Scale properties of sea ice deformation and fracturing

The sea ice cover, which insulates the ocean from the atmosphere, plays a fundamental role in the Earth's climate system. This cover deforms and fractures under the action of winds, ocean currents and thermal stresses. Along with thermodynamics, this deformation and fracturing largely controls the amount of open water within the ice cover and the distribution of ice thickness, two parameters of high climatic importance, especially during fall and winter (no melting). Here we present a scaling analysis of sea ice deformation and fracturing that allows us to characterize the heterogeneity of fracture patterns and of deformation fields, as well as the intermittency of stress records. We discuss the consequences of these scaling properties, particularly for sea ice modelling in global climate models. We show how multifractal scaling laws can be extrapolated to small scales to learn about the nature of the mechanisms that accommodate the deformation. We stress that these scaling properties preclude the use of homogenisation techniques (i.e. the use of mean values) to link different scales, and we discuss how these detailed observations should be used to constrain sea ice dynamics modelling. To cite this article: J. Weiss, D. Marsan, C. R. Physique 5 (2004).     

Some aspects of strength in sea ice mechanics

The paper provides estimates of the relation between the strength of sea ice and its porosity in the framework of a porous solid model proposed earlier. The influence of the ice capillary structure filled with brine on the ice strength is analyzed, showing that the rise of local stresses in the ice changes its limit equilibrium conditions. Comparison is made to elucidate how the sea ice strength responds to loading length- and crosswise the capillary axes. Patterns by which ice structure elements merge resulting mostly in main cracks or in multiple ordered crack systems under compression are discussed. The tendency toward generation of ordered crack systems or propagation of single main cracks in the structured material is determined by the activity of its structural elements involved in fracture. A method is proposed for estimating the average strength of sea ice cover relative to horizontal compression with regard to partial loss of the bearing capacity of ice layers during deformation.     

A dislocation-based model for creep recovery in ice

Creep recovery strain is significant in polycrystalline ice, and its stress dependence is strongly dependent on ice type and deformation history. Although it is generally recognized that creep recovery strain is largely attributable to dislocations, a dislocation-based model that rectifies the observed differences in two important ice types (freshwater and sea ice) has not previously emerged; and the development of such a model forms the goal of the present effort. The model considers basal dislocation distributions, employs a dislocation density–stress relationship from previous work, and uses an empirical expression for the decrease in slip-line spacing with increasing stress. The dislocation processes are taken to operate over a subgrain-sized domain, and the strain associated with the relative motion of neighbouring domains is considered. The model accounts for dislocation multiplication and the decrease in slip-line spacing if the applied stress is sufficiently high. The model explains the observed differences in creep recovery strain for freshwater and sea ice and adequately reproduces both the stress dependence and the limiting value of the experimentally observed behaviour. It also reproduces the shift from nonlinear to linear behaviour that has been observed in prestrain experiments on freshwater and sea ice cores.     

Polarization ratio characteristics of electromagnetic scattering from sea ice in polar areas

In the global climate system, the polar regions are sensitive indicators of climate change, in which sea ice plays an important role. Satellite remote sensing is a significant tool for monitoring sea ice. The use of synthetic aperture radar(SAR) images to distinguish sea ice from sea water is one of the current research hotspots in this topic. To distinguish sea ice from the open sea, the polarization ratio characteristics of sea ice and sea water are studied for L-band and C-band radars, based on an electromagnetic scattering model of sea ice derived from the integral equation method(IEM) and the radiative transfer(RT) model. Numerical experiments are carried out based on the model and the results are given as follows. For L-band, the polarization ratio for sea water depends only on the incident angle, while the polarization ratio for sea ice is related to the incident angle and the ice thickness. For C-band, the sea water polarization ratio is influenced by the incident angle and the root mean square(RMS) height of the sea surface. For C-band, for small to medium incident angles,the polarization ratio for bare sea ice is mainly determined by the incident angle and ice thickness. When the incident angle increases, the RMS height will also affect the polarization ratio for bare sea ice. If snow covers the sea ice, then the polarization ratio for sea ice decreases and is affected by the RMS height of snow surface, snow thickness, volume fraction and the radius of scatterers. The results show that the sea ice and the open sea can be distinguished by using either L-band or C-band radar according to their polarization ratio difference. However, the ability of L-band to make this differentiation is higher than that of C-band.     

