干扰破坏, 致使工程区及其影响区冻土上限下降速度加快, 同时得出了冻结层上水埋深(1～3 m)与冻土层厚度变化的关系以及冻结层上水水位随冻土上限下降而变化的规律. 指出工程活动对植被的影响不仅与工程本身有关, 也与工程区冻土厚度、地下水、土壤等因素有密切关系.     

估算冻结(融化)深度方法的比较及在中国地区的修正和应用

利用中国地区的冻土观测资料计算比较了常用冻土活动层深度估算的几种方法,并对影响估算的主要因子进行了讨论.结果表明:考虑了土壤水分变化、土壤水分相变潜热变化、积雪和植被效应的指数与实际观测值之间有较好的拟合;在采用分区插值方法对温度进行处理,并应用数字化高程、数字化植被和土壤数据的基础上,采用和观测值相关较高的Kudryavtsev指数计算、以ArcGIS为工具获取的中国地区冻土分布更为合理.     

青藏高原西大滩多年冻土活动层土壤性状与地表植被的关系

以青藏高原多年冻土区北界的西大滩为研究基点, 选取多年冻土不同退化阶段的两个样地, 对植被分布特征、 冻土活动层和土壤特性等进行调查监测, 同时分析不同活动层状态下土壤水热、 养分和地表植被特征变化及相互间的作用关系. 结果表明: 西大滩地区的植被以浅根系植物为主, 0~10 cm的表层土壤中地下生物量约占地下生物总量的63%和62.2%之多. 在气候条件基本一致的情况下, 多年冻土的存在情况及活动层土壤水热状况对植被生态系统的演变起决定性作用. 高地表植被覆盖下的冻土土壤水热调节能力强, 有助于延缓冻土退化过程. 西大滩土壤全氮、 碱解氮、 速效钾与有机质含量密切相关, 含量随冻土退化明显减少, 且随土层深度的变化曲线表现为相同的趋势. 土壤表层养分和速效养分受冻土退化程度的影响较大.     

黄河源区冻土对植被的影响

黄河源区由于近年来气候变化的影响,打破了高寒植被与冻土环境之间稳定的适应性关系,由此引发了一系列生态环境退化的现象.在黄河源区多年野外工作的基础上,定量分析了冻土与植被之间的关系.研究表明:多年冻土埋深通过影响浅层土壤含水量影响植被生长的,多年冻土的埋深与浅层土壤含水率和植被的覆盖率具有良好的相关性规律.冻土埋深<2 m时,冻土埋深决定浅层土壤含水率,成为影响植被的生长主要因素;埋深>2 m时,冻结层上水水位低、补给量少,冻结层上水水量小,毛细上升高度不能达到植被根系分布的浅层土壤中,植被生长环境干旱化,多数植被生长受限制,这时只有少量根系发达的耐旱植被存活,覆盖率小,一般不超过35%.因此,2 m的多年冻土埋深为“生态冻土埋深”.近20 a来,黄河源区地温长期处于增温状态,多年冻土出现表层融化,形成深埋的或少冰的冻土等现象;部分地带完全融化消失,连续多年冻土变成不连续冻土或岛状冻土.多年冻土退化后,土壤含水量减少,导致植被物种更替、“黑土滩”等退化现象.     

青藏高原冻土及水热过程与寒区生态环境的关系

从青藏高原冻土及水热过程出发,利用青藏高原活动层监测数据,讨论了冻土水热过程与植被生长环境的关系,比较了季节冻土区与多年冻土区水热过程的差异及与植被的关系,同时讨论了不同草地生态冻融过程的变化.结果表明,冻土及水热过程与寒区生态环境有着密切的关系,冻土及水热过程不仅控制着地表状态的变化,影响着植被的发育程度,同时二者之间也存在着强烈的相互作用的关系.一旦地表条件被破坏,干扰了冻土水热过程与地表植被生长间的平衡关系,将引起生态环境的退化,出现荒漠化,甚至沙漠化.     

