Measuring the Distribution of Ocean Mass Using GRACE

The Gravity Recovery and Climate Experiment (GRACE), which was successfully launched March 17, 2002, has the potential to create a new paradigm in satellite oceanography with an impact perhaps as large as was observed with the arrival of precision satellite altimetry via TOPEX/Poseidon (T/P) in 1992. The simulations presented here suggest that GRACE will be able to monitor non-secular changes in ocean mass on a global basis with a spatial resolution of ≈500 km and an accuracy of ≈3 mm water equivalent. It should be possible to recover global mean ocean mass variations to an accuracy of ≈1 mm, possibly much better if the atmospheric pressure modeling errors can be reduced. We have not considered the possibly significant errors that may arise due to temporal aliasing and secular gravity variations. Secular signals from glacial isostatic adjustment and the melting of polar ice mass are expected to be quite large, and will complicate the recovery of secular ocean mass variations. Nevertheless, GRACE will provide unprecedented insight into the mass components of sea level change, especially when combined with coincident satellite altimeter measurements. Progress on these issues would provide new insight into the response of sea level to climate change. This revised version was published online in August 2006 with corrections to the Cover Date.     

Civilizations and Sea Level Change

Climate and sea level changes have been relatively stable for the last few centuries. The delicate balance between global and oceanic systems, however, may be altered due to the increased warming of the earth's average surface temperature. A few degrees increase could cause a 15-17 cm sea level rise by the year 2000, resulting in wide spread flooding of low lying areas. Recent studies indicate the global sea level is rising and extensive coastal flooding is anticipated. If these changes continue through the next century, we may be faced with a loss of our present culture. Oscillating sea levels since the late Pleistocene at times have left the floors of the continental margins and inland seas exposed, increasing the total earth's land surface by 8%. It was during these events that North America and island refugia including England, Indonesia, Japan and Australia were invaded and colonized by man. However, the swinging sea levels and shoreline displacements of as much as 120 m per year must have been very disruptive to the inhabitants of the coastal region and evidence of their cultures must have eroded away due to the relentless cycle of the coastal processes.     

A geopause satellite system concept

The forthcoming 10 cm range tracking accuracy capability holds much promise in connection with a number of Earth and ocean dynamics investigations. These include a set of earthquake-related studies of fault motions and the Earth's tidal, polar and rotational motions, as well as studies of the gravity field and the sea surface topography which should furnish basic information about mass and heat flow in the oceans. The state of the orbit analysis art is presently at about the 10 m level, or about two orders of magnitude away from the 10 cm range accuracy capability expected in the next couple of years or so. The realization of a 10 cm orbit analysis capability awaits the solution of four kinds of problems, namely, those involving orbit determination and the lack of sufficient knowledge of tracking system biases, the gravity field, and tracking station locations. The Geopause satellite system concept offers promising approaches in connection with all of these areas. A typical Geopause satellite orbit has a 14 hour period, a mean height of about 4.6 Earth radii, and is nearly circular, polar, and normal to the ecliptic. At this height only a relatively few gravity terms have uncertainties corresponding to orbital perturbations above the decimeter level. The orbit s, in this sense, at the geopotential boundary, i.e., the geopause. The few remaining environmental quantities which may be significant can be determined by means of orbit analyses and accelerometers. The Geopause satellite system also provides the tracking geometery and coverage needed for determining the orbit, the tracking system biases and the station locations. Studies indicate that the Geopause satellite, tracked with a 2 cm ranging system from nine NASA affiliated sites, can yield decimeter station location accuracies. Five or more fundamental stations well distributed in longitude can view Geopause over the North Pole. This means not only that redundant data are available for determining tracking system biases, but also that both components of the polar motion can be observed frequently. When tracking Geopause, the NASA sites become a two-hemisphere configuration which is ideal for a number of Earth physics applications such as the observation of the polar motion with a time resolution of a fraction of a day. Geopause also provides the basic capability for satellite-to-satellite tracking of drag-free satellites for mapping the gravity field and altimeter satellites for surveying the sea surface topography. Geopause tracking a coplanar, drag-free satellite for two months to 0.03 mm per second accuracy can yield the geoid over the entire Earth to decimeter accuracy with 2.5° spatial resolution. Two Geopause satellites tracking a coplanar altimeter satellite can then yield ocean surface heights above the geoid with 7° spatial resolution every two weeks. These data will furnish basic boundary condition information about mass and heat flows in the oceans which are important in shaping weather and climate.     

