Modeling and forecast of the polar motion excitation functions for short-term polar motion prediction

Short-term forecast of the polar motion is considered by introducing a prediction model for the excitation function that drives the polar motion dynamics. The excitation function model consists of a slowly varying trend, periodic modes with annual and several sub-annual frequencies (down to the 13.6-day fortnightly tidal period), and a transient decay function with a time constant of 1.5 days. Each periodic mode is stochastically specified using a second-order auto-regression process, allowing its frequency, phase, and amplitude to vary in time within a statistical tolerance. The model is used to time-extrapolate the excitation function series, which is then used to generate a polar motion forecast dynamically. The skills of this forecast method are evaluated by comparison to the C-04 polar motion series. Over the lead-time horizon of four months, the proposed method has performed equally well to some of the state-of-art polar motion prediction methods, none of which specifically features forecasting of the excitation function. The annual mode in the ₂ component is energetically the most dominant periodicity. The modes with longer periods, annual and semi-annual in particular, are found to contribute more significantly to forecast accuracy than those with shorter periods.     

Evidence and cause of climate cycles in polar motion

 The long-term variation of polar motion contains a number of periods similar to climate cycles. Two possible causes for these long-term variations are mass redistributions produced by variations of atmospheric and oceanic circulation, and mass exchanges between the cryosphere and hydrosphere. Inner-core wobble, which can be inferred from the observed motion of the geomagnetic pole, is another phenomenon with periods similar to climate cycles. Only observations relating to mass redistributions caused by atmosphere dynamics and inner-core wobble are available for sufficiently long periods of time to investigate their influence on climate cycles in polar motion. Both processes contribute to climate cycles in polar motion, but they cannot completely explain these cycles. Possible sources of climate cycles are discussed. Received: 20 December 1999 / Accepted: 28 August 2000     

Parameter variability of the observed periodic oscillations of polar motion with smaller amplitudes

Compared to the Chandler and annual wobbles, the higher-frequency components of polar motion (PM) have substantially smaller amplitudes. Therefore, their study has had to wait until higher-quality time series with high temporal resolution, as measured by space geodetic techniques, have become available. Based on the combined Earth orientation series SPACE99 computed by the Jet Propulsion Laboratory (JPL) from 1976 to 2000 at daily intervals, the periodic PM terms, in particular at the quasi-biennial, 300-day, semi-Chandler, semi-annual, 4-month, 90-day, 2-month and 1.5-month periods, have been separated by band-pass filtering and it has been found that the persistence of oscillations becomes less with increasing frequency. In order to quantify and better describe the parameter variability of these PM components over time, the radii, direction angles and period lengths were computed from the periodic terms filtered out from the time series. The results clearly show the characteristics and time evolution of the periodic PM components. The largest elliptic oscillation is the semi-annual wobble with a maximum semi-major axis of up to 13 mas (milliarc seconds). The other wobbles are smaller. They have maximum semi-major axes of between 3 and 8 mas. If the oscillations have period lengths of 4 months and less, then they are elapsed not only progradly, but also retrogradly. AcknowledgementsThis paper was presented at the 27th General Assembly of the European Geophysical Society in Nice, France, 22–26 April 2002. Thanks go to Kevin Fleming for his linguistic advice. The author would also like to thank Barbara Koaczek for suggesting some valuable improvements.     

A study of gravity variations caused by polar motion using superconducting gravimeter data from the GGP network

Long-term continuous gravity observations, recorded at five superconducting gravimeter (SG) stations in the Global Geodynamic Project (GGP) network, as well as data on orientation variations in the Earths rotation axis (i.e. polar motion), have been used to investigate the characteristics of gravity variations on the Earths surface caused by polar motion. All the SG gravity data sets were pre-processed using identical techniques to remove the luni-solar gravity tides, the long-term trends of the instrumental drift, and the effects of atmospheric pressure. The analysis indicates that the spectral peaks, related to the Chandler and annual wobbles, were identified in both the power and product spectral density estimates. The magnitude of gravity variations, as well as the gravimetric amplitude factor associated with the Chandler wobble, changed significantly at different SG stations and during different observation periods. However, when all the SG observations at these five sites were combined, the gravimetric parameters of the Chandler wobble were retrieved accurately: 1.1613 ± 0.0737 for the amplitude factor and –1°.30 ± 1°.33 for the phase difference. The value of the estimated amplitude factor is in agreement with that predicted theoretically for the zonal tides of an elastic Earth model.     

