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。     

大气光学湍流估算模式及其相似性函数

地表热通量和水汽通量对全球气候变化和大气环流有着重要而广泛的影响,而Monin-Obukhov(M-O)相似性函数在计算近地层热通量和水汽通量的过程中扮演着重要的角色;同时M-O相似性函数是大气光学湍流估算模式中不可或缺的因子。通过对合肥西郊35 m铁塔上气象数据的分析,利用非线性最小二乘法拟合得到了一套全新的M-O相似性函数。将之与前人提出的相似性函数作对比,展现出了较好的一致性。同时,该函数大气光学湍流的估算模式中,估算值与实测值具有较好的吻合度。分析结果表明:在稳定条件和非常不稳定条件下,可以认为温度相似性函数等于湿度相似性函数,在弱不稳定条件下两者不再满足这种相似性。     

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...     

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。      

Can climate sensitivity be estimated from short-term relationships of top-of-atmosphere net radiation and surface temperature?

Increasing the knowledge in climate radiative feedbacks is critical for current climate studies. This work focuses on short-term relationships between global mean surface temperature and top-of-atmosphere (TOA) net radiation. The relationships may be used to characterize the climate feedback as suggested by some recent studies. As those recent studies, an energy balance model with ocean mixed layer and both radiative and non-radiative heat sources is used here. The significant improvement of current model is that climate system memories are considered.Based on model simulations, short-term relationship between global mean surface temperature and TOA net radiation (or the linear striation feature as suggested by previous studies) might represent climate feedbacks when the system had no memories. However, climate systems with the same short-term feedbacks but different memories would have a similar linear striation feature. This linear striation feature reflects only fast components of climate feedbacks and may not represent the total climate feedback even when the memory length of climate systems is minimal. The potential errors in the use of short-term relationships in estimations of climate sensitivity could be big. In short time scales, fast climate processes may overwhelm long-term climate feedbacks. Thus, the climate radiative feedback parameter obtained from short-term data may not provide a reliable estimate of climate sensitivity. This result also suggests that long-term observations of global surface temperature and TOA radiation are critical in the understanding of climate feedbacks and sensitivities.     

Propagation of ocean surface waves on a spherical multiple-cell grid

Satellite observations have established that the Arctic ice is retreating faster than expected and global ocean surface wave models have to be extended to cover the polar region in the future. The major obstacle preventing the wave model extension is that the diminishing longitude grid-length at high latitudes exerts a severe restriction on time steps and leads to polar singularity. A spherical multiple-cell (SMC) grid is installed in a global wave model to overcome the polar problems. A 2nd order upstream non-oscillatory advection scheme and a rotation scheme for wave spectral refraction are used. The unstructured SMC grid allows time step to be relaxed and land cells to be removed, saving over 1/3 of the total computation time in comparison with the original latitude–longitude grid model. It also allows multi-resolutions within one model domain so that coastlines and small islands can be resolved at refined resolutions. It also makes it possible to merge regional models into a single global model, replacing nested models in operational forecasting systems. Validations with satellite and buoy observations show that the SMC grid wave model performs as well as the latitude–longitude grid model and yields better swell predictions if coastlines and small islands are resolved at refined resolutions. Due to the ice coverage in the Arctic, an ideal wave spectral propagation in an ice-free Arctic is used to illustrate a map-east reference direction method for extension of the wave model over the whole Arctic.     

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.     

Reminiscences on the study of wind waves

The wind blowing over sea surface generates tiny wind waves. They develop with time and space absorbing wind energy, and become huge wind waves usually referred to ocean surface waves. The wind waves cause not only serious sea disasters but also take important roles in the local and global climate changes by affecting the fluxes of momentum, heat and gases (e.g. CO₂) through the air-sea boundary. The present paper reviews the selected studies on wind waves conducted by our group in the Research Institute for Applied Mechanics (RIAM), Kyushu University. The themes discussed are interactions between water waves and winds, the energy spectrum of wind waves, nonlinear properties of wind waves, and the effects of surfactant on some air-sea interaction phenomena.     

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).     

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.     

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.     

Radiative forcing of aerosols and gases over tropical ocean and its impact on the global atmospheric environment: GCM investigations during INDOEX campaigns

Radiative forcings due to aerosols and the pollutant gases accumulated as haze which are transported from nearby continent to the tropical ocean are essentially important elements of the world climate system. Vertical transport of aerosols and gaseous species takes place within the deep convective cloud clusters of the inter-tropical convergence zone and subsequently these are distributed by the upper atmospheric zonal wind flow, thereby have impact on the global atmospheric environment. The comprehensive global atmospheric models have shown capability of simulating the climate of the atmosphere with proper forcing. We have deduced the radiative forcing, optical depth and the global energy balance components by a global atmospheric model. The results are validated well with INDOEX and other available observational findings.     

