科学家警告:北极冰盖正在迅速缩小

2012年9月,北极海冰的范围下降到自1979年有卫星记录以来的最低点.海冰的面积340万平方公里仅相当于1979-2000年间最小平均覆盖值的一半.根据2011年MIT模型计算,海冰减薄的程度相当于2007年联合国关于气候变迁政府间圆桌会议的估计值的4倍.一些气象学家提醒,没有冰层的北冰洋之夏或许会在近几年出现,将比现有气候模型估计的时间提早10年.     

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。     

Toward a new elastic-decohesive model of Arctic sea ice

This paper argues for a new constitutive model, an elastic-decohesive model for sea ice. The model is motivated by examining satellite observations of the Arctic processed to show ice deformation in the form of divergence, shear and vorticity. The model is implemented numerically in the material-point method and used to predict motion and deformation of sea ice by simulating a region of the Beaufort Sea. The model is able to capture the qualitative and statistical behavior of localized deformation seen in the observations.     

Laboratory-scale measurement of scattering from modelled Arctic ice ridges

An experiment is performed to measure acoustic scattering from scale modelled ice ridges in both specular (forward) and non-specular (backward) directions, for comparison with predictions from theoretical models. The experiment uses a 100 kHz transmitter emitting sinusoidal bursts. An array of miniature transducers is used to measure the scattered field as a function of scattering angle. Experimental results are obtained for scattering from different types of rods simulating ice ridges, and also the reflection coefficient from an acrylic block. The results show good agreement with the Twersky model predictions. This experiment establishes an effective technique, using scale models in the laboratory, to compare theoretical predictions and field experimental data.     

北极冰下双轴声道传播特性研究

本文利用第六次北极科学考察的所得的楚科奇海温盐深数据,开展了北极海域声传播特性研究。通过引入Diachok海冰散射模型,将海冰界面模拟为无限椭圆形半圆柱体在自由界面的随机分布,利用Twersky散射理论计算了海冰的反射系数。使用简正波中的KRAKENC模型,结合海冰参数,进行了传播损失仿真分析。研究表明,北极的双声道波导中表面声道的传播特性与声源频率和收发深度紧密相关。深海声道与北极典型声速结构的传播特性一致,具有频率选择特性,在20 Hz左右时传播特性较为理想。此外,在远程传播时,深海声道的传播特性要优于表面声道。     

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.     

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,某些频率声波的反射率会明显降低,其中93%的反射率低点超过-10 dB,有的甚至会达到-20 dB以上。(2)冰层反射率低点对应的频段声波,能被冰层上方检波器接收到,并显示出较强的合振速谱,且与该站位自由冰层模型中同时产生的A2和S2模态Lamb波在频段上相符。这些结果可为跨冰层水声信号拾取和水声传播研究提供参考。     

Light scattering evidence of a central component in ice Ih

High resolution Raman spectra of ice I_h single crystal in the frequency range 0–30 cm^?1 are reported, at various temperatures. For the first time evidence is given for the existence of a Lorentzian central component in all scattering configurations. A polarization analysis of the spectra clearly shows that the central mode cannot be ascribed to the high frequency tails of the Brillouin doublet, nor to the disorder induced phonon background even accounting for renormalization of the phonon propagator. The epectral features indicate the existence of a direct coupling between the light and some relaxation mechanism of the crystal.A model is suggested to account for the excess scattering and for the activation of the acoustic phonon density of states.     

Analysis of the variation on underwater acoustic signal across ice layer in the Arctic

Information transmission between the seawater and the air was blocked by the sea ice in the Arctic.In order to obtain the effect of ice on underwater acoustic s...     

Sound velocity in shallow water bodies with gas-saturated water-bottom (ice) interfaces

Sound velocity variations in shallow water bodies with gas-saturated water-bottom (ice) interfaces are investigated. The effect of air inclusions in water and water-like bottoms (ice) on the velocity of longitudinal sound waves is qualitatively and quantitatively estimated. It is shown that changes in the sound velocity are mainly governed by the radial resonance, which at low frequencies depends on the quality factor of the zeroth mode of bubble oscillation. For real concentrations of bubbles, the velocity of longitudinal waves may become very low. This may lead to considerable distortions of boundary conditions at rough surfaces and, hence, to enhancement of scattering and absorption of sound waves and additional leakage of acoustic energy into the bottom (ice), as well as considerable changes in the sound velocity profile in surface layers with a change of sign of the velocity gradient from negative to positive, which results in the formation of a subsurface channel or an increase in its power. It is found that water-like bottoms (sediments) and ice (“liquid” ice), which are characterized by shear wave velocities of an order of 15 m/s or less, behave in the kilohertz range almost as a vacuum (C _l → 0) when the air content in them reaches several percent. As a result, the propagation of first normal modes in shallow water or in subsurface layers of arctic and oceanic waveguides noticeably changes.     

