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41.
Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves Hs>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max Hs: 8.4 m) and the second on 20–22 February 2004 (max Hs: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events. 相似文献
42.
P. Stauning Oleg Troshichev Alexander Janzhura 《Journal of Atmospheric and Solar》2008,70(18):2246-2261
The polar geomagnetic activity resulting from solar wind–magnetosphere interactions can be characterized the Polar Cap (PC) indices, PCN and PCS. PC index values are derived from polar magnetic variations calibrated on a statistical basis such that the index approximate values in units of mV/m of the interplanetary “geo-effective” (or “merging”) electric field (EM) conveyed by the solar wind. The timing and amplitude relations of the PC index to solar wind plasma and magnetic field parameters are reported. The solar wind effects are parameterized in terms of the geo-effective electric field (EM) and the dynamical pressure (PDYN). The PC index has a delayed and damped response to EM variations and display saturation-like effects for EM values exceeding 10 mV/m. Steady or slowly varying levels of solar wind dynamical pressure have little or no impact on the PC index above the effects related to EM for which the solar wind velocity is also a factor. Sharp increases in the dynamical pressure generate impulsive variations in the PC index comprising a initial negative impulse of 5–10 min duration followed by a positive impulse lasting 10–20 min. Typical amplitudes of both the negative and the positive impulses are 0.2–0.5 units. A sharp decrease in the pressure produces the inverse sequence of pulses in the PC index. Auroral substorm activity represented by the AL index level has a marked influence on the average PC/EM level at the transition from very quiet (AL0 nT) to disturbed conditions while more or less disturbed conditions (AL<−100 nT) have no systematic effect on the average PC/EM values. At distinct substorm events the PC/EM ratio has a minimum (0.8) in the pre-onset phase at around 20 min before substorm onset. The average ratio gradually increases in the expansion phase to reach a maximum value (1.1) at around 40 min after substorm onset (or 20 min after the largest (negative) peak in AL). At substorm recovery during the next 2 h the PC/EM ratio decreases. Finally, we report on the application of polar magnetic variations to model the disturbance storm time (Dst) index development during magnetic storms by using the PC index as a source function to quantify the energy input to the ring current representing accumulated storm energy and characterized by the Dst index. 相似文献
43.
The Cartesian moments of the mass density of a gravitating body and the spherical harmonic coefficients of its gravitational
field are related in a peculiar way. In particular, the products of inertia can be expressed by the spherical harmonic coefficients
of the gravitational potential as was derived by MacCullagh for a rigid body. Here the MacCullagh formulae are extended to
a deformable body which is restricted to radial symmetry in order to apply the Love–Shida hypothesis. The mass conservation
law allows a representation of the incremental mass density by the respective excitation function. A representation of an
arbitrary Cartesian monome is always possible by sums of solid spherical harmonics multiplied by powers of the radius. Introducing
these representations into the definition of the Cartesian moments, an extension of the MacCullagh formulae is obtained. In
particular, for excitation functions with a vanishing harmonic coefficient of degree zero, the (diagonal) incremental moments
of inertia also can be represented by the excitation coefficients. Four types of excitation functions are considered, namely:
(1) tidal excitation; (2) loading potential; (3) centrifugal potential; and (4) transverse surface stress. One application
of the results could be model computation of the length-of-day variations and polar motion, which depend on the moments of
inertia.
Received: 27 July 1999 / Accepted: 24 May 2000 相似文献
44.
V. M. Hamza R. R. Cardoso C. F. Ponte Neto 《International Journal of Earth Sciences》2008,97(2):205-226
A reappraisal of the international heat flow database has been carried out and the corrected data set was employed in spherical
harmonic analysis of the conductive component of global heat flow. Procedures used prior to harmonic analysis include analysis
of the heat flow data and determination of representative mean values for a set of discretized area elements of the surface
of the earth. Estimated heat flow values were assigned to area elements for which experimental data are not available. However,
no corrections were made to account for the hypothetical effects of regional-scale convection heat transfer in areas of oceanic
crust. New sets of coefficients for 12° spherical harmonic expansion were calculated on the basis of the revised and homogenized
data set. Maps derived on the basis of these coefficients reveal several new features in the global heat flow distribution.