Linear drift and periodic variations observed in long time series of polar motion

 Two long time series were analysed: the C01 series of the International Earth Rotation Service and the pole series obtained by re-analysis of the classical astronomical observations using the HIPPARCOS reference frame. The linear drift of the pole was determined to be 3.31 ± 0.05 milliarcseconds/year towards 76.1 ± 0.80° west longitude. For the least-squares fit the a priori correlations between simultaneous pole coordinates x _p , y _p were taken into account, and the weighting function was calculated by estimating empirical variance components. The decadal variations of the pole path were investigated by Fourier and wavelet analysis. Using sliding windows, the periods and amplitudes of the Chandler wobble and annual wobble were determined. Typical periods in the variable Chandler wobble and annual wobble parameters were obtained from wavelet analyses. Received: 21 January 2000 / Accepted: 28 August 2000     

An improved empirical model for the effect of long-period ocean tides on polar motion

Because the tide-raising potential is symmetric about the Earth’s polar axis it can excite polar motion only by acting upon non-axisymmetric features of the Earth like the oceans. In fact, after removing atmospheric and non-tidal oceanic effects, polar motion excitation observations show a strong fortnightly tidal signal that is not completely explained by existing dynamical and empirical ocean tide models. So a new empirical model for the effect of the termensual (Mtm and mtm), fortnightly (Mf and mf), and monthly (Mm) tides on polar motion is derived here by fitting periodic terms at these tidal frequencies to polar motion excitation observations that span 2 January 1980 to 8 September 2006 and from which atmospheric and non-tidal oceanic effects have been removed. While this new empirical tide model can fully explain the observed fortnightly polar motion excitation signal during this time interval it would still be desirable to have a model for the effect of long-period ocean tides on polar motion that is determined from a dynamical ocean tide model and that is therefore independent of polar motion observations.     

The impact of errors in polar motion and nutation on UT1 determinations from VLBI Intensive observations

The earth’s phase of rotation, expressed as Universal Time UT1, is the most variable component of the earth’s rotation. Continuous monitoring of this quantity is realised through daily single-baseline VLBI observations which are interleaved with VLBI network observations. The accuracy of these single-baseline observations is established mainly through statistically determined standard deviations of the adjustment process although the results of these measurements are prone to systematic errors. The two major effects are caused by inaccuracies in the polar motion and nutation angles introduced as a priori values which propagate into the UT1 results. In this paper, we analyse the transfer of these components into UT1 depending on the two VLBI baselines being used for short duration UT1 monitoring. We develop transfer functions of the errors in polar motion and nutation into the UT1 estimates. Maximum values reach 30 [μs per milliarcsecond] which is quite large considering that observations of nutation offsets w.r.t. the state-of-the-art nutation model show deviations of as much as one milliarcsecond.     

对分量进行联合解算的极移预报方法研究

为了提高极移预报的精度,该文提出结合极移的两个分量进行统一求解的极移联合预报方法:将极移的两个分量组成一个观测方程,并采用LS+AR模型对联合分量的确定项与随机项进行拟合。实验结果表明,文中采用的联合预报方法可以提高极移的预报精度。     

Atmospheric excitation of polar motion

The polar motion excited by the fluctuation of global atmospheric angular momentum (AAM) is investigated. Based on the global AAM data, numerical results demonstrate that the fluctuation of AAM can excite the seasonal wobbles (e.g., the 18-month wobble) and the Chandler wobble, which agree well with previous studies. In addition, by filtering the dominant low frequency components, some distinct polar wobbles corresponding to some great diurnal and semi-diurnal atmospheric tides are found.     

Atmospheric excitation of polar motion

The polar motion excited by the fluctuation of global atmospheric angular momentum (AAM) is investigated. Based on the global AAM data, numerical results demonstrate that the fluctuation of AAM can excite the seasonal wobbles (e.g., the 18-month wobble) and the Chandler wobble, which agree well with previous studies. In addition, by filtering the dominant low frequency components, some distinct polar wobbles corresponding to some great diurnal and semi-diurnal atmospheric tides are found.     

Comparison of polar motion with oceanic and atmospheric angular momentum time series for 2-day to Chandler periods

During a 4-year period starting in July 1996 and using intervals ranging from 3 days to 4 years, four precise polar motion (PM) series have been compared to excitation by atmospheric angular momentum (AAM) augmented with oceanic angular momentum (OAM) data. The first three series (C03, C04 and Bulletin A) are multi-technique combinations generated by the International Earth Rotation and Reference Systems Service (IERS) and the fourth combined series (IGS00P02) is produced by the International GPS Service (IGS) using only GPS data. The IGS PM compared the best with the combined excitations of atmosphere and oceans (AAM+OAM) at all intervals, showing high overall correlation of 0.8–0.9. Even for the interval of only three days, the IGS PM gave a significant correlation of about 0.6. Moreover, during the interval of February 1999 – July 2000, which should be representative of the current precision of the IGS PM, a significant correlation (>0.4) extended to periods as short as 2.2 days and 2.5 days for the x_p and y_p PM components, respectively. When using the IERS Bulletin B (C04) PM and an interval of almost 6 years, starting in November 1994, the combined OAM+AAM accounted for practically all the annual, semi-annual and Chandler wobble (CW) PM signals. When only AAM was used, either the US National Centers for Environment Prediction reanalysis data, which were used throughout this study, or the Japanese Meteorological Agency data, two large and well-resolved amplitude peaks of about 0.1 mas/day, remained at the retrograde annual and CW periods.     

