Accumulation over the Greenland Ice Sheet as Represented in Reanalysis Data

Annual precipitation,evaporation,and calculated accumulation from reanalysis model outputs have been investigated for the Greenland Ice Sheet （GrIS）,based on the common period of 1989-2001.The ERA-40 and ERA-interim reanalysis data showed better agreement with observations than do NCEP-1 and NCEP-2 reanalyses.Further,ERA-interim showed the closest spatial distribution of accumulation to the observation.Concerning temporal variations,ERA-interim showed the best correlation with precipitation observations at five synoptic stations,and the best correlation with in situ measurements of accumulation at nine ice core sites.The mean annual precipitation averaged over the whole GrIS from ERA-interim （363 mm yr 1） and mean annual accumulation （319 mm yr 1） are very close to the observations.The validation of accumulation calculated from reanalysis data against ice-core measurements suggests that further improvements to reanalysis models are needed.

Accumulation and geochemical evidence for the Little Ice Age episode in eastern Antarctica

Data on accumulation and concentration of chemical compounds recorded in an essentially unexplored area(Dome Argus)of the Indian Ocean sector of eastern Antarctica during the past 2,680 years(680 B.C. to 1999 A.D.) are presented. During the first 1, 700 years(680 B. C. to 1000 A. D.), the accumulation data shows a slightly decreasing trend, while chemical ions appear to be stable, representing a stable climatic condition. An intensive increasing trend of the accumulation occurred during the 12^(th) to 14^(th) century. The period from 15^(th) to 19^(th) century was characterized by a rapid reducing accumulation and concentrations of volatile compounds suffering post-depositional loss linked to sparse precipitation amount,which was temporally consistent with the Little Ice Age(LIA) episode. Comparison between observed accumulation rates with other eastern Antarctic ice cores show a consistent decreasing trend during LIA, while sea salt and dust-originated ions increased due to sea ice extent and intensified atmospheric transportation. Distribution of volcanic originated sulfate over the Antarctic continent show a significant change during the 15^(th) century, coincident with the onset of the LIA. These results are important for the assessment of Antarctic continent mass balance and associated interpretation of the Dome A deep ice core records.

Evaluation of the Antarctic Mesoscale Prediction System Based on Snow Accumulation Observations over the Ross Ice Shelf

Recent snow height measurements(2008–15) from nine automatic weather stations(AWSs) on the Ross Ice Shelf are used to examine the synoptic and seasonal variability in snow accumulation,and also to evaluate the performance of the Antarctic Mesoscale Prediction System(AMPS) for precipitation. The number of snow accumulation events varies from one station to another between 2008 and 2015,thus demonstrating geographic dependence. The interannual variability in snow accumulation is too high to determine its seasonality based on the current AWS observations with limited time coverage.Comparison between the AMPS and AWS snow height measurements show that approximately 28% of the AWS events are reproduced by AMPS. Furthermore,there are significant correlations between AMPS and AWS coincident event sizes at five stations(p < 0.05). This finding suggests that AMPS has a certain ability to represent actual precipitation events.

外部环境温度与内部气压对HVDC GIL表面电荷积聚的影响

盆式绝缘子的表面电荷积聚是制约高压直流GIL发展的重要因素之一,而GIL内部热传递过程会加剧表面电荷的积聚进程。本文建立了电荷积聚的电-热场耦合数学模型,模拟了不同外部环境温度和内部气体压力下高压直流GIL内部温度随时间的变化情况,进一步研究了外部环境温度恒定及时变情况下的表面电荷积聚特征,并分析了其对表面电荷积聚的影响。结果表明:在外部环境温度为恒定值的情况下,外部温度每升高10℃,绝缘子表面的稳态温度会升高9.2%以上,绝缘子的稳态表面电荷密度也会因此增加17.3%以上。当考虑到外部环境温度随时间变化时,绝缘子表面的温度在持续上升一段时间后,最终会在特定温度范围内波动,此时绝缘子表面电荷密度与外部环境平均温度下的表面电荷密度值近似相等。此外,绝缘子的表面温度和电荷密度会随着气压的升高而减小。研究结果有望为直流GIL的设计和运行提供参考,从而提高直流GIL运行的安全性和稳定性。