The magnitudes of heat flow anomalies of the ocean ridge segments are found to have mean values of less than 150 mW/m2. Also, the mean global heat flow values for the raw and binned data are found to fall in the range of 56–67 mW/m2, down by nearly 25% compared to the previous estimate of 1993, but similar to earlier assessments based on raw data alone.
To improve the spatial resolution of the heat flow anomalies, the spherical harmonic expansions have been extended to higher
degrees. Maps derived using coefficients for 36° harmonic expansion have allowed identification of new features in regional
heat flow fields of several oceanic and continental segments. For example, lateral extensions of heat flow anomalies of active
spreading centers have been outlined with better resolution than was possible in earlier studies. Also, the characteristics
of heat flow variations in oceanic crust away from ridge systems are found to be typical of conductive cooling of the lithosphere,
there being little need to invoke the hypothesis of unconfined hydrothermal circulation on regional scales. Calculations of
global conductive heat loss, compatible with the observational data set, are found to fall in the range of 29–34 TW, nearly
25% less than the 1993 estimate, which rely on one-dimensional conductive cooling models. 相似文献
45.
Computation of spherical harmonic coefficients and their error estimates using least-squares collocation 总被引:4,自引:0,他引:4
C. C. Tscherning 《Journal of Geodesy》2001,75(1):12-18
Equations expressing the covariances between spherical harmonic coefficients and linear functionals applied on the anomalous
gravity potential, T, are derived. The functionals are the evaluation functionals, and those associated with first- and second-order derivatives
of T. These equations form the basis for the prediction of spherical harmonic coefficients using least-squares collocation (LSC).
The equations were implemented in the GRAVSOFT program GEOCOL. Initially, tests using EGM96 were performed using global and
regional sets of geoid heights, gravity anomalies and second-order vertical gravity gradients at ground level and at altitude.
The global tests confirm that coefficients may be estimated consistently using LSC while the error estimates are much too
large for the lower-order coefficients. The validity of an error estimate calculated using LSC with an isotropic covariance
function is based on a hypothesis that the coefficients of a specific degree all belong to the same normal distribution. However,
the coefficients of lower degree do not fulfil this, and this seems to be the reason for the too-pessimistic error estimates.
In order to test this the coefficients of EGM96 were perturbed, so that the pertubations for a specific degree all belonged
to a normal distribution with the variance equal to the mean error variance of the coefficients. The pertubations were used
to generate residual geoid heights, gravity anomalies and second-order vertical gravity gradients. These data were then used
to calculate estimates of the perturbed coefficients as well as error estimates of the quantities, which now have a very good
agreement with the errors computed from the simulated observed minus calculated coefficients. Tests with regionally distributed
data showed that long-wavelength information is lost, but also that it seems to be recovered for specific coefficients depending
on where the data are located.
Received: 3 February 2000 / Accepted: 23 October 2000 相似文献
46.
Spatially restricted data distributions on the sphere: the method of orthonormalized functions and applications 总被引:1,自引:0,他引:1
In many geoscientific applications data are irregularly distributed and not globally available, e.g. caps around the poles
which are uncovered due to non-polar satellite orbits, or signals being defined solely on bounded regions on the globe. Starting
from a sequence of base functions with global support, which in the present case is composed of spherical harmonics being
initially non-orthogonal on a bounded subdomain, a set of functions is generated that constitutes an orthonormal basis. Different
approaches to realize this transformation are studied and compared with respect to numerical stability and computational effort,
and the corresponding effects on the coefficient recovery are investigated. A number of synthetic tests demonstrate the applicability,
the benefit, but also the limitations, of this method.
Received: 24 March 2000 / Accepted: 9 October 2000 相似文献
47.
The constant and harmonic parts of the global ocean tide are modeled by up to nine major tidal constituents, namely, S2, M2,
N2, K1, P1, O1, Mf, Mm, and Ssa. Our computations start with the Fourier sine and cosine series expansion for the tidal constituents,
including the constant Mean Sea Level (MSL). Although the frequencies of the tidal constituents are considered known, the
coefficients of the sine and cosine functions are assumed to be unknown. Subsequently, the coefficients of the sine and cosine
functions, as well as the constant part of the Fourier expansion, were expanded into spherical harmonics up to degree and
order n, where n corresponds to the number of linearly independent spherical harmonic base functions needed to model the tidal constituents,
determined via independent columns of the Gram matrix. The unknown coefficients of the spherical harmonic expansions are computed
using sea level observations within cycles #1–#350 of the TOPEX/Poseidon satellite altimetry over 11 years of its mission.