国家气候中心大气环流模式冬季模式误差特征分析

本文利用1982—2010年国家气候中心第二代月动力延伸预测系统中大气环流模式(BCC_AGCM)的回报资料和美国国家环境预测中心和美国国家大气研究中心(NCEP/NCAR)再分析资料,分析了该模式对冬季气温预测的误差特征及其与外强迫的联系.结果表明模式能够在整体上较好地反映出欧亚区域冬季气温的变化趋势,能抓住东亚冬季风区气温年际变化的主要空间模态,对东亚冬季风区冬季气温具有一定的预报能力.预报误差的空间分布和时间演变特征的结果表明,误差在陆地大于海洋,高纬地区大于低纬地区,同时与海拔高度也有密切关系.预报误差的主要模态与一些关键区域的海温和海冰存在显著的相关性,表明模式对外强迫异常的响应能力存在缺陷.这为结合模式对关键区海温和海冰异常的响应能力,有针对性地改进模式对东亚冬季风区冬季气温的预测能力提供了依据.     

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。      

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.     

Remote sensing of Arctic Fjords by Raman lidar: Heat transfer screening by layer of glacier’s relict water

West Spitsbergen fjords have been probed (on shipboard) by a compact Raman lidar (Wave Research Center, Prokhorov General Physics Institute, Russian Academy of Sciences), based on a pulsed diode-pumped Nd³⁺:YVO₄ laser with frequency doubling, in August, 2011. The evolution of lidar signal spectra (Raman, fluorescence, and elastic scattering) when approaching the Paulabreen glacier of the Rinders Fjord has been measured. A new phenomenon was discovered: the formation of an immiscible layer of relict thaw water from the glacier on the sea surface. An increase in the layer thickness (to 5?C10 cm near the glacier) was accompanied by a decrease in the layer temperature (which was determined by measuring the distortion of the OH Raman band) and the content of phytoplankton and dissolved organic matter to zero, with a simultaneous increase in the Rayleigh scattering amplitude and a change in the surface color from blue to gray. Smoothing of the rippled sea surface (covered by relict water) at the border of the wake trace (which it rolled down from) was observed for the first time, as far as we know. The mechanism of the formation of a supercooled (t ? 0°C) layer and screening the heat diffusion, which reduces the contribution of the relatively warm sea water (??3°C) to the thermal balance of Arctic and global climate, is discussed.     

Propagation of acoustic waves in a coupled ice-water system for the Arctic Ocean

We studied the propagation of acoustic waves in a coupled ice-water system for the Arctic Ocean.The sea ice is considered as homogeneous as the seawater.The dis...     

Some factors controlling the climatological evolution of the upper-layer sea temperature at Trieste

Summary The long-period evolution of the sea temperature at Trieste is analysed, taking into account the contributions from global solar irradiance, offshore wind speed (20 years observations) and atmospheric pressure modulation (45 years observations). Sea temperature data are taken at a location in the harbour that can be assumed representative, from the climatological point of view, of the entire Gulf of Trieste. The results exhibit at least two time scales, one of the order of months, related to heat fluxes, and the other of the order of few years, due to southerly water mass advection forced by the inverted barometer effect.     

Singing behavior of fin whales in the Davis Strait with implications for mating, migration and foraging

Most baleen whales undertake migrations between low-latitude breeding grounds and high-latitude feeding grounds. Though little is known about the timing of their migration from the Arctic, fin whales are assumed to undertake a similar migratory pattern. To address questions about habitat use and migrations, the acoustic activity of fin whales in Davis Strait, between Greenland and Canada, was monitored continuously for two years using three bottom-moored acoustic recorders. The acoustic power in the fin whale call frequencies peaked in November-December, showing that fin whales are present in Davis Strait much later in the year than previously expected. The closely timed peaks in song activity and conception time imply that not all fin whales migrate south to mate, but rather start mating at high latitudes rather than or before migrating. Singing activity was strongly linked to daylight hours, suggesting that fin whales might feed during the few daylight hours of the late fall and early Arctic winter. A negative correlation between the advancing sea ice front and power in fin whale frequencies indicates that future changes in sea ice conditions from global warming might change the distribution and migratory patterns of fin whales near the poles.     

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...