Analysis method and experimental study of ice accumulation detection signal based on Lamb waves

A quantitative identification method for in-flight icing has the capability to significantly enhance the safety of aircraft operations.Ultrasonic guided waves have the unique advantage of detecting icing in a relatively large area,but quantitative identification of ice layers is a challenge.In this paper,a quantitative identification method of ice accumulation based on ultrasonic guided waves is proposed.Firstly,a simulation model for the wave dynamics of piezoelectric coupling in three dimensions is established to analyze the propagation characteristics of Lamb waves in a structure consisting of an aluminum plate and an ice layer.The wavelet transform method is utilized to extract the Time of Flight(ToF)or Time of Delay(ToD)of S_(0)/B_(1) mode waves,which serves as a characteristic parameter to precisely determine and assess the level of ice accumulation.Then,an experimental system is developed to evaluate the feasibility of Lamb waves-based icing real-time detection in the presence of spray conditions.Finally,a combination of the Hampel median filter and the moving average filter is developed to analyze ToF/ToD signals.Numerical simulation results reveal a positive correlation between geometric dimensions(length,width,thickness)of the ice layer and ToF/ToD of B1 mode waves,indicating their potential as indicators for quantifying ice accumulation.Experimental results of real-time icing detection indicate that ToF/ToD will reach greater peak values with the growth of the arbitrary-shaped ice layer until saturation to effectively predict the simulation results.This study lays a foundation for the practical application of quantitative icing detection via ultrasonic guided waves.

250 years of accumulation, oxygen isotope and chemical records in a firn core from Princess Elizabeth Land, East Antarctica

A 51.85-m tim core collected from site DT001 （accumulation rate 127 kgm^-2a^-1, mean annual temperature -33.1 ℃） on Princess Elizabeth Land, East Antarctica, during the 1996-97 Chinese First Antarctic Inland Expedition has been analyzed for chemical composition and oxygen isotope ratio. A comparison between the seasonal variations of major ions was carried out in order to reduce the dating uncertainty, using the volcanic markers as time constrains. A deposition period of 251 years was determined. The calculated accumulation rates display an increasing trend before 1820, while after 1820, the trend of the accumulation is not obvious. Overall, temperature change in the region shows a slight increasing trend over the past 250 years. But, notably, a temperature decline of -2 ℃ is observed from 1860 to the present. This feature, at odds with the warming trend over the past century recorded in both hemispheres, likely reflects a regional characteristic related to the lack of a high latitude/low latitude link in the Southern Hemisphere circulation patterns. The results of the glaciochemical records of the firn core show that the mean concentrations of Cl^-, Na^＋ and Mg^2＋ are similar to those reported from other sites in East Antarctica. However, the mean concentration of Ca^2＋ is much higher than that reported from other regions, suggesting the influence of the strong local terrestrial sources in Princess Elizabeth Land. There is no evidence of a positive correlation between NO3^- concentrations and solar activity （11-year solar cycle and solar cycle length）, although solar proton events may account for some of the NO3^- peak values in the record.

Measurement of surface charges on the dielectric film based on field mills under the HVDC corona wire

The ion flow field on the ground is one of the significant parameters used to evaluate the electromagnetic environment of high voltage direct current（HVDC） power lines.HVDC lines may cross the greenhouses due to the restricted transmission corridors.Under the condition of ion flow field,the dielectric films on the greenhouses will be charged,and the electric fields in the greenhouses may exceed the limit value.Field mills are widely used to measure the groundlevel direct current electric fields under the HVDC power lines.In this paper,the charge inversion method is applied to calculate the surface charges on the dielectric film according to the measured ground-level electric fields.The advantages of hiding the field mill probes in the ground are studied.The charge inversion algorithm is optimized in order to decrease the impact of measurement errors.Based on the experimental results,the surface charge distribution on a piece of quadrate dielectric film under a HVDC corona wire is studied.The enhanced effect of dielectric film on ground-level electric field is obviously weakened with the increase of film height.Compared with the total electric field strengths,the normal components of film-free electric fields at the corresponding film-placed positions have a higher effect on surface charge accumulation.

