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51.
A numerical simulation of residual circulation in Tampa Bay. Part I: Low-frequency temporal variations 总被引:1,自引:0,他引:1
Steven D. Meyers Mark E. Luther Monica Wilson Heather Havens Amanda Linville Kristin Sopkin 《Estuaries and Coasts》2007,30(4):679-697
The residual (time-average) salinity and circulation in a numerical ocean model of the Tampa Bay estuary are shown to experience
significant temporal variation under realistic forcing conditions. A version of the Estuarine Coastal Ocean Model developed
for Tampa Bay with 70 by 100 horizontal grid points and 11 sigma levels is examined for the years 2001–2003. Model output
variables are averaged over the entire time of the simulation to generate long-term residual fields. The residual axial current
is found to be dominated by the buoyancy-driven baroclinic circulation with an outflow (southwestward) at the surface and
to the sides of the shipping channel, and an inflow (northeastward) usually occurring subsurface within or above the shipping
channel. Averages over 30 d are used to examine variations in the residual fields. During the simulation the average surface
salinity near the head of Tampa Bay varies with the freshwater inflow, from 12‰ to 33%. At the bay mouth salinity varies from
30%. to 36%.. A localized measure of the baroclinic circulation in the shipping channel indicates the residual circulation
can vary strongly, attaining a magnitude triple the long-term mean value. The baroclinic circulation can be disrupted, going
to near zero or even reversing, when the buoyancy-driven flow is weak and the surface winds are to the northeast. Three time
periods, representing different environmental conditions, are chosen to examine these results in detail. A scaling argument
indicates the relative strength of buoyancy versus wind as ΔρgH2(LC
Dω2)−1, where δρ is head-to-mouth density difference across the bay,g is gravitational acceleration,H is depth,L is bay length,C
D is the surface wind drag coefficient, andw is wind speed. Tampa Bay is usually in the buoyancy dominated regime. The importance of winds in the weak-buoyancy case is
demonstrated in an additional simulation without wind stress. 相似文献
52.
Darrell A. Norris 《The Professional geographer》1987,39(1):23-32
The commercial non-metropolitan exit clusters of America's interstate highway system exhibit regularities of basic form, scale, mix of services provided, and the ordinal placement of establishments relative to the exit proper. An examination of 354 exits serving 1-75 indicates the typical morphology of interstate exit clusters. 相似文献
53.
We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r∼118,800 km, 4.5 m at r∼120,700 km) and the inner A Ring (10-15 m for r<127,000 km). 相似文献
54.
Some remarks on the Gaussian beam summation method 总被引:1,自引:0,他引:1
Summary. Recently, a method using superposition of Gaussian beams has been proposed for the solution of high-frequency wave problems. The method is a potentially useful approach when the more usual techniques of ray theory fail: it gives answers which are finite at caustics, computes a nonzero field in shadow zones, and exhibits critical angle phenomena, including head waves. Subsequent tests by several authors have been encouraging, although some reported solutions show an unexplained dependence on the 'free' complex parameter ε which specifies the initial widths and phases of the Gaussian beams.
We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly ε-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means. 相似文献
We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly ε-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means. 相似文献
55.