IV: GEODESY: Remarks on the Role of Height Datum in Altimetry-Gravimetry Boundary-Value Problems

The problem of global geoid determination is usually solved using satellite altimetry data on the oceans, together with an oceanographic model of sea surface topography, and gravity anomaly data on the continents. Such data, however, enable to obtain only potential differences with respect to a reference surface whose absolute potential is unknown. This situation suggests to modify the classical mixed boundary-value problem of physical geodesy by inserting into the boundary conditions an unknown additive constant, that must be determined by imposing a suitable additional constraint. Yet, such formulation of the boundary-value problem, from the point of view of its mathematical properties, is not unconditionally well-posed, and, furthermore, does not reflect faithfully the available physical model, as the present knowledge of ocean circulation does not allow to connect along coastlines the reference surfaces defined on the oceans and on the continents. The introduction of two different unknown additive constants, one for the oceans and one for the earth, to be determined by imposing two additional constraints, gives rise to a more faithful picture of the present physical knowledge, and, at the same time, to a new well-posed formulation of the boundary-value problem. This revised version was published online in August 2006 with corrections to the Cover Date.     

TOPEX/Poseidon: Electrical power system-lessons learned

The main objective of the TOPEX/Poseidon Spacecraft is to monitor the world's oceans for scientific study of weather and climate prediction, coastal storm warning and maritime safety. The operational conditions of this satellite imposed challenging requirements for the on-board Electrical Power System (EPS), Going through various phases of its development and on-orbit performance verifications, there were certain events and/or circumstances we would have liked to avoid. Some circumstances were avoided with preventative measures, other potentially detrimental events were not. Thus, a number of valuable lessons were learned which are presented in this paper     

V: SEA LEVEL: Benefits of GRACE and GOCE to sea level studies

The recently published Third Assessment Reports of the Intergovernmental Panel on Climate Change have underlined the scientific interest in, and practical importance of past and potential future sea level changes. Space gravity missions will provide major benefits to the understanding of the past, and, thereby, in the prediction of future, sea level changes in many ways. The proposal for the GOCE mission described well the improvements to be expected from improved gravity field and geoid models in oceanography (for example, in the measurement of the time-averaged, or ‘steady state’, ocean surface circulation and better estimation of ocean transports), in geophysics (in the improvement of geodynamic models for vertical land movements), in geodesy (in positioning of tide gauge data into the same reference frame as altimeter data, and in improvement of altimeter satellite orbits), and possibly in glaciology (in improved knowledge of bedrock topography and ice sheet mass fluxes). GRACE will make many important steps towards these ‘steady state’ aims. However, its main purpose is the provision of oceanographic (and hydrological and meteorological) temporally-varying gravity information, and should in effect function as a global ‘bottom pressure recorder’, providing further insight into the 3-D temporal variation of the ocean circulation, and of the global water budget in general. This paper summaries several of these issues, pointing the way towards improved accuracy of prediction of future sea level change. This revised version was published online in August 2006 with corrections to the Cover Date.     

The impact of solar forcing on the variability in a coupled climate model

The impact of variations in solar irradiance on the variability of climate is still a topic of debate. Herein we assess the response of a coupled General Circulation Model (GCM) of intermediate complexity to an estimate of the solar variability since 1700 and to a series of idealized sinusoidal solar forcings. On the continental to global scale and averaged over periods longer than 30 years, the solar-induced variability dominates internal variability in the annual global mean surface air temperature. Locally and on the regional scale, the internal variability dominates. The dominant patterns of natural variability and explained variance are not affected by a variable solar forcing, the spectra however are sensitive. The control run shows a preferred decadal time scale of 18 year in a sea surface temperature mode associated with the North Atlantic Oscillation. The preferred decadal time scale disappears for a variable solar forcing. This is caused by small changes in oceanic circulation resulting in subsurface oceanic modes with modified structure and time scale.     