Recent change of the ice core accumulation rates on the Qinghai-Tibetan Plateau

Three ice cores recovered from the Himalayas (i.e. the East Rongbuk Glacier and the Far East Rongbuk Glacier at Mt. Qomolangma (Everest), and the Dasuopu Glacier at Xixiabangma) show a sharp decline in the accumulation rates since the 1950s, which is consistent with the precipitation fluctuation over India and the low northern latitude zone (5°-35癗). Correspondingly, an increasing trend is observed for the ice core accumulations from the central and northern Qinghai-Tibetan Plateau (i.e. the Xiao Dongkemadi Glacier in the central Tanggula Mountains, the Guliya Ice Cap in the western Kunlun Mountains, and the Dunde Ice Cap in the Qilian Mountains) since the 1950s, which is consistent with the precipitation fluctuation over the middle-high northern latitude zone (35°-70°N). However, the variation magnitude of the high-elevation ice core accumulations is more significant than that of precipitation at the low-eleva- tion places, suggesting its extra sensitivity of high-elevation areas to climatic change.

Accumulation in Dasuopu ice core in Qinghai-Tibet Plateau and solar activity

The time series of accumulation in recent 300 years correlated well with solar activity in Dasuopu ice core. Results of spectrum analysis on the accumulation time series of the Dasuopu ice core shows that there are some periods that coincide with the periods of solar activity. By comparing the long-time change trend of the accumulation in the Dasuopu ice core with various kinds of indexes of solar activity intensity, a negative correlation is found between the trend and solar activity.

PROGRESS IN STUDIES ON ICE ACCUMULATION IN RIVER BENDS

River ice is an important hydraulic element in temperate and polar environments and would affect hydrodynamic conditions of rivers through changes both in the boundary conditions and the thermal regime. The river bend has been reported as the common location for the initiation of ice jams because the water flow along a river bend is markedly affected by the channel curvature. In this article, the experimental studies about the ice accumulation in a river bend are reviewed. Based on experiments conducted so far, the criteria for the formation of ice jams in the river bend, the mechanisms of the ice accumulation in the river bend and the thickness profile of the ice accumulation in the river bend are discussed. The two-equation turbulence model is used to simulate the ice accumulation under an ice cover along a river bend. A formula is proposed for describing the deformation of the ice jam bottom. Our results indicate that all simulated thickness of the ice accumulation agrees reasonably well with the measured thickness of the ice accumulation in the laboratory.

Use of Tephrochronology in the Evaluation of Accumulation Rates on Nelson Ice Cap,South Shetland Isands,Antarctica

A volcanic ash layer was observed in the 3 ice cores on the Nelson Ice Cap,Antarctica.A comparison of major elemental composition of glass shards from the 3 tephra layers with average whole-rock compositions of 1967-1970 eruptions on the Deception Island and of glass shards from other suspected De- ception Island eruptions collected from Antarctic ice and firn indicate that the most reasonable source for the tephra in the Nelson Ice Cap cores is the 1970 eruption on the Deception Island.From the depth of the vol- canic ash layer and measured density profile of the cores,the net accumulation rate at the summit,Core GW,is 1200 g·cm^(-2)·a^(-1) during the past 20 years,and the net accumulation rates are 700 g·cm^(-2)·a^(-1) and 6 g·cm^(-2)·a^(-1) at sites N30 and N50,respectively.In the eastern part of the Nelson Ice Cap,the accu- mulation rate may be higher than that in the northern part,because no volcanic ash in ice cores was observed at similar depths collected from this region.

高压直流GIL中盆式绝缘子表面电荷积聚与消散的实验研究

研究直流电压下绝缘子表面电荷积聚及其抑制措施,是开发直流气体绝缘管道输电线路(GIL)的一项关键技术。因此建立了一套盆式绝缘子表面电荷测量系统,采用静电探头法,在空气中对施加了直流电压后的环氧树脂盆式绝缘子进行了表面电位的测量,研究了不同极性、不同幅值电压以及极性反转情况下表面电荷的积聚现象,并对表面电荷的消散进行了测量。实验结果表明:绝缘子表面电荷分布与所施电压极性密切相关;在0.5 MPa空气中,随着施加电压幅值(+40^+70 kV)增加,绝缘子表面电荷急剧增加(负电位最大处从-200 V增加到-3 000 V);在0.5 MPa空气中,先后施加+70 kV及-40 kV电压,绝缘子局部表面电荷激增现象明显(正电位最大处由500 V增大到超过2 500 V);在0.1 MPa空气中施加+40 kV电压,在0~300 min内,绝缘子表面电荷消散近似指数衰减过程,时间常数约为104 s数量级。