Modeling of radiation transport in coupled atmosphere-snow-ice-ocean systems

A radiative transfer model for coupled atmosphere-snow-ice-ocean systems (CASIO-DISORT) is used to develop accurate and efficient tools for computing the bidirectional reflectance distribution function (BRDF) of sea ice for a wide range of situations occurring in nature. These tools include a method to generate sea ice inherent optical properties (IOPs: single-scattering albedo, extinction optical depth, and scattering asymmetry parameter) for any wavelength between 300 and 4000 nm as a function of sea ice physical parameters including real and imaginary parts of the sea ice refractive index, brine pocket concentration and effective brine pocket size, air bubble concentration and effective air bubble size, volume fraction of ice impurities and impurity absorption coefficient, and sea ice thickness. The CASIO-DISORT code was used to compute look-up tables (LUTs) of the Fourier expansion coefficients of the BRDF as a function of angles of illumination and observation, sea ice IOPs, and ocean albedo. By interpolation in the LUTs one efficiently obtains accurate BRDF values. To include snow on the ice we modified DISORT2 to accept Fourier expansion coefficients for the BDRF as input instead of the BRDF itself, thereby reducing the computation time by a factor of about 60. The BRDF computed by CASIO-DISORT or retrieved from the LUTs applies to diffuse light only. To remedy this shortcoming we added a specular Gaussian beam component to the new BRDF tool and verified that it works well for BRDFs for bare and snow-covered sea ice.     

双基地冰-水界面混响强度的理论预报

针对北极冰层冰水界面具有小尺度粗糙界面的情况,给出由冰层造成的混响平均强度的理论预报公式。首先将北极冰层等效为具有粗糙界面的弹性介质,并且采用小粗糙度微扰理论在精细拟合粗糙度谱的基础上,建立粗糙冰层的三维散射强度模型,然后建立三维双基地声呐几何模型确定有效散射区域,最后给出双基地冰下混响平均强度的估算公式,并在所拟合的粗糙度谱的情况下,利用该预报公式计算双基地声呐配置参数和海冰物理及声学特性对冰下混响强度的影响算例。数值仿真表明该公式能够估算出北极冰下双基地声呐产生的混响平均强度,并且分析出了时延、基线长度等声呐配置参数和冰层声速比、密度比等海冰物理参数会对混响强度造成的影响。声呐的配置参数主要影响散射的有效面积,海冰的物理参数则影响着冰层的散射强度。其中,冰层声速比不仅影响混响强度的大小,还会影响混响强度随时间的衰减速度。      