The Elusive Stationary Geoid

We discuss the various problems occurring when trying to fix a geoid or geopotential model using sea level observations sampled during a limited time span from a bounded geographical domain. Such problems are on the one hand aliasing and spectral leakage, and on the other, the non-conservation of matter over only part of the world ocean. In the light of these issues we discuss whether it is sensible to include in a definition of the global geoid the radially symmetric part of either the mean sea level field itself, or its linear or nonlinear time dependence, arriving at a negative conclusion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Long Term Climate Records from Polar Ice

One of the great challenges in climate research is to investigate the principal mechanisms that control global climatic changes and an effective way to learn more about it, is the reconstruction of past climate changes. The most important sources of information about such changes and the associated composition of the atmosphere are the two large ice caps of Greenland and Antarctica. Analysis of ice cores is the most powerful means we have to determine how climate has changed over the last few climatic cycles, and to relate this to changes in atmospheric composition, in particular to concentrations of the principal greenhouse gases – CO₂, CH₄ and N₂O (carbon dioxide, methane, and nitrous oxide).Transitions from cold ice age climates to warmer interstadials have always been accompanied by an increase of the atmospheric concentration of the three principal greenhouse gases. This increase has been, at least for CO₂, vital for the ending of glacial epochs. A highly simplified course of events for the past four transitions would then be as follows: first, changing orbital parameters initiated the end of the glacial epoch; second, an increase in greenhouse gases then amplified the weak orbital signal; third, in the second half of the transition, warming was further amplified by decreasing albedo, caused by melting of the large ice sheets in the Northern Hemisphere going parallel with a change of the ocean circulation.The isotopic records of Greenland ice cores show evidence for fast and drastic climatic changes during the last glacial epoch. Possible causes and mechanisms of such changes and their significance as global climatic events are discussed here. Ice core results also enable the reaction of the environment to past global changes to be investigated.It will also be discussed how reliable stable isotope records are as a local temperature proxy and how representative paleoclimatic results from Greenland and Antarctica are in relation to global climate.     

界面相对3D-C/SiC复合材料热膨胀性能的影响

利用减压化学气相浸渗（LPCVI）技术制备了3D C/SiC复合材料,从热解碳（PyC）界面相厚度对界面结合强度和热应力的影响出发,研究了界面相对复合材料热膨胀性能的影响。结果表明:①界面相厚度对3D C/SiC复合材料热膨胀性能的影响主要归因于其对界面结合强度和脱黏面上的滑移阻力的影响。在一定厚度范围（约70～220nm）内,材料的热膨胀系数随热解碳厚度的增加而逐渐降低;②热处理可提高材料的热稳定性,并通过改变材料内部结构,使热应力重新分布,对复合材料的高温热膨胀产生显著影响,但是,并没有改变基体裂纹的愈合温度（900℃）。     

Climate Observations - The Instrumental Record

A survey is given of the available instrumental data for monitoring and analysis of climatic variations. We focus on temperature measurements, both over land and ocean, at the surface and aloft.Over land, the older observations were subject to exposure changes which may not have been fully compensated. The effects of urbanization have been largely avoided in studies of climatic change over the last 150 years. There are few records for pre-1850 outside Europe and eastern North America, and the global network shows a recent decline. Over the ocean, sea surface temperature (SST) has been measured using buckets, engine intakes, hull sensors, buoys, and satellites. Many of these data have been effectively homogenized, but new challenges arise as observing systems evolve. Available SST and marine air temperature datasets begin in the 1850s. The data are concentrated in shipping lanes especially before 1900, and very sparse during the world wars, but additional historical data are being digitized.The radiosonde record is short (40 years) and has major gaps over the oceans, tropics and Southern Hemisphere. Instrumental heterogeneities are beginning to be assessed and removed using physical and statistical techniques. The MSU record is complete but only began in 1979, and is not highly resolved in the vertical: major biases, mainly affecting the lower-tropospheric retrieval, have been reduced as a result of recent analyses.Advanced interpolation or data-assimilation techniques are being applied to these data, but the results must be interpreted with care.     