高压直流绝缘子积污特性数值模拟与预测

为探讨高压直流线路复合绝缘子在不同环境下的积污特性,建立了基于流体力学原理的高压直流绝缘子积污仿真模型。通过自然积污试验验证了仿真模型的可行性,根据仿真数据建立了基于神经网络算法的粒子撞击率预测模型,使模型更具有实用性。结果表明:雾霾环境下,风速低时电场力作用明显,风速大于1 m/s时电场力影响较小;大风环境下,粒径小于80μm时风速对上表面积污影响不大,粒径大于80μm时上表面粒子撞击率随风速的增大而减小,下表面粒子撞击率几乎为0,粒径对整体粒子撞击率的影响比风速大;风向不水平时,气流向上使下表面粒子撞击率增加,气流向下使上表面粒子撞击率增加。研究成果可用于高压直流绝缘子积污机理研究,并为外绝缘设计及线路运行维护提供依据。

直流电压下聚合物表面电荷测量方法及积聚特性

固体绝缘介质表面电荷积聚现象是研发高压直流气体绝缘装置的重要考虑因素,这些积聚的表面电荷会导致绝缘介质局部电场畸变,大大降低装置的绝缘水平。因此,研究直流电压下聚合物表面电荷积聚现象具有重要意义,如何实现对绝缘子表面电荷分布进行准确的测量,成为该领域一个重要的课题。针对这一问题,采用静电探头法测量缩比气体绝缘输电线路(gas-insulated line,GIL)中圆锥绝缘子的表面电位,研究针对这种"平移改变"系统的表面电荷的反演计算方法。研究采用数值模拟的方法获得了从电荷到电位的传递函数矩阵,应用了基于维纳滤波的数字图像处理技术,对传递函数矩阵的病态特性进行了改善,大大降低了系统噪声,提高了反演计算的稳定性。研究分析了测量结果的空间分辨率和计算精度,发现该系统的空间分辨率可以达到1.8mm。采用该算法,分别研究了空气和SF6中直流电压下绝缘子表面电荷的积聚情况,发现绝缘子表面电荷呈均匀分布的"基本模式"和随机分布的"电荷斑"两种模式。"基本模式"的极性与所加电压极性相同,表明固体侧体电流是电荷积聚的主要来源。

输电线路覆冰阶段性防御策略(英文)

根据覆冰形成和发展的物理过程,提出了冰灾发展过程的 3 个阶段理论:迎风面覆冰阶段、冰壳覆冰阶段和冰灾阶段,分析了各阶段的特点,并提出了冰冻前调度潮流抗冰和冰冻后利用直流电源融冰的构想。设计了冰灾综合防御框架,并介绍了构成框架的预警信息系统的功能,提出了融冰装置设计的要求和融冰过程中要遵循的 3 个约束条件。

直流电压下C_(4)F_(7)N/CO_(2)混合气体中环氧复合绝缘子表面电荷积聚特性

SF中的表面电荷积聚现象已有较多报道,而CFN作为目前最受关注的环保型绝缘气体,其混合气体中的表面电荷积聚却报道较少。为研究直流电压下C_(4)F_(7)N/CO_(2)混合气体中绝缘子表面电荷积聚现象,选择缩比型绝缘子作为实验样品,采用同轴圆柱电极结构,施加直流高压,测量0.1MPa、0.2MPa和0.3MPa以及5%、10%、15%和20%摩尔含量下C_(4)F_(7)N/CO_(2)混合气体中的绝缘子表面电荷分布。实验结果表明,C_(4)F_(7)N/CO_(2)混合气体中表面电荷形态呈现3种分布:高压电极附近不规则形状的同极性电荷注入区、高压电极附近的异极性电荷斑和绝缘子表面云状分布的双极性杂散电荷。随着气压升高,异极性电荷斑增大,注入区范围略有减少,云状电荷消失。随着CFN含量升高,注入区电荷密度、异极性电荷斑和云状电荷范围均提高。与SF相比,C_(4)F_(7)N/CO_(2)混合气体对电场更敏感,增强了气体电离和气体传导,使得气体侧产生的异极性电荷在法向电场作用下大量沉积在绝缘子表面上。在C_(4)F_(7)N/CO_(2)混合气体中,绝缘子表面积聚了更多电荷,对于其实际应用,应采取一定的表面电荷抑制措施。