基于新参考系的极移改正

本文利用IAU2000决议中关于CIP的定义阐述了极移坐标系和极移坐标的定义,在总结微分旋转矩阵的性质的基础上给出了极移旋转矩阵的详细推导,结合极移产生的主要原因,详细介绍了目前国际上关于极移模型化工作的最新进展,最后给出了获取极移坐标和TIO位置的方法和途径。     

Hydrological excitation of polar motion by different variables from the GLDAS models

Continental hydrological loading by land water, snow and ice is a process that is important for the full understanding of the excitation of polar motion. In this study, we compute different estimations of hydrological excitation functions of polar motion (as hydrological angular momentum, HAM) using various variables from the Global Land Data Assimilation System (GLDAS) models of the land-based hydrosphere. The main aim of this study is to show the influence of variables from different hydrological processes including evapotranspiration, runoff, snowmelt and soil moisture, on polar motion excitations at annual and short-term timescales. Hydrological excitation functions of polar motion are determined using selected variables of these GLDAS realizations. Furthermore, we use time-variable gravity field solutions from the Gravity Recovery and Climate Experiment (GRACE) to determine the hydrological mass effects on polar motion excitation. We first conduct an intercomparison of the maps of variations of regional hydrological excitation functions, timing and phase diagrams of different regional and global HAMs. Next, we estimate the hydrological signal in geodetically observed polar motion excitation as a residual by subtracting the contributions of atmospheric angular momentum and oceanic angular momentum. Finally, the hydrological excitations are compared with those hydrological signals determined from residuals of the observed polar motion excitation series. The results will help us understand the relative importance of polar motion excitation within the individual hydrological processes, based on hydrological modeling. This method will allow us to estimate how well the polar motion excitation budget in the seasonal and inter-annual spectral ranges can be closed.     

Temporal variations of the polar moment of inertia and the second-degree geopotential

In order to study the geodynamic behaviour of the Earth over short (elastic Earth) and long (almost perfectly liquid Earth) geological periodic variations, the changes of the moment of inertia are decomposed into two parts: the first, described by a volume integral, explains the effect of the density variations, while the second gives the impact of the surface variations using a surface integral. It is shown that both components have physical significance, but their contribution is different in case of short (lunisolar) and long (connected to secular despinning) periods.     

Period variations of the Chandler wobble

Variations in the period of the Chandler wobble have been discussed since its discovery by Chandler in 1892. Various authors engaged in the investigation of polar motion time series suggest both a variable and an invariable period. It cannot be resolved by the analysis of time series whether the Chandler period is variable. By studying the influence of mass redistributions on the Chandler period it has been found that it is in fact variable, but the magnitude of such variation is much smaller than that found by polar motion time series analysis. For the currently available time series of polar motion, it is sufficient to assume an invariable Chandler period. AcknowledgmentsUseful discussions with Dr. F. Barthelmes and Dr. K. Fleming are gratefully appreciated.     

Optimising the accuracy of a low-cost photogrammetric motion study system

Generally, a low-cost image-based motion study system consists of a set of two or more video imaging cameras and a set of object space control targets. The control targets are required to provide for the computation of exterior parameters of the video frames of objects in motion. Subsequently, the computed exterior orientation parameters are used to compute the position of the motion targets. In general, the accuracy of the motion data is dependent largely on the accuracy of the 3D coordinates of the control targets, the computed camera and lens parameters and the frame rate of the camera. Obviously, it is difficult to improve the frame rate of a low-cost camera; however, the other factors may be optimised analytically. Optimising the accuracy of the control targets is a straightforward process and is discussed briefly in the paper. Optimising the computed camera and lens parameters was the main focus of the research. Consequently, the paper provides the detail of the developed optimising technique. The results show that an optimal principal distance and other lens parameters can be determined by analysing the error of a set of highly accurate object distances. The evaluation shows that the accuracy of the video motion study system can be improved by as much as six times or a reduction of scaling error from 1·06 to 1·01.     

