Continental drift and true polar wandering

Summary . Evidence in the form of 75 yr of ILS data is accumulating which suggests that true polar wander may be currently taking place. It seems likely that true wander of some magnitude must always accompany plate motions, but the extrapolated ILS rate is an order of magnitude larger than the rate of true polar wander deduced from palaeomagnetic data over the past 55 Myr. The conflict between palaeomagnetic and latitude data provides the motivation for investigating one possible excitation of polar wander, the mass redistribution which accompanies continental drift.
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 10⁶yr of plate motions is found to be negligibly small; even consideration of episodic plate movements, anelasticity, or a decoupled lithosphere cannot boost the effect to the ILS rate of polar wander. These conclusions are strengthened by the fact that any one of several absolute plate velocity models, based on extremely diverse assumptions, yields the same results.
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander.     

改进的热惯量模式及遥感土壤水分

本文提出一个考虑地表显热通量及潜热通量的热惯量模式。模式利用了地面定标的方法并充分利用了热象图的空间分布信息,因而较大幅度地提高了估算土壤水分含量的精度。     

Evaluation of paleohydrologic models for terrestrial inverted channels: Implications for application to martian sinuous ridges

Fluvial systems can be preserved in inverted relief on both Earth and Mars. Few studies have evaluated the applicability of various paleohydrological models to inverted fluvial systems. The first phase of this investigation focused on an extensive (spanning  12 km) inverted paleochannel system that consists of four sandstone-capped, carbonate-cemented, sinuous ridges within the Early Cretaceous Cedar Mountain Formation located southwest of Green River, Utah. Morphologic and sedimentologic observations of the exhumed paleochannels were used to evaluate multiple numerical models for reconstructing paleofluvial hydrological parameters. Another objective of the study was to determine whether aerial or orbital observations yield model results that are consistent with those constrained by field data. The models yield an envelope of plausible dominant discharge values (100–500 m³/s), reflecting the limitations of the approach, and no single model can be used to reliably estimate paleodischarge. On Mars, landforms with attributes consistent with inverted channels have been identified. In spite of differences in the formation history between these martian landforms and the terrestrial analog described here, including potential differences in cement composition and the erosional agent that was responsible for relief inversion, these numerical models can be applied (with modification) to the martian landforms and yield an envelope of plausible values for dominant discharge.     

基于DEM的地球与火星格状沙丘对比分析

沙丘是柴达木盆地可类比火星的重要地貌类型,沙丘形态是类火星风沙地貌研究的重要内容。基于数字地形分析（DTA）的方法,采用高程、坡度、坡向及地表复杂程度4个地形计量学指标对火星（北极地区）和地球（柴达木盆地）格状沙丘的地貌形态特征进行定量对比分析。结果表明：（1）两个研究区的高程剖面、坡度、坡向的地理学空间分布格局具有较大相似性;（2）高程、高程梯度、坡度和坡向的直方图相似度指数均大于0.7;（3）不同尺度上的分形维数近似相等,即地表复杂程度相似。用数字地形分析与直方图相似度指数结合的方法,定量或半定量地分析两个研究区沙丘地貌形态的相似性,这对类火星风沙地貌研究中科学选择试验点是一种新的尝试,以期为反演火星风沙地貌的形成与演化提供科学依据。     

Error assessment of grid-based flow routing algorithms used in hydrological models

This paper reports an investigation on the accuracy of grid-based routing algorithms used in hydrological models. A quantitative methodology has been developed for objective and data-independent assessment of errors generated from the algorithms that extract hydrological parameters from gridded DEM. The generic approach is to use artificial surfaces that can be described by a mathematical model, thus the ‘true’ output value can be pre-determined to avoid uncertainty caused by uncontrollable data errors. Four mathematical surfaces based on an ellipsoid (representing convex slopes), an inverse ellipsoid (representing concave slopes), saddle and plane were generated and the theoretical ‘true’ value of the Specific Catchment Area (SCA) at any given point on the surfaces could be computed using mathematical inference. Based on these models, tests were made on a number of algorithms for SCA computation. The actual output values from these algorithms on the convex, concave, saddle and plane surfaces were compared with the theoretical ‘true’ values, and the errors were then analysed statistically. The strengths and weaknesses of the selected algorithms are also discussed.     

Revised value of the dynamical ellipticity of the Earth

Summary. A new value of the Earth's dynamical ellipticity H , defined as the ratio of the difference between the Earth's polar and mean equatorial moments of inertia to its polar moment of inertia, is derived from the most recent, and accurate, values of the Earth's equinoxial precession, the Earth—Moon mass ratio μ and other appropriate data, and a re-evaluation of the numerical procedures involved. This value is an order of magnitude more accurate than its previous values and yields an equivalent improvement in accuracy in other geodynamical quantities derived from H . The new value is consistent with the new System of Astronomical Constants and the new Geodetic Reference System 1980 and is suitable for use in the many astronomical, geophysical and geodetic applications of H .     