基于物理模型的被动微波遥感反演土壤水分

利用土壤水分和海洋盐度(SMOS)卫星进行土壤水分反演的算法中,对地表发射率的描述仍采用半经验Q/H模型,该模型描述地表粗糙度对有效发射率在V和H极化下影响相同.基于微波散射理论模型-高级积分方程模型(AIEM)建立了一个针对SMOS传感器的参数配置,包含各种地表粗糙度和介电特性的裸露地表辐射模拟数据库,发展了L波段多角度地表辐射参数化模型.在此基础上,利用SMOS多角度双极化特点,建立了土壤水分反演算法.该算法可以消除粗糙度对土壤水分反演的影响,同时最小化反演过程中辅助信息引入带来影响.反演算法通过美国农业部提供的L波段多角度地基辐射计数据(BARC)进行验证,在20°~50°入射角,土壤水分反演精度在4%左右.     

地表冻融过程被动微波遥感机理研究进展

地表冻融过程及参数特征是陆表过程、气候模式、全球变化等研究的重要方面。被动微波遥感由于具有对土壤水分敏感、高时间分辨率等特点,非常适合用于地表冻融过程的监测和相关参数的反演。在分析地表冻融过程被动微波遥感机理研究发展现状的基础上,面向全球范围内大尺度陆表冻融过程微波遥感算法发展的需求,针对由土壤、积雪和植被组合的复杂地...     

Research advances in passive microwave remote sensing of surface freeze-thaw state北大核心CSCD

Soil freeze-thaw cycles have important effects on surface water and energy balance,and then affect vegetation growth,soil water content,carbon cycle and terrestrial ecosystem. Passive microwave plays an important role in monitoring global and regional surface freeze-thaw processes due to its high temporal resolution,abundant data and sensitivity to soil moisture. With the launch of passive microwave sensors at home and abroad,it provides conditions for the study of permafrost interannual variation,seasonal variation,diurnal variation and long time series of near-surface soil freeze-thaw cycle. In recent years,the study of surface freeze-thaw cycle using passive microwave data has gradually increased. Based on previous studies,this paper summarizes the types of passive microwave remote sensing data and the characteristics of the bands contained in them. Expounded the principle of passive microwave monitoring data used for freezing and thawing,focus on passive microwave data in five categories in the study of freezing and thawing monitoring algorithms,including double index algorithm,the decision tree algorithm,freeze-thaw discriminant algorithm,seasonal threshold algorithm and based on the freezing L-band relative factors discriminant algorithm threshold,and analysis of 5 kinds of algorithms are compared;The freeze-thaw products based on different algorithms and passive microwave data were combed. Finally,the problems and future research directions of passive microwave remote sensing in surface freeze-thaw applications are summarized. In the acquisition of passive microwave data,it is found that the passive microwave data is missing due to the physical characteristics of the sensor,the shape and orbit of the earth,and the low resolution of passive microwave data leads to the low precision of freeze-thaw discrimination. For the problem of missing passive microwave data,it is proposed to use the average value of passive microwave data before and after two days to fill the missing brightness temperature data,or establish statistical function to complement the missing data. For the problem of low passive microwave resolution,the current development trend is to scale down based on passive microwave data and combine with multiple data products,such as ground temperature and active microwave data,or perform probability discrimination on surface freezing-thawing state in pixels,so as to better describe surface freeze-thaw state. In terms of the algorithm for discriminating surface freezing-thawing,based on the problem that dual-index algorithm,decision tree algorithm,freezing-thawing discriminant algorithm and seasonal threshold algorithm cannot accurately distinguish snow and frozen soil,this paper proposes to adopt the method of data assimilation or start from the snow radiation and frozen soil dielectric model. Optimization of the algorithm for the snow covered surface can further improve the accuracy of freeze-thaw classification. Based on existing freeze-thaw products,Although SMAP freeze-thaw products continue to be updated,SAMP satellite was launched late,and SAMP freeze-thaw products have a short time series. In the future,the time span of this algorithm for freezing-thawing products can be extended by combining L-band data provided by SMOS satellite. The problems mentioned above and the direction of further research are of great significance for improving the accuracy of freezing and thawing discrimination and improving the understanding of the variation law of freezing and thawing cycles,and also have certain research space. © 2022 Science Press (China).     