船运铁矿粉污染海冰反射光谱特征研究

国际防止船舶造成污染公约（MARPOL）中关于防止船舶垃圾污染的附则V规定,对含有海洋环境有害的物质（HME）的干燥残留物必须在适当的港口接收设施中排放,但因费用等因素影响,很多船只将废弃物直接抛弃于海洋中。散落于海冰表面的船运铁矿粉会污染海冰品质,并加速海冰融化,对海洋环境造成一定的污染。对铁矿粉污染海冰的光谱反射率特征的研究,能够为利用光学遥感影像进行海冰品质监测提供数据基础。通过现场测量覆盖不同面积比例铁矿粉颗粒的海冰和清洁海冰之间光谱特征差异,为港口附近铁矿粉污染的范围以及沉降量的估算提供参考和依据。该研究是在渤海海域天然海冰条件下进行的,获取并分析了不同污染程度的海冰光谱特征,探讨这些光谱特征与铁矿粉颗粒几何截面积比例的相关性。为了反演海冰表面铁矿粉面积比例,利用光谱矢量角余弦值和光谱吸收指数阈值法进行了海冰、铁矿粉端元提取,根据混合像元线性解混模型理论,建立基于特征光谱波段反射率值的海冰表面铁矿粉面积比例反演模型。所研究海冰表面铁矿粉面积比例分为0 （洁净海冰）,25.8%,37.2%,46.1%,52.1%,65.1%,72.5%,82.3%,92.3%,93.1%和100%（纯铁矿粉）等,数据采集波长范围350～2 500 nm。结果表明,利用1 460 nm波段处的吸收指数（SAI）进行海冰和铁矿粉端元提取效果最佳,反射率与铁矿粉面积在918～1 400,1 500～1 780和2 250～2 300 nm波段,相关系数都大于0.90,86%以上波段范围的反射率与铁矿粉面积的相关系数在0.90以上,91.75%以上的波段范围相关系数在0.80以上。选取1 610～1 630 nm波段的平均反射率估算铁矿粉面积比例,几何截面积较小的误差一般大于几何截面积较大的,在克服海冰背景反射率的影响下,比较精确地估算了海冰-铁矿粉像元中铁矿粉颗粒的几何截面积比例,平均反演精度达到94.23%。     

北极海冰的光谱特征分析

北极海冰对全球气候变化有着举足轻重的作用,它的快速变化及其影响是国内外关注的焦点。采用ASD FieldSpec 3便携式地物光谱仪,在2010年中国第4次北极科学考察的长期冰站观测期间对不同类型的海冰进行了野外光谱测量,系统分析了光谱特征。结果表明积雪覆盖海冰的光谱反射率最高,裸露海冰其次,表层消融的海冰(有融池)最低。厚雪、薄雪、湿雪、雪晶的光谱曲线峰谷特征明显,基本都随波长的增加反射率降低。天然海冰、白冰、蓝冰的反射率随波长的变化基本一致,反射率居于中等,灰冰的反射率远低于天然海冰、白冰和蓝冰。分析北极海冰的波谱特性,探索北极海冰的定量遥感研究,进而分析其对全球变暖的响应,具有重要的科学意义。     

渤海海冰反射光谱基本特征的观测研究

针对渤海海冰监测的需要,在分析前人相关研究的基础上, 从洁净平整冰入手, 通过控制实验,在辽东湾鲅鱼圈港内获取不同厚度的洁净平整冰、有积雪覆盖海冰和含有泥沙海冰的样本,开展海冰现场反射光谱观测实验。根据两年的观测数据, 在可观测到的小于30 cm厚度范围内,洁净平整冰的光谱曲线呈现出“一大一小” 的双峰特征,且主反射峰值与海冰厚度有较好的对应关系;有积雪覆盖海冰的主反射峰光谱值要大大高于相同厚度的没有积雪覆盖的洁净平整冰主反射峰光谱值;含有泥沙的海冰的光谱曲线在675～725 nm波长范围内表现为先略有抬升,然后再继续下降的特征,而且其主反射峰值也大大升高。积雪覆盖和泥沙是产生渤海海冰“同厚异谱”或“同谱异厚”现象的重要原因。     

北极水下声信号跨冰层变化分析

北极海冰阻碍了海水和空气两个空间的信息传输.为获得冰层对水下声信号跨冰层传输的影响,采用三维检波器在北冰洋中心区开展了水下声信号的跨冰层实验.利用水冰界面反射模型和自由冰层Lamb波模型,对水下声信号小角度(小于10°)入射冰层时测量数据进行分析,结果表明:(1)20 Hz～1 kHz声信号入射到光滑冰层时,某些频率声...     