基于遥感影像的雪线高程提取方法对比分析——以天山北坡为例

利用2010-2014年6~8月MOD10A1、MOD10A2及HJ-1/CCD数据,以天山北坡为研究区,对比分析了积雪持续时间比率法及最大值合成法在雪线高程提取中的精度和适用性。结果表明:当MOD10A1以旬合成,积雪持续时间比率阈值为40%时,提取的雪线高程与"真值"最为接近;基于混淆矩阵法,以目视解译得到的HJ-1/CCD积雪面积为"真值",分析得出,积雪持续时间比率法提取积雪面积的总体精度较高,为98.59%,khat系数为0.90,最大值合成法的总体精度仅为85.29%,khat系数为0.70;基于统计学的方法,计算得出,积雪持续时间比率法提取的雪线高程与"真值"的相关系数较高,为0.77,其平均误差、正、负误差、绝对平均误差以及均方根误差相对较低;最大值合成法提取的雪线高程与"真值"的相关系数较低,仅为0.54,其平均误差、正、负误差、绝对平均误差以及均方根误差相对较高;积雪持续时间比率法提取的雪线高程结果优于最大值合成法。     

火星风沙地貌研究进展

作为最活跃的地貌塑造外营力,风力作用在火星表面塑造了各种各样的地貌形态。对火星风沙活动及风沙地貌的认识是随着观测和探测技术的进步而逐渐明确的。火星沙丘在北极地区连绵成带状分布,在其他地区分布比较零散,且主要与撞击坑、河谷和山谷相伴分布。火星风沙地貌与地球上相应的地貌类型形态相似而规模更大,表明两个星球上塑造风沙地貌的动力系统具有一定的相似性。火星沙丘主要由玄武岩质（北极部分沙丘为石膏质）颗粒堆积而成,沉积物粒度比地球粗,主要是由中沙和粗纱组成。随着获取影像时空分辨率的提高和着陆器在火星近表面的观测,发现火星上部分区域的沙丘和沙波纹比较活跃。同地球一样,火星风沙地貌也可指示火星气候的变化。     

A CA-based land system change model: LANDSCAPE

Cellular automata (CA) models are widely used to simulate land-use changes because of their simplicity, flexibility, intuitiveness and ability to incorporate the spatial and temporal dimensions of processes. A small number of CA-based models have been developed to simulate changes in multiple land uses, most of which use the hierarchical allocation strategy and/or inertia factors to enable these CA models to do so accurately. However, only some of these models allow explicit determination of the allocation sequence for active land uses according to the hierarchical allocation strategy and the objective calculation of inertia factors. In this paper, we proposed a CA-based model, i.e. the LAND System Cellular Automata model for Potential Effects (LANDSCAPE), with a hierarchical allocation strategy and resistances, to simulate changes in multiple land uses. Furthermore, we introduced effective ways to objectively determine the allocation sequence for active land uses and calculate resistances for individual land uses. The results show that the LANDSCAPE model, with a calibrated allocation sequence and resistances, is reliable and accurate for simulating multiple land-use changes.     

矢量数据在多尺度栅格化中的精度损失模型探讨

在进行空间分析时,由于栅格数据便于空间分析,因而通常将矢量数据转化成栅格数据进行空间分析。在转化过程中,采用不同尺度的栅格大小会造成怎样的精度损失?精度损失、栅格大小和平均斑块大小之间的关系是否可以用模型来表达?本文正是针对这样的问题,以重庆市1∶10万的土地利用矢量数据为例,探讨了在不同尺度栅格大小(从30m×30m到1000m×1000m)的情况下,各种土地利用类型在转化过程中的精度损失情况,对精度损失与栅格大小和平均斑块大小之间的关系进行了定量分析。研究表明,精度损失与地类的平均斑块大小和栅格尺度之间的关系可以用模型来表达。该模型为:Y=5.366-0.179X-0.978ln(S)+0.0348Xln(S)。模型中Y为精度损失,S为地类的平均斑块大小,X表示栅格的大小。该模型的复相关系数为0.93。     

A comparison of methods used to estimate the height of sand dunes on Mars

The collection of morphometric data on small-scale landforms from other planetary bodies is difficult. We assess four methods that can be used to estimate the height of aeolian dunes on Mars. These are (1) stereography, (2) slip face length, (3) profiling photoclinometry, and (4) Mars Orbiter Laser Altimeter (MOLA). Results show that there is good agreement among the methods when conditions are ideal. However, limitations inherent to each method inhibited their accurate application to all sites. Collectively, these techniques provide data on a range of morphometric parameters, some of which were not previously available for dunes on Mars. They include dune height, width, length, surface area, volume, and longitudinal and transverse profiles. The utilization of these methods will facilitate a more accurate analysis of aeolian dunes on Mars and enable comparison with dunes on other planetary surfaces.     

