Harmonic signals with a fundamental period near the GPS draconitic year (351.2 days) and overtones up to at least the sixth multiple have been observed in the power spectra of nearly all products of the International GNSS Service (IGS), including station position time series, apparent geocenter motions, orbit jumps between successive days, and midnight discontinuities in earth orientation parameter (EOP) rates. Two main mechanisms have been suggested for the harmonics: mismodeling of orbit dynamics and aliasing of near-sidereal local station multipath effects. Others have studied the propagation of local multipath errors into draconitic position variations, but orbit-related processes have been less examined. We elaborate our earlier analysis of GPS day-boundary orbit discontinuities where we observed some draconitic features as well as prominent spectral bands near 29-, 14-, 9-, and 7-day periods. Finer structures within the sub-seasonal bands fall close to the expected alias frequencies for 24-h sampling of sub-daily EOP tide lines but do not coincide precisely. While once-per-revolution empirical orbit parameters should strongly absorb any sub-daily EOP tide errors due to near-resonance of their respective periods, the observed differences require explanation. This has been done by simulating EOP tidal errors and checking their impact on a long series of estimated daily GPS orbits and EOPs. Indeed, simulated tidal aliases are found to be very similar to the observed IGS orbital features in the sub-seasonal bands. Moreover and unexpectedly, some low draconitic harmonics were also produced, potentially a source for the widespread errors in most IGS products. The results from this study are further evidence for the need of an improved sub-daily EOP tide model. 相似文献
We explore the causes and predictability of extreme low minimum temperatures (Tmin) that occurred across northern and eastern Australia in September 2019. Historically, reduced Tmin is related to the occurrence of a positive Indian Ocean Dipole (IOD) and central Pacific El Niño. Positive IOD events tend to locate an anomalous anticyclone over the Great Australian Bight, therefore inducing cold advection across eastern Australia. Positive IOD and central Pacific El Niño also reduce cloud cover over northern and eastern Australia, thus enhancing radiative cooling at night-time. During September 2019, the IOD and central Pacific El Niño were strongly positive, and so the observed Tmin anomalies are well reconstructed based on their historical relationships with the IOD and central Pacific El Niño. This implies that September 2019 Tmin anomalies should have been predictable at least 1–2 months in advance. However, even at zero lead time the Bureau of Metereorolgy ACCESS-S1 seasonal prediction model failed to predict the anomalous anticyclone in the Bight and the cold anomalies in the east. Analysis of hindcasts for 1990–2012 indicates that the model's teleconnections from the IOD are systematically weaker than the observed, which likely stems from mean state biases in sea surface temperature and rainfall in the tropical Indian and western Pacific Oceans. Together with this weak IOD teleconnection, forecasts for earlier-than-observed onset of the negative Southern Annular Mode following the strong polar stratospheric warming that occurred in late August 2019 may have contributed to the Tmin forecast bust over Australia for September 2019.
In order to better predict the geometry and spatial distribution of reservoir facies and permeability barriers, it is critical to understand stratigraphic geometries and architecture at all scales within a basin. Stratigraphic forward modelling (SFM) allows geologists to forward project the deposition and evolution of sedimentary facies within a stratigraphic framework with given prior boundary conditions. The method can approximate depositional processes while taking into consideration a range of factors that affect basin evolution and sedimentation. This paper presents an overview of recent developments in stratigraphic forward models that are currently available and have been successfully employed in simulating stratigraphic development, including geometric models, diffusion models, Fuzzy Logic models and hydraulic models. In addition, the paper discusses SFM used in the simulation of the behaviour of natural sedimentary systems and verification of the potential for hydrocarbon entrapment and accumulation in a basin using an example from the Ordos Basin, western China. 相似文献
A brief summary is presented of requirements for the measurements of extragalactic γ-ray lines. The electron-positron annihilation line at 511 keV represents the best prospect, and although this line is greatly broadened in active galactic nuclei, a narrow line should be present in clusters of galaxies and radio lobes as a result of prior AGN activity. The strongest fluxes should be of the order of 10−4 photons cm−2 s−1 from the closest extended sources. 相似文献
A finite element approach is used to assess the transient stability of excavation in elasto-plastic soils. Stability is shown to be a function of the rate of excavation, the soil permeability and the drainage path lengths. Sequential excavation has been modelled rigorously in the finite element analysis, together with the transient effects through a fully coupled Biot formulation. Results are presented which demonstrate the effects of the rate of excavation and permeability on stability. Both drained and undrained behaviour of the problem are retrieved as special cases of the transient analysis, and comparisons made with classical solutions where available. 相似文献
Hydrographic data from the Iceland-Faeroes Ridge region, covering an area of 59°–69° north and 0°–20° west, have been subjected to a spatial data analysis. The analysis consists of the two-dimensional spectral method (2D-FFT) and the empirical eigenfunction method (EOF). Results from the two methods show good agreement indicating that the significant length scales for horizontal variability present in the data have wavelengths of approximately 339 km, 72 km and 37 km. These wavelengths relevant to the Iceland-Faeroes front are interpreted as the sizes respectively of the warm water intrusion from the south of the front with a diameter of half the wavelength (i.e.,?169 km), and the meander-like and eddy-like features. Predictions of the same frontal system have also been made using a realistic numerical model to provide hydrographic outputs similar to the observations. The hydrographic outputs from the model have been subjected to the same 2D-FFT method to establish the spatial length scale present in the model predictions. A comparison of results from a spectral data analysis between the field measurements and the model predictions shows that the model can predict the sizes of the meander-like and the eddy-like features quite accurately. However, predictions of frontal orientation, frontal slopes and size of the warm water intrusion still require further study. The work presented also demonstrates the importance of spatial statistics in oceanographic research, particularly in ocean predictability studies. 相似文献
Abstract A non‐linear response in current‐meter compasses is examined in terms of its effect on the measured residual current. It is shown that, even for a compass response within typical manufacturers’ specifications, the induced errors are important for regions where the residual speed is less than about 10% of the peak tidal current. In an M2 current, the non‐linearity also induces an M4 signal, but this is not sufficiently large to be easily detected by tidal analyses. 相似文献