Stellar occultations by Saturn’s rings observed with the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty features such as the F ring and the ringlets in the Encke and the Laplace Gaps have distinctive infrared transmission spectra. These spectra show a narrow optical depth minimum at wavelengths around 2.87 μm. This minimum is likely due to the Christiansen Effect, a reduction in the extinction of small particles when their (complex) refractive index is close to that of the surrounding medium. Simple Mie-scattering models demonstrate that the strength of this opacity dip is sensitive to the size distribution of particles between 1 and 100 μm across. Furthermore, the spatial resolution of the occultation data is sufficient to reveal variations in the transmission spectra within and among these rings. In both the Encke Gap ringlets and F ring, the opacity dip weakens with increasing local optical depth, which is consistent with the larger particles being concentrated near the cores of these rings. The Encke Gap ringlets also show systematically weaker opacity dips than the F ring and Laplace Gap ringlet, implying that the former has a smaller fraction of grains less than ∼30 μm across. However, the strength of the opacity dip varies most dramatically within the F ring; certain compact regions of enhanced optical depth lack an opacity dip and therefore appear to have a greatly reduced fraction of grains in the few-micron size range. Such spectrally-identifiable structures probably represent a subset of the compact optically-thick clumps observed by other Cassini instruments. These variations in the ring’s particle size distribution can provide new insights into the processes of grain aggregation, disruption and transport within dusty rings. For example, the unusual spectral properties of the F-ring clumps could perhaps be ascribed to small grains adhering onto the surface of larger particles in regions of anomalously low velocity dispersion. 相似文献
56.
Dale P. Cruikshank Allan W. Meyer Roger N. Clark Katrin Stephan Scott A. Sandford Gianrico Filacchione Philip D. Nicholson Thomas B. McCord J. Brad Dalton Dennis L. Matson 《Icarus》2010,206(2):561-572
Several of the icy satellites of Saturn show the spectroscopic signature of the asymmetric stretching mode of C-O in carbon dioxide (CO2) at or near the nominal solid-phase laboratory wavelength of 4.2675 μm (2343.3 cm−1), discovered with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft. We report here on an analysis of the variation in wavelength and width of the CO2 absorption band in the spectra of Phoebe, Iapetus, Hyperion, and Dione. Comparisons are made to laboratory spectra of pure CO2, CO2 clathrates, ternary mixtures of CO2 with other volatiles, implanted and adsorbed CO2 in non-volatile materials, and ab initio theoretical calculations of CO2 * nH2O. At the wavelength resolution of VIMS, the CO2 on Phoebe is indistinguishable from pure CO2 ice (each molecule’s nearby neighbors are also CO2) or type II clathrate of CO2 in H2O. In contrast, the CO2 band on Iapetus, Hyperion, and Dione is shifted to shorter wavelengths (typically ∼4.255 μm (∼2350.2 cm−1)) and broadened. These wavelengths are characteristic of complexes of CO2 with different near-neighbor molecules that are encountered in other volatile mixtures such as with H2O and CH3OH, and non-volatile host materials like silicates, some clays, and zeolites. We suggest that Phoebe’s CO2 is native to the body as part of the initial inventory of condensates and now exposed on the surface, while CO2 on the other three satellites results at least in part from particle or UV irradiation of native H2O plus a source of C, implantation or accretion from external sources, or redistribution of native CO2 from the interior.The analysis presented here depends on an accurate VIMS wavelength scale. In preparation for this work, the baseline wavelength calibration for the Cassini VIMS was found to be distorted around 4.3 μm, apparently as a consequence of telluric CO2 gas absorption in the pre-launch calibration. The effect can be reproduced by convolving a sequence of model detector response profiles with a deep atmospheric CO2 absorption profile, producing distorted detector profile shapes and shifted central positions. In a laboratory blackbody spectrum used for radiance calibration, close examination of the CO2 absorption profile shows a similar deviation from that expected from a model. These modeled effects appear to be sufficient to explain the distortion in the existing wavelength calibration now in use. A modification to the wavelength calibration for 13 adjacent bands is provided. The affected channels span about 0.2 μm centered on 4.28 μm. The maximum wavelength change is about 10 nm toward longer wavelength. This adjustment has implications for interpretation of some of the spectral features observed in the affected wavelength interval, such as from CO2, as discussed in this paper. 相似文献
57.
Heather Katherine Handley Colin G. Macpherson Jon P. Davidson 《Contributions to Mineralogy and Petrology》2010,159(6):885-908
The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive
groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and
Sr–O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis,
as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence
for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning
observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma
properties (e.g. P, T, and PH2O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent
with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested
to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity
and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation
involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene
provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation
temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to ~13–24 km depth beneath the volcanoes (mid- to lower
crust). Low, mantle-like clinopyroxene δ18O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation
of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the
moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and
strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary,
is distinct from the other GVC groups. 相似文献
58.