被动微波遥感近地表土壤冻融状态数据产品对比及分析

近地表土壤冻融循环过程是陆面过程的“开关”,对地表能量过程、水文过程、植被动态、温室气体交换和生态系统功能等都具有重要影响。除了依靠传统观测手段判别地表冻融,被动微波卫星遥感技术已被成熟运用于全球尺度地表冻融研究中。利用被动微波卫星遥感亮温监测地表冻融大致经历了理论基础和算法的研制、算法参数的验证和应用以及建立数据集三个主要过程,并基于不同的算法针对不同目的分别发布了不同地区、不同时间序列的近地表土壤冻融数据集。以中国气象局435个气象台站观测的0 cm土壤温度为标准,对比分析了基于目前较为通用的双指标算法、单指标算法、决策树算法和判别式算法建立的近地表土壤冻融数据集的分类结果。结果表明：单指标算法数据集冻结指标的平均值与观测资料最接近,其偏差和均方根误差最小,而决策树算法由于全局采用单一的37 GHz垂直极化亮温阈值,低估了地表冻结,导致偏差和均方根误差最大。近地表土壤冻融数据集和观测资料均表明随着全球变暖,近地表土壤呈冻结首日推迟、冻结终日提前和冻结天数减少的趋势。通过对不同冻融数据集的比较研究,为未来被动微波卫星遥感算法改进、近地表土壤冻融数据的合理使用提供了可靠的科学依据。     

冻结土壤介电常数混合模型机理研究

介电常数是微波辐射传输理论所需的最重要、最基本的参数。通过研究平均温度低于-4℃,组分中含有冰的土壤——冻结土壤,建立了一种适用于微波波段冻结土壤的介电常数混合模型。该模型考虑了土壤温度、微波频率、土壤质地以及其中未冻水含量的影响。模型使用一点法计算未冻水含量,引入Stern双电层理论计算冻结土壤未冻水的介电常数,引入Maxwell-Garnett介电常数混合理论、介电常数容积混合理论构建冻结土壤介电常数混合模型。模型计算结果与实测资料进行了详细的对比分析,冻结土壤介电常数混合模型给出的介电常数实部、虚部的曲率与实验结果很好吻合,模型结果比较满意,对比分析证实了模型的可靠性。     

海北高寒草甸的季节冻土及在植被生产力形成过程中的作用

海北高寒矮嵩草草甸区植被下的草毡寒冻雏形土属季节性冻土,因温度低,冻土在年内的每个月均可发生.一般在11月中旬可形成稳定的季节冻结层,至翌年3～4月冻土层厚度最大可达230cm.从3月下旬到4月中旬开始,土壤开始消融,至6月下旬到7月上旬冻土全部消失.分析发现,季节冻土在高寒草甸植被生产力形成过程中有着积极的影响作用,主要表现在:1)季节冻土的存在和维持将给高寒植物生长提供良好的土壤水分,对植物初期营养生长发育有利,可弥补春夏之交时降水不足所引起的干旱胁迫影响;2)季节冻土的长时间维持,有利于植物残体和土壤有机质留存于土壤,并随土壤冻结和融化过程发生迁移,可提高土壤肥力;3)较高的土壤水分有利于土壤胡敏酸的形成,可保证植物生长所需的其它有机元素的供给;4)冻土层所形成较高的土壤水分使土体热容量加大,从而调节因气候异常波动引起的土壤温度变化;5)季节冻土的变化对植物地上年生产量形成有一定的影响作用,表现出从10月或11月开始,土壤冻结速率快,对提高植物地上年生产量有利.这也证实,在未来气候变暖的趋势下,土壤有机质将加快分解速度,土壤水分因受温度升高、冻结期缩短,其贮存能力降低;受温度升高的影响,地表蒸发能力加大,若降水仍保持目前的水平,土壤水分将明显减少,将导致高寒草甸植被生产力有下降的可能.     