基于星载辐射计AMSR-E不同波段的北极海冰折射系数提取

海冰折射系数是一个重要的地球物理参数。利用Hong Approximation(HA)算法和星载辐射计AMSR-E数据,基于不同微波频段(6.9,10.7,23,37,89 GHz)实现北极地区不同季节海冰折射系数反演。首先对反演所用关键数据海冰密集度反演算法的系点进行优化,反演结果与验证数据平均偏差分别为3.00%和-0.50%,低于其它常用算法。结果表明：冬季一年冰和夏季海冰的折射系数反演结果在所有微波频段的平均值范围为(1.78~1.75)和(1.72~1.70),与前人研究结果基本一致,但冬季多年冰在较高频率的折射系数与前人结果偏差较大,高于10.7 GHz的微波频段不能用于冬季多年冰折射系数的提取;综合利用不同波段海冰折射系数反演结果能够有效识别沿岸冰间湖(比如冬季6.9 GHz),区分多年冰与一年冰(冬季89 GHz),并能确定多夏季海冰边缘融化范围(夏季6.9 GHz)。     

Multipath structure of the typical under-ice sound channel in the Arctic:theory and experiment

Considering the typical acoustic environment of the Arctic, this paper proposes a method based on OASES-Bellhop coupled model to rapidly analyze the multipath structure of the under-ice sound channel. Firstly the proposed model refers to ice plate as stratified elastic media with some roughness. Secondly,it uses the perturbation method and Kirchhoff approximation theory to solve the scattering loss due to the sea ice inhomogeneity. Finally, the model predicts the multipath structure of the under-ice channel through Ray theory. The results of the numerical simulation and experiment indicate that the typical Arctic sound channel presents multipath structures due to the sea ice and seabed in the range of 6 km, respectively.The sea ice reflection paths are stable,with a short multipath spread within 14 ms. The seabed reflection paths have relatively weak strength. The proposed OASES-Bellhop coupled model successfully predicts the amplitude and delay of the multipath structure with small error,which indicates the proposed model is able to analyze and predict the multipath structure of the observed acoustic environment in experiment.     

时间域混合像元分析在海冰密集度变化研究中的应用

传统意义混合像元分析方法是对有不同光谱特征的地物进行分解(spectral mixture analysis,SMA),得到各个组分在该像元内所占的百分数。而将光谱域的混合像元分析延伸至时间域内(temporalmixture analysis,TMA),提取的表征时间特性的端元用于像元分解所得残差小于8.5%,说明TMA提取南极地区海冰密集度时间变化特性具有可行性。将获取的多年平均端元用于2005年和2010年海冰密集度数据,得到残差分别为(1.4±2.42)%和(1.7±2.87)%,高于多年平均残差精度(1±1.53)%,在一定程度上反映某年海冰密集度数据相对多年平均值的变化。因此,TMA为全球变暖背景下的海冰密集度的时空特性研究提供了新思路。     

Regular network model for the sea ice-albedo feedback in the Arctic

The Arctic Ocean and sea ice form a feedback system that plays an important role in the global climate. The complexity of highly parameterized global circulation (climate) models makes it very difficult to assess feedback processes in climate without the concurrent use of simple models where the physics is understood. We introduce a two-dimensional energy-based regular network model to investigate feedback processes in an Arctic ice-ocean layer. The model includes the nonlinear aspect of the ice-water phase transition, a nonlinear diffusive energy transport within a heterogeneous ice-ocean lattice, and spatiotemporal atmospheric and oceanic forcing at the surfaces. First results for a horizontally homogeneous ice-ocean layer show bistability and related hysteresis between perennial ice and perennial open water for varying atmospheric heat influx. Seasonal ice cover exists as a transient phenomenon. We also find that ocean heat fluxes are more efficient than atmospheric heat fluxes to melt Arctic sea ice.     