Acoustically induced light gyration in nickel nanoparticles

Circularly polarized acoustically induced light gyration (AILG) in nickel nanoparticles (NiNPs) attached to indium tin oxide (ITO) substrates was observed to be enhanced by nanosecond UV laser excitation at a wavelength in the surface plasmon resonance region. The AILG was observed during exposure to two acoustical waves with frequencies of 2 and 4 MHz and power densities of up to 5 W/cm². The maximum value of the AILG observed for NiNPs of average size ca. 8.7 nm, attached to an ITO substrate was about 2.8°/mm without UV-light illumination. Additional irradiation by 5 ns pulse UV laser light (λ: 337 nm) at the surface plasmon resonance region was found to favour the additional enhancement of the AILG up to 11°/mm. The effect was optimized at a temperature of 120 K. This increase was not observed when the size of NiNPs was 16.8 nm.     

Acoustically induced potential dots in GaAs quantum wells

Dynamic dots (DDs) consisting of confined and mobile potentials are realized by the interference of orthogonal surface acoustic wave (SAW) beams in GaAs quantum wells. Photoluminescence spectroscopy reveals that the DDs are characterized by a peculiar distribution of strain and piezoelectric fields dictated by the lattice symmetry, which is quite different from the one induced by a single SAW. We demonstrate the unique ability of DDs to control the flow of photogenerated electron-hole pairs and of photons by realizing an electronic switch based on SAWs.     

Acoustically induced transparency in optically dense resonance medium

It is shown that mechanical vibration (acoustical oscillation) of a solid medium along the propagation of multifrequency laser radiation enables one to control the resonant absorption. There exists an optimal spectral structure of the incident field dependent on vibration amplitude as well as the number and intensity of the frequency components that provides the full resonant transparency. A mechanism of the transparency is discussed. Transparency of this kind is shown to appear also via adiabatic modulation of the atomic transition frequency by an external microwave field.     

Acoustically induced stark effect for excitons in intrinsic semiconductors

A Stark effect for excitons parametrically driven by coherent acoustic phonons is proposed. Our scheme refers to a low-temperature intrinsic semiconductor or semiconductor nanostructure pumped by an acoustic wave (frequency band nu(ac) approximately equal to 1-40 GHz and intensity range I(ac) approximately equal to 10(-2)-10(2) W/cm(2)) and probed by low-intensity light. Tunable optical band gaps, which strongly change the spectral shape of the exciton line, are induced in the polariton spectrum by acoustic pumping. We develop an exactly solvable model of the acoustic Stark effect and apply our results to GaAs driven by bulk or surface acoustic waves.     

Acoustically modified spatial coherence in optical fresnel diffraction region

Theoretical and experimental interpretations are given to the fact that in the optical Fresnel diffraction region acoustically modified spatial coherence is independent of the distance from the acoustic cell, but takes an identical expression across any planes parallel to the propagation direction of the acoustic wave.     

冰盘管排列密度对蓄冷速率影响的实验研究

设计了蒸发器盘管排列密度可以改变的直接蒸发冰盘管蓄冷装置,在高、中、低三种盘管排列密度情况下,进行了蓄冷槽内载冷剂在自然对流和强制对流下的静态和动态蓄冷实验,研究了盘管排列密度和蓄冷状态对蓄冷速率的影响。结果显示:高密度系统平均蓄冷速率较低密度系统提高27.9%以上,动态系统平均蓄冷速率较静态系统提高2%左右。     

Acoustically coupled gas bubbles in fluids: time-domain phenomena.

In previous work [C. Feuillade, J. Acoust. Soc. Am. 98, 1178-1190 (1995)] a coupled oscillator formalism was introduced for describing collective resonances, scattering, and superresonances, of multiple gas bubbles in a fluid. Subsequently, time-domain investigations of the impulse response of coupled systems have disclosed the exact conditions which determine whether the ensemble scattering behavior should be described using: either (a), a multiple scattering; or (b), a self-consistent methodology. The determining factor is the Q of the individual scatterers, and their typical spatial separations in the medium. For highly damped or sparse systems, e.g., scattering from loose schools of swimbladder fish, or from a gassy seabed containing entrained bubbles, the multiple scatter counting approach should be applicable. For more strongly coupled systems, e.g., a dense cloud of resonating bubbles in the water column, energy exchange may be due primarily to radiative cycling rather than scattering, in which case a self-consistent approach is indicated. The result has implications for both volume and bottom scattering applications.     

Nonlinear Spherical Standing Waves in an Acoustically Excited Liquid Drop

Nonlinear evolution of a standing acoustic wave in a spherical resonator with a perfectly soft surface is analyzed. Quadratic approximation of nonlinear acoustics is used to analyze oscillations in the resonator by the slowly varying amplitude method for the standing wave harmonics and slowly varying profile method for the standing wave profile. It is demonstrated that nonlinear effects may lead to considerable increase in peak pressure at the center of the resonator. The proposed theoretical model is used to analyze the acoustic field in liquid drops of an acoustic fountain. It is shown that, as a result of nonlinear evolution, the peak negative pressure may exceed the mechanical strength of the liquid, which may account for the explosive instability of drops observed in experiments.