基于GPS数据的MODIS近红外水汽线性回归模型

针对地基GPS反演水汽空间不连续以及MODIS近红外水汽产品精度不足的问题,利用2010年香港地区地基GPS水汽数据和MODIS近红外水汽数据,提出了一种基于地基GPS订正MODIS水汽产品继而得到高精度空间连续分布可降水汽的方法。通过比较3种干延迟模型,选取最适合香港地区的Hopfield模型并利用高精度软件GAMIT解算GPS水汽,建立GPS水汽与MODIS水汽之间的线性模型,以实现对MODIS近红外水汽产品的逐月订正。结果表明,经过订正的MODIS水汽产品在各月的MRE、RMSE、PBIAS均有所改善,该方法能够有效融合地基GPS和MODIS的优点,可以得到连续区域的高精度水汽数据,这对天气预报、气候监测等有重要意义。     

Comment on 'Estimation of resolution and covariance for large matrix inversions' by J. Zhang and G. A. McMechan

When inverting large matrices, iterative techniques are necessary because of their speed and low memory requirements, as opposed to singular value decomposition (SVD). Recently, there have been attempts to obtain information on the quality of the solutions calculated using conjugate gradient (CG) methods such as LSQR. The purpose of this note is to comment on the paper titled "Estimation of resolution and covariance for large matrix inversions' by Zhang & McMechan (1995), who extend Paige and Saunders' LSQR algorithm to obtain an orthonormal basis used to approximate resolution and covariance. We show that for larger problems, where the number of orthogonal vectors is several orders of magnitude smaller than the number of model parameters, the vectors obtained do not adequately span the range of the model space. We use a synthetic borehole experiment to illustrate the differences between the singular value spectrum obtained through the more complete method of SVD and the Ritz value spectrum that results from a simple extension of LSQR, We also present a trivial numerical example to illustrate the differences between Zhang & McMechan's approximate resolution matrix and the true resolution.     

On the amplitude of the Chandler wobble

The change in the inertia tensor of the Earth, due to the mass shift following a seismic event, has been computed by several authors for non-rotating earth models. Rotation is taken into account in the present paper, and the additional change in the inertia tensor is computed for an equivalent earth model, in which the axis of geometrical symmetry becomes tilted instead of the axis of greatest inertia. Rotation is thus seen to produce an increase by a factor 1.4 in the amplitude variation of the Chandler wobble, with respect to the non-rotating case, which, when added to the 1.4 amplitude increase due to the precessional re-adjustment of the equatorial bulge, gives a factor of 2 increase of the Chandler wobble amplitude with respect to the case of a rigid earth model.     

Coseismic rotation changes from the 2004 Sumatra earthquake: the effects of Earth's compressibility versus earthquake induced topography

Polar motion is modelled for the large 2004 Sumatra earthquake via dislocation theory for an incompressible elastic earth model, where inertia perturbations are due to earthquake-triggered topography of density–contrast interfaces, and for a compressible model, where inertia perturbation due to compression-dilatation of Earth's material is included; density and elastic parameters are based on a multilayered reference Earth. Both models are based on analytical Green's functions, propagated from the centre to the Earth's surface. Preliminary and updated seismological solutions are considered in elucidating the effects of improving earthquake parameters on polar motion. The large Sumatra thrust earthquake was particularly efficient in driving polar motion since it was responsible for large material displacements occurring orthogonally to the strike of the earthquake and to the Earth's surface, as imaged by GRACE gravity anomalies over the earthquake area. The effects of earthquake-induced topography are four times larger than the effects of Earth's compressibility, for l = 2 geopotential components. For varying compressional Earth properties and seismic solution, modelled polar motion ranges from 8.6 to 9.4 cm in amplitude and between 117° and 130° east longitude in direction. The close relationship between polar motion direction, earthquake longitude and thrust nature of the event, are established in terms of basic physical concepts.     