典型北极冰下声信道多途结构分析及实验研究

针对北极海域典型声场环境,提出了基于OASES-Bellhop耦合模型的冰下声信道多途结构快速分析方法。模型将海冰等效为具有粗糙界面的弹性分层介质,利用微扰法与Kirchhoff近似,估计海冰界面不均匀造成的散射损失,结合射线传播理论对典型北极冰下声信道多途结构进行分析与预报。数值仿真与实验结果表明,在6 km距离处,典型北极冰下声信道由于海冰与海底反射分别形成多途结构,海冰多次反射路径叠加形成的多途结构较为稳定,时延扩展在14 ms范围内,海底反射路径强度相对较弱。OASES-Bellhop模型对冰下声信道多途结构幅度和时延预测误差较小,能够较好的解释及预报实验观测到的多途结构环境特性。      

Scale dependence and localization of the deformation of Arctic sea ice

A scaling analysis of the deformation of Arctic sea ice over a 3-day time period is performed for scales of 10 to 1000 km. The deformation field is derived from satellite radar data; it allows us to study how a very large solid body-the Arctic sea-ice cover-deforms under the action of heterogeneous forcing winds and ocean currents. The deformation is strongly localized at small scales, and can be characterized as multifractal. This behavior is well known for turbulent flows, and is here also observed for a deforming solid. A multiscaling extrapolation to the meter scale (laboratory scale) shows that, at the 3-day time scale, about 15% of the deformation is larger than 10(-4) s(-1), implying brittle failure, over 0.2% of the total area.     

Pressure-induced transformation plasticity of H(2)O ice

Transformation plasticity is a deformation mechanism which occurs during phase transformation of an externally stressed material. Pressure-induced transformation plasticity of ice could be relevant to the geology of the moons of the outer solar system, and has long been postulated to occur in olivine in the earth's interior. In the present work, ice specimens were subjected to isostatic pressure cycling between 0 and 300 MPa to induce reversible polymorphic transformation between ice I and II at 230 K. When a small uniaxial compressive stress was applied during cycling, the specimens exhibited a compressive strain (as large as 18.5% after a single cycle) proportional to the applied stress, in agreement with observations and theory for transformation plasticity of metals induced by polymorphic thermal cycling.     

辽东湾海冰反射特性

海冰反照率是研究海冰表面特性、海冰分布、海冰厚度和极地气候的重要光学量之一。将卫星于不同位置所测量到的海冰双向反射率准确地转化为相应的反照率的值,对于获得大规模海冰的时空特性是非常重要的。研究了辽东湾海冰反照率与海冰双向反射分布函数之间的相关关系。结果表明：(1)反照率与海冰组分密切相关,海冰表层颗粒物浓度越高,反照率α(λ)的值越小,气泡的含量越高,α(λ)越大;(2)当仪器探头天顶角θ为0°时,所测量的双向反射因子R_f与反照率α(λ)的值是相似的;随着θ的增大,R_f的值逐渐增大,且与探头方位角的相关性逐渐增强;当θ的值等于太阳天顶角63°时,R_f的值最大,且与探头方位角的相关性最强;(3)不同类型的冰具有不同的各向异性反射因子。     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

有冰海区油膜光谱特征研究

近些年有冰海区石油的勘探、开发以及运输频度逐年上升,这增加了冰区溢油发生的风险。试验通过测量海水、碎冰、整冰等不同背景条件下轻柴油和原油油膜的可见光-近红外光谱反射率曲线,并与洁净的海水、碎冰和平整冰光谱曲线进行比较,得到能够有效识别冰区溢油的波段。测量结果表明：同一油种在不同背景下油膜的光谱反射率曲线有所差异,同一油种同一背景下由于分布形态不同光谱反射率曲线也有所差异。但同一油种的油膜仍呈现出许多区别于背景的共同特征：以海水、碎冰或整冰为背景的轻柴油油膜的光谱反射率曲线呈现“先低后高”、部分“相伴而行”的特点;海水、碎冰和整冰中原油油膜的光谱反射率曲线在750～770 nm区间出现楔状反射峰的典型特点。根据以上特点可以将溢油污染海水/海冰与洁净海水/海冰很好的区分开来,从而为遥感监测冰区溢油时的波段选择和溢油识别提供参考和依据。