Planetary Magnetic Fields: Achievements and Prospects

The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of planetary interior properties and evolutions. Most planetary magnetic fields arise from dynamos, past or present, and planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or “snow”). Although planetary dynamos mostly appear to operate with an internal field that is not very different from (2ρΩ/σ)^1/2 in SI units where ρ is the fluid density, Ω is the planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of planetary magnetic fields depends as much on our understanding of the history and material properties of planets as it does on our understanding of the dynamo process. Future developments can be expected in our understanding of the criterion for a dynamo and on planetary properties, through a combination of theoretical work, numerical simulations, planetary missions (MESSENGER, Juno, etc.) and laboratory experiments.     

原子干涉技术在惯性领域中的应用

惯性技术因其强自主性、不依赖外界信号、适应全天候等特性在导航领域备受关注,为了提升惯性导航的精度,数十年来人们在如何提高惯性传感器性能方面进行了大量的攻关工作并研制出了多种基于不同原理的惯性传感器。得益于量子效应,原子传感器能在诸如时间、加速度、转动、磁场等领域提供比现有技术更高的测量灵敏度、精度和速度。通过研制基于原子干涉技术的高精度原子惯性器件,实现重力/重力梯度数据实时补偿匹配的量子导航将是新一代高精准军用惯性导航的首选。本文简要介绍了以物质波干涉为基础的原子干涉惯性器件的原理,回顾了以原子重力仪、原子干涉陀螺为主的技术发展历程及现状,并结合我国目前在该领域的发展态势,表达了对我国原子惯性设备实装应用的迫切性。     

碳纤维增强铝合金层合板的残余热应力分析

碳纤维增强铝合金层合板（CARALL）是由碳纤维复合材料和铝合金薄板制成的,由于碳纤维复合材料和铝合金的热膨胀系数相差很大,加热固化后会产生较大的残余热应力。本文对碳纤维增强铝合金层合板中残余热应力进行了分析,并在碳纤维增强铝合金层合板的加热固化阶段,使用二次加热法来降低碳纤维复合材料和铝合金薄板的粘结温度,从而降低残余热应力。     

Planetary Radiation Budgets

The energy state of a planet depends fundamentally on its radiation budget. Measurements made from space over past decades have led to significant revisions of ground-based estimates, both of the reflected fraction (the Bond albedo) of solar radiative flux and of the emitted thermal infrared radiation flux, for the Earth as well as for the other planets. After a brief survey of methods and difficulties in accurately determining planetary radiation budgets, we note contradictions in existing tabulations of global parameters, in particular Bond albedo. For the Earth, such contradictions are unjustified, considering that global and annual means as well as the seasonal cycle of Earth Radiation Budget components have now been determined with high accuracy. The Earth's Bond albedo is close to 0.3. Net storage of energy in the Earth-ocean system is close to zero, with a well-established annual cycle of amplitude close to ±12 Wm⁻². Some contradictions remain for the other terrestrial planets. For the giant planets, modern reduced values of the Bond albedo imply reduced but still significant internal energy generation.     

Physical reasons and consequences of a three-dimensionally structured heliosphere