西北地区气候因素对沙尘暴影响的模型研究

气候因素是沙尘暴形成的必要条件之一,建立适合于西北地区衡量气候因素对沙尘暴影响的定量模型极为重要。在前人的沙尘天气模型基础上,强调在西北地区利用温度、降水量、蒸发量综合考虑地区水分均衡来计算干燥指数,并与大风日数和风速计算的风速影响指数共同建立新的沙尘暴气候影响指数模型。将模型实际应用于新疆、青海、甘肃、陕西、宁夏和内蒙古等西北6省区,利用6省区1961—1980年的气象资料进行回归分析,结果表明,气候影响指数D和沙尘暴日数S之间具有良好的相关性。并选取1981—2005年陕西、甘肃和内蒙古3地气象资料计算的沙尘暴日数预测值与3省区的实际值作比较,发现模型拟合程度较好,且在揭示气候影响因子对沙尘暴影响作用方面,效果较为显著。     

Paleolakes on Mars

Observational evidence such as outflow channels and valley networks suggest that in the past there was flowing water on Mars. The images of fluvial features on Mars logically suggest that there must exist downstream locations in which the water pooled and the sediment load deposited (i.e. lakes). Sediments and morphological features associated with the martian paleolakes are believed to occur in Valles Marineris, and several large basins including Amazonis, Chryse and Elysium planitia. As Mars became progressively colder over geological time, any lakes on its surface would have become seasonally, and eventually perennially ice-covered. We know from polar lakes on Earth that ice-covered lakes can persist even when the mean annual temperature falls below freezing. Thus, the most recent lacustrine sediments on Mars were probably deposited in ice-covered lakes. While life outside of the Earth's atmosphere has yet to be observed, there is a general consensus among exobiologists that the search for extraterrestrial life should be based upon liquid water. The inference that there was liquid water on Mars during an earlier epoch is the primary motivation for considering the possibility of life during this time. It would be of enormous interest from both an exobiological and paleolimnological perspective to discover lakes or the evidence of former lakes on another planet such as Mars. Limnology would then become an interplanetary science.     

Mars landscape evolution: influence of stratigraphy on geomorphology in the north polar region

Lithology and physical properties of strata exposed at the Earth's surface have direct influence on the erosion and geomorphic expression of landforms. While this is well known on our planet, examples on Mars are just coming to light among the tens of thousands of airphoto-quality images (resolutions 1.5–12 m/pixel) acquired since 1997 by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). Specific examples occur among martian north polar layered materials, which MOC images reveal are divided into two distinct stratigraphic units: a lower, dark-toned layered unit and a younger, upper, lighter-toned layered unit. The lower unit is less resistant to wind erosion than the upper unit. The upper unit most likely consists of stratified dust and ice, while the lower unit contains abundant, poorly cemented sand. Sand is more easily mobilized by wind than dust; the lower resistance to erosion of the lower unit results from the presence of sand. Where wind erosion in polar troughs has penetrated to the lower unit, geomorphic change has proceeded more rapidly: sand has been liberated from the lower unit, and arcuate scarps have formed as the upper unit has been undermined. Wind erosion of the lower unit thus influences the geomorphology of the north polar region; this result likely explains the genesis of the large polar trough, Chasma Boreale, and the relations between dunes and arcuate scarps that have puzzled investigators for nearly three decades. The properties of the stratigraphic units suggest that the upper limit for the amount of water contained in the north polar layered materials may be 30–50% less than previously estimated.     

On the density distribution within the Earth

The distribution of density as a function of position within the Earth is much less well constrained than the seismic velocities. The primary information comes from the mass and moment of inertia of the Earth and this information alone requires that there be a concentration of mass towards the centre of the globe. Additional information is to be found in the frequencies of the graver normal modes of the Earth which are sensitive to density through self-gravitation effects induced in deformation.
  The present generation of density models has been constructed using linearized inversion techniques from earlier models, which ultimately relate back to models developed by Bullen and based in large part on physical arguments. A number of experiments in non-linear inversion have been conducted using the PREM reference model, with fixed velocity and attenuation, but with the density model constrained to lie within fixed bounds on both density and density gradient. A set of models is constructed from a uniform probability density within the bound and slope constraints. Each of the resultant density models is tested against the mass and moment of inertia of the Earth, and for successful models a comparison is made with observed normal mode frequencies. From the misfit properties of the ensemble of models the robustness of the density profile in different portions of the Earth can be assessed, which can help with the design of parametrization for future reference models. In both the lower mantle and the outer core it would be desirable to allow a more flexible representation than the single cubic polynomial employed in PREM.     

遥感估算热惯量研究的回顾与展望

热惯量是影响地表温度变化的一个重要因子,随着热红外数据反演地表温度算法的不断完善,遥感技术已 成为估算热惯量的主要手段。本文回顾了近些年来遥感估算热惯量研究的进展,对其中3 种主要方法即地表温度 日较差法、日出日落温差法及模型反演法进行了评述,重点讨论了地表温度日较差估算热惯量方法,并提出从加强 机理研究、尺度转换及采用多时相多光谱数据等方面进一步完善遥感估算热惯量方法、提高其估算精度的设想。