高压直流输电线路复合绝缘子的积污特性研究

为研究高压直流输电线路复合绝缘子的积污规律,建立了±660k V输电线路复合绝缘子流体力学模型,研究了绝缘子表面污秽撞击率及其积污特性,并进行为期一年的自然积污试验,对比试验数据与仿真结果。结果表明:绝缘子串积污规律呈现两端高中间低的趋势,绝缘子两端积污较为严重;绝缘子串两端下表面的积污比上表面严重,而绝缘子串中间部分上表面的积污比下表面严重;试验结果与仿真结果一致。

基于差分积聚量的柔性直流线路单端高速保护

柔性直流输电系统的快速推广迫切需求提出高性能的柔直线路保护原理。该文推导了柔性直流线路在不同性质故障下电气量的暂态特征,根据直流线路边界特征限制电气量突变陡度的特点,提出利用采样值差分积聚量提取线路故障波动特征;从防止正向超越、提升高阻故障识别和故障选极能力及防止雷击干扰造成保护误动等方面入手,构造高灵敏性、高可靠性的柔性直流线路电压电流复合量单端高速保护原理。大量仿真分析表明,该文方法运算量小,可灵敏、快速识别区内故障,在各种区外故障及扰动下可靠不误动,动作性能优异。

The Rapidly Changing Arctic and Its Impact on Global Climate

Arctic sea ice has significant seasonal variability. Prior to the 2000 s, it retreated about 15% in summer and fully recovered in winter. However, by the year 2007, Arctic sea ice extent experienced a catastrophic decline to about 4.28×10^6 km^2, which was 50% lower than conditions in the 1950 s to the 1970 s(Serreze et al., 2008). That was a record low over the course of the modern satellite record, since 1979(note that the year 2012 became the new record low). This astonishing event drew wide-ranging attention in 2007-2009 during the 4 th International Polar Year. The dramatic decline of sea ice attracts many scientists’ interest and has become the focus of intense research since then. Currently, sea ice retreat is not only appearing around the marginal ice zone, but also in the pack ice inside the central Arctic(Zhao et al., 2018). In fact, premonitory signs had already been seen through other evidence. Before the disintegration of the Soviet Union, US naval submarines had been conducting an extensive survey under the sea ice and taking measurements of sea ice thickness. Their measurements revealed a gradual decrease of ice thickness to 1.8 m during winter by the end of the 20 th century, in contrast to the climatological mean of 3.1 m(Rothrock et al., 1999). However, this alarming result did not draw much attention since the Arctic was still severely cold at that time.

基于突变电流积聚量的高压直流输电线路伪同步差动保护新原理

为解决现有高压直流线路电流差动保护动作延时长(秒级)的问题,提出基于突变电流积聚量的伪同步差动保护新原理。分析两端电流在区内外故障时的突变特征,提出突变电流积聚量以有效捕捉电流突变方向,利用两端突变电流积聚量分别构造动作电流和制动电流,提出全新原理的直流差动保护,并通过伪同步算法进一步缩短保护动作时间。此外,比较两端电流数据在伪同步和同步算法下的时序关系,进一步分析伪同步算法的优越性。仿真验证结果表明,保护新原理具有高快速性和灵敏性,考虑20ms线路通道延时,区内金属性故障时保护的动作时间不大于20ms,经高阻故障(高达800Ω)时保护动作时间不大于27.6ms。保护新原理不需对分布电容电流进行补偿,两端数据不需完全同步,算法计算量小且对数据采样率没有高要求,工程实用性强。