A set of orthonormal base functions was generated for the marine areas covered by TOPEX/Poseidon observations from the spherical
harmonics using a Gram-Schmidt orthogonalization process. These were used for modeling the dominant tidal constituents. The
computed models based on orthonormal base functions for the nine tidal constituents and the constant part of the Fourier expansion,
were tested numerically for their validity and accuracy, proving centimeter accuracy. 相似文献
48.
Since the release of the ETOPO1 global Earth topography model through the US NOAA in 2009, new or significantly improved topographic data sets have become available over Antarctica, Greenland and parts of the oceans. Here, we present a suite of new 1′ (arc-min) models of Earth’s topography, bedrock and ice-sheets constructed as a composite from up-to-date topography models: Earth2014. Our model suite relies on SRTM30_PLUS v9 bathymetry for the base layer, merged with SRTM v4.1 topography over the continents, Bedmap2 over Antarctica and the new Greenland bedrock topography (GBT v3). As such, Earth2014 provides substantially improved information of bedrock and topography over Earth’s major ice sheets, and more recent bathymetric depth data over the oceans, all merged into readily usable global grids. To satisfy multiple applications of global elevation data, Earth2014 provides different representations of Earth’s relief. These are grids of (1) the physical surface, (2) bedrock (Earth’s relief without water and ice masses), (3) bedrock and ice (Earth without water masses), (4) ice sheet thicknesses, (5) rock-equivalent topography (ice and water masses condensed to layers of rock) as mass representation. These models have been transformed into ultra-high degree spherical harmonics, yielding degree 10,800 series expansions of the Earth2014 grids as input for spectral modelling techniques. As further variants, planetary shape models were constructed, providing distances between relief points and the geocenter. The paper describes the input data sets, the development procedures applied, the resulting gridded and spectral representations of Earth2014, external validation results and possible applications. The Earth2014 model suite is freely available via http://ddfe.curtin.edu.au/models/Earth2014/. 相似文献
49.
50.
We examine the dependence of glacial-isostatic adjustment (GIA) due to changes in the Vatnajökull Ice Cap, Iceland, on the underlying viscosity structure. Iceland offers a unique case study for GIA research, with a thinner elastic lithosphere underlain by a low-viscosity zone or asthenosphere, as opposed to regions such as Fennoscandia or North America described by a thicker lithosphere, while not necessarily featuring an asthenosphere.A laterally homogeneous spherical earth model is used consisting of an elastic lithosphere, a viscoelastic asthenosphere, a viscoelastic upper and lower mantle and a fluid core. We examine the response of the earth model to three ice models with circular plans and cross-section profiles based on the assumption of perfectly plastic material, but with different load histories. These are: (1) A history where the ice cap grows from a AD 900 minimum to a maximum at 1890, followed by a uniform decrease until 1991, continuing to the present day at an average rate based on recent mass-balance measurements, (2) a history that is the same as the first, except for constant ice volumes prior to 1890, and (3) a history that is again the same as the first model, except that the post-1991 changes correspond to the measured mass-balance values. We first compare the response to each ice model using typical earth-model parameters for Iceland presented in the literature. We then undertake a parameter-space search, where we assess the importance of lithosphere thickness, asthenosphere viscosity and basal asthenosphere depth, to predicted vertical-displacement rates, and compare them to rates determined from GPS measurements obtained from campaigns conducted between 1991 and 1999.The earth-viscosity structure that provides the optimum predictions with respect to the GPS-derived vertical-displacement rates consists of an elastic lithosphere with a thickness of between 20 and 30 km, an asthenosphere viscosity between 1 and 2 × 1018 Pa s, and a basal asthenosphere depth between 250 km and possibly greater than 400 km. We find that the very low asthenosphere viscosity values of ca. 1017 Pa s sometimes suggested in the literature are not necessary to account for the rapid vertical-displacement rates observed, which are the result of the contemporary decrease in the mass of the ice cap not considered previously. 相似文献