军用车辆运动速度数学模型

本文以军事地形分析应用为背景,分析了决定军用(轮式、履带)车辆运动速度的主要因素;以动力平衡为基础,建立了各有关因素与运动速度之间的函数关系,给出了求解方法,并指出了实际运用时应注意的问题。试算表明,该方法计算结果与试验统计数据基本吻合。     

A transboundary study of urban sprawl in the Pacific Coast region of North America: The benefits of multiple measurement methods

Sprawling urban development has emerged as a primary concern of policy makers, land preservationists and both urban and rural communities in developing regions across the globe. For the first time in history, more global residents lived in urban areas than not and the trend to urbanization is in full force at the start of the 21st century. An understanding of the nature and character of urban sprawl is complicated by a failure to satisfactorily define it and by the limitations of measurement techniques designed to characterize complex landscape forms. Like other landscape patterns, the quantification of urban sprawl is highly spatially and temporally scale-dependent. This paper summarizes a recent project to measure urban sprawl in the transboundary region of the Pacific Coast of North America. The metropolitan centers of Portland, OR, Seattle, WA and Vancouver, BC, span two nations, three state/provincial governments and dozens of cities. As a region, this was a global leader in population growth in the 1990s. The study relied on three separate methods – an impervious surface metric, a neighborhood density metric and a building permit metric – for quantifying urban growth. The results provide insight on the strengths and shortcomings of different methods with respect to the challenges posed by data availability and format. Taken together they demonstrate the richer understanding that combined methods may offer in characterizing phenomena as difficult to communicate and agree upon as urban sprawl.     

卫星高度计的北太平洋中尺度涡时空分析

本文利用1993年—2012年AVISO卫星高度计融合数据,识别和追踪了北太平洋海域(100°E—77°W,0°N—70°N)20年的中尺度涡。统计分析了北太平洋中尺度涡的生命周期、振幅、移动速度等属性特征、空间分布和运动特征、季节、年际和年代际的变化趋势及其与ENSO的关系。结果表明:北太平洋中尺度涡的平均寿命为6.9周,平均振幅为8.44 cm,平均速度为6.4 cm/s;随着纬度的增加,反气旋涡的生命周期和振幅差异较大,气旋涡差异较小,涡旋的移动速度随纬度的增加逐渐减少。在空间分布上,日本东部的黑潮延伸区、加利福尼亚海岸和阿拉斯加湾是涡旋的高发区,其中黑潮延伸区分布最为密集;涡旋大多数向西运动,只有少数涡旋向东传播,移动过程中反气涡旋表现为向赤道方向偏移,气旋涡向极地方向偏移,且具有明显的非线性特征。在季节、年际尺度上,春季和夏季是涡旋高发的两个季节,秋季和冬季生成的涡旋相对最少,在加利福尼亚沿岸涡旋的季节差异较明显;涡旋数量的年际变化与ENSO事件具有明显的相关性,1993年—2002年厄尔尼诺年生成的涡旋较多,而拉尼娜年生成的涡旋较少,2003年—2012年则是厄尔尼诺年生成的涡旋较少,而拉尼娜年生成的涡旋较多。     

北太平洋中尺度涡海表温度和叶绿素浓度特征分析

本文利用AVISO卫星高度计资料识别并追踪了北太平洋2007年—2012年的中尺度涡,并利用OSTIA的海表温度SST(Sea Surface Temperature)资料与MODIS的叶绿素a浓度(Chl-a)资料,研究了北太平洋2007年—2012年中尺度涡SST和Chl-a浓度的时空分布特征,并分析北太平洋典型中尺度涡SST与Chl-a浓度的变化特征,主要结论如下:本文共识别出992个中尺度涡,其中442个气旋涡,550个反气旋涡。中尺度涡SST时空分布特征为:气旋涡温度强度(ICE)月变化特征比反气旋涡温度强度(IAE)更强。ICE年际变化显著,IAE则不明显。温度强度较强的气旋涡和反气旋涡集中分布在黑潮延伸区。中尺度涡Chl-a浓度时空分布特征如下:气旋涡和反气旋涡Chl-a浓度月变化特征明显,且二者的变化趋势一致;年际变化则均不明显。Chl-a浓度值高的中尺度涡主要分布在高纬海域。中尺度涡SST与海洋动力参数(振幅、涡度和涡动能(EKE))的相互关系为:反气旋涡SST与振幅的相关性亦正亦负,且在空间上均匀分布。气旋涡SST与振幅的负相关系数主要分布在黑潮延伸区。正相关性强的反气旋涡多于气旋涡。反气旋涡SST与涡度的相关性亦正亦负,气旋涡SST与涡度呈负相关。反气旋涡SST与EKE的相关性亦正亦负;气旋涡的相关性为正。中尺度涡Chl-a浓度与海洋动力参数的相互关系为:反气旋涡Chl-a浓度与振幅的相关性为正,且在空间上均匀分布;气旋涡在黑潮延伸区与阿拉斯加湾呈正相关。反气旋涡和气旋涡Chl-a浓度与涡度均呈正相关。反气旋涡Chl-a浓度与EKE呈正相关;气旋涡Chl-a浓度与EKE相关性亦正亦负。