In this article we have discussed reasons both of solar and of interstellar origin giving rise to a pronounced three-dimensional structure of the expanding solar wind and thus of the global configuration of the heliosphere. Our present observational knowledge on these structurings is reviewed, and all attempts to theoretically model these solar wind structures are critically analysed with respect to their virtues and flaws. It is especially studied here by what mechanisms interstellar imprints on the actual type of solar wind expansion can be envisaged. With concern to this aspect it hereby appears to be of eminent importance that the solar system maintains a relative motion with a submagnetosonic velocity of about 23km/sec with respect to the ambient magnetized interstellar medium corresponding to a magnetosonic Mach number of about 0.5.A heliopause closing the distant heliospheric cavity within a solar distance of about 100AU on the upwind side and opening it into an largely extended tail on the downwind side results as a first consequence from this relative motion. As a second consequence an asymmetric heliospheric shockfront with upwind distances smaller than downwind distances by ratios between 1/3 and 2/3 is most likely provoked which gives rise to at least two important upwind-downwind asymmetric processes influencing the supersonic solar wind expansion downstream from the shock: the anomalous cosmic ray diffusion into the solar wind, and high energetic jet electrons originating at the shock and moving inwards up to an inner critical point at around 20AU. As we shall demonstrate both processes are influencing the solar wind expansion beyond 20AU, however, more efficiently in the upwind hemisphere as compared to the downwind hemisphere. In the region inside 20AU other mechanisms are operating to propagate the interstellar imprint on the solar wind expansion further downstream into the inner heliosphere because here even the original solar wind electrons, in view of the solar wind bulk velocities, behave as a subsonic plasma constituent which can modify the solar wind solutions by means of an appropriate detuning of the circumsolar electric polarisation field. We give quantitative estimates for these effects.What concerns the theory of a solar wind expansion into a counterflowing ambient interstellar medium, some flaws of the present theoretical attempts are identified impeding that the interstellar influence on the actual solar wind solutions can become visible. We thus conclude that there is a clear need for three-dimensional and time-dependent solar wind models with a free outflow geometry taking into account the multisonicity of the solar wind plasma with different eigenmodes for a perturbation propagation.     

Error Characteristics Estimated from Champ,GRACE and GOCE Derived Geoids and from Satellite Altimetry Derived Mean Dynamic Topography

This paper presents a review of geoid error characteristics of three satellite gravity missions in view of the general problem of separating scientifically interesting signals from various noise sources. The problem is reviewed from the point of view of two proposed applications of gravity missions, one is the observation of the mean oceanic circulation whereby an improved geoid model is used as a reference surface against the long term mean sea level observed by altimetry. In this case we consider the presence of mesoscale variability during assimilation of derived surface currents in inverse models. The other experiment deals with temporal changes in the gravity field observed by GRACE in which case a proposed experiment is to monitor changes in the geoid in order to detect geophysical interesting signals such as variations in the continental hydrology and non-steric ocean processes. For this experiment we will address the problem of geophysical signal contamination and the way it potentially affects monthly geoid solutions of GRACE. This revised version was published online in August 2006 with corrections to the Cover Date.     

Invar钢模具制造工艺研究

复合材料零件固化后型面几乎不再做加工,因此复合材料成型模具的优劣对制品的外形及表面质量起着重要作用。采用GB／T4339～1999标准测试了几种模具材料的热膨胀系数,结果表明Invar钢与复合材料相匹配。通过对Invar钢焊接性能与切削性能进行研究,试验结果表明：采用坡口间隙2mm、堆焊电流170A、保护气体15L／min焊接工艺可得到良好的焊接接头;粗加工采用转速1300～1500r／min、进给速度400mm／min、切深2．5mm,机床运行无异常,刀具未见明显磨损;精加工采用转速2200-2500r／min、进给速度480mm／min、切深0．5mm可得到良好的切削表面。     

三维编织复合材料热物理性能的有限元分析

根据三维编织复合材料的细观结构,分别建立了三维四向和五向编织复合材料热物理性能的有限元模型.采用周期性的非绝热温度边界条件和位移边界条件,计算了三维四向和五向编织复合材料的整体等效热传导系数和热膨胀系数,计算结果同已有文献相比与实验值符合得更好.在此基础上,迸一步研究了编织角、纤维体积分数、编织结构等参数对材料热物理性...     

国外隐身材料研究进展

简单回顾了从二次世界大战至今几十年间,国外隐身材料的发展历程;系统综述了国外在陶瓷材料,导电高分子材料,晶须材料,纳米材料,手征材料等新型隐身材料研究方面所取得的进展,从耐高温隐身材料,智能隐身材料,等离子体隐身三个方面入手,重点介绍了国外近年来在隐身材料领域的最新进展;并且指出了隐身技术未来的发展方向。