Influences of alpine ecosystem degradation on soil temperature in the freezing-thawing process on Qinghai–Tibet Plateau

The alpine ecosystem is very sensitive to environmental change due to global and local disturbances. The alpine ecosystem degradation, characterized by reducing vegetation coverage or biomass, has been occurring in the Qinghai–Tibet Plateau, which alters local energy balance, and water and biochemical cycles. However, detailed characterization of the ecosystem degradation effect is lack in literature. In this study, the impact of alpine ecosystem degradation on soil temperature for seasonal frozen soil and permafrost are examined. The vegetation coverage is used to indicate the degree of ecosystems degradation. Daily soil temperature is monitored at different depths for different vegetation coverage, for both permafrost and seasonal frozen soils. Results show that under the insulating effort of the vegetation, the freezing and thawing process become quicker and steeper, and the start of the freezing and thawing process moves up due to the insulating effort of the vegetation. The influence of vegetation coverage on the freezing process is more evident than the thawing process; with the decrease of vegetation coverage, the integral of frozen depth increases for seasonal frozen soil, but is vice versa for permafrost.     

The variation of soil temperature and water content of seasonal frozen soil with different vegetation coverage in the headwater region of the Yellow River, China

The variation and distribution of temperature and water moisture in the seasonal frozen soil is an important factor in the study of both the soil water cycle and heat balance within the source region of the Yellow River, especially under the different conditions of vegetation coverage. In this study, the impact of various degrees of vegetation coverage on soil water content and temperature was assessed. Soil moisture (θ _v) and soil temperature (T _s) were monitored on a daily basis. Measurements were made under different vegetation coverage (95, 70–80, 40–50 and 10%) and on both thawed and frozen soils. Contour charts of T _s and θ _v as well as a θ _v–T _s coupling model were developed in order to account for the influence of vegetation cover and the interaction between T _s and θ _v. It was observed that soil water content affected both the overall range and trend in the soil temperature. The regression analysis of θ _v versus T _s plots indicated that the soil freezing and thawing processes were significantly affected by vegetation cover changes. Vegetation coverage changes also caused variations in the θ _v–T _s interaction. The effect of soil water content on soil temperature during the freezing period was larger than during the thawing period. Moreover, the soil with higher vegetation coverage retained more water than that with lower coverage. In the process of freezing, the higher vegetation coverage reduced the rate of the reduction in the soil temperature because the thermal capacity of water is higher than that of soil. Areas with higher vegetation coverage also functioned better for the purpose of heat-insulating. This phenomenon may thus play an important role in the environmental protection and effective uses of frozen soil.     

冻土静力学室内试验研究进展

冻土力学是冻土工程学的理论基础,以解决冻土工程问题为归宿。冻土力学分为冻土静力学和冻土动力学两个方面,而冻土静力学是冻土力学的重要部分。为此,对常规冻土静力学室内试验的研究进展进行了总结,系统地阐述了其在冻土的强度特性、变形特性及理论模型等方面所取得的重要研究成果,并指出研究中存在的一些不足之处。最后结合冻土静力学研究的特点及实际冻土工程建设的需要,对冻土静力学研究的未来发展进行了展望。     

估算冻结（融化）深度方法的比较及在中国地区的修正和应用

利用中国地区的冻土观测资料计算比较了常用冻土活动层深度估算的几种方法,并对影响估算的主要因子进行了讨论。结果表明：考虑了土壤水分变化、土壤水分相变潜热变化、积雪和植被效应的指数与实际观测值之间有较好的拟合;在采用分区插值方法对温度进行处理,并应用数字化高程、数字化植被和土壤数据的基础上,采用和观测值相关较高的Kudryavtsev指数计算、以ArcGIS为工具获取的中国地区冻土分布更为合理。