John A. Izbicki Christen E. Petersen Kenneth J. Glotzbach Loren F. Metzger Allen H. Christensen Gregory A. Smith David O’Leary Miranda S. Fram Trevor Joseph Heather Shannon 《Applied Geochemistry》2010
About 1.02 × 106 m3 of chlorinated municipal drinking water was injected into a confined aquifer, 94–137 m below Roseville, California, between December 2005 and April 2006. The water was stored in the aquifer for 438 days, and 2.64 × 106 m3 of water were extracted between July 2007 and February 2008. On the basis of Cl− data, 35% of the injected water was recovered and 65% of the injected water and associated disinfection by-products (DBPs) remained in the aquifer at the end of extraction. About 46.3 kg of total trihalomethanes (TTHM) entered the aquifer with the injected water and 37.6 kg of TTHM were extracted. As much as 44 kg of TTHMs remained in the aquifer at the end of extraction because of incomplete recovery of injected water and formation of THMs within the aquifer by reactions with free-chlorine in the injected water. Well-bore velocity log data collected from the Aquifer Storage Recovery (ASR) well show as much as 60% of the injected water entered the aquifer through a 9 m thick, high-permeability layer within the confined aquifer near the top of the screened interval. Model simulations of ground-water flow near the ASR well indicate that (1) aquifer heterogeneity allowed injected water to move rapidly through the aquifer to nearby monitoring wells, (2) aquifer heterogeneity caused injected water to move further than expected assuming uniform aquifer properties, and (3) physical clogging of high-permeability layers is the probable cause for the observed change in the distribution of borehole flow. Aquifer heterogeneity also enhanced mixing of native anoxic ground water with oxic injected water, promoting removal of THMs primarily through sorption. A 3 to 4-fold reduction in TTHM concentrations was observed in the furthest monitoring well 427 m downgradient from the ASR well, and similar magnitude reductions were observed in depth-dependent water samples collected from the upper part of the screened interval in the ASR well near the end of the extraction phase. Haloacetic acids (HAAs) were completely sorbed or degraded within 10 months of injection. 相似文献
59.
Karin M. Kettenring Melissa K. McCormick Heather M. Baron Dennis F. Whigham 《Estuaries and Coasts》2010,33(1):118-126
The invasion and expansion of the introduced haplotype of Phragmites australis across North America is of growing concern. Previous studies in the Chesapeake Bay region found that Phragmites was more abundant, had higher foliar nitrogen, and produced more viable seeds in brackish wetland subestuaries with more
anthropogenic development of the watershed. Here, we focus on a different scale and address issues related to the invasion
of Phragmites within a single subestuary, the Rhode River. We evaluated patterns in seed viability, foliar nutrient concentrations, patch
size, and genetic variation in ten Phragmites patches in wetlands that occur in the side of the subestuary that is surrounded by forest and 10 patches in wetlands that
are in the side of the subestuary that has extensive anthropogenic development. Seed viability varied from 0–60% among the
20 patches but did not differ significantly between patches on the developed vs. forested sides of the Rhode River. Foliar
nutrients also did not differ between patches on the two sides of the Rhode River. Seed viability, however, was negatively
related to foliar nutrients. Most Phragmites patches consisted of >1 genotype. Larger patches had multiple genotypes, and patches with more genotypes produced more viable
seeds. Our study indicates that the Rhode River subestuary behaves as one system with no differences in the measured Phragmites variables between the forested vs. developed sides of the watershed. Our findings also suggest a cyclical process by which
Phragmites can spread: larger patches contain more genetic diversity, which increases the chances for cross-fertilization. The subsequent
increased production of viable seeds can increase local levels of genetic diversity, which can further facilitate the spread
of Phragmites by seed. 相似文献
60.