Fractal properties of whitecaps     

B.R. Kerman  K. Szeto 《大气与海洋》2013,51(3):531-551

Abstract

Images of a field of breaking waves over the ocean obtained using a line‐scanner on an aircraft are analyzed for a possible fractal geometry. The cumulative probability function of the intensity is shown to be self‐similar for sufficiently large intensities occupying about 10% of an imaged area. This structure is invariant to successive averaging over successively larger boxes. A box‐counting technique was applied to images of one representative flight. The estimated fractal dimension decreases from about 2.25 for a 10% a real coverage to about 1.7 for 0.1% coverage. It is concluded that the spatial distribution of the scattered light from foam and whitecaps is not monofractal, but is instead multifractal.  相似文献   

In situ identification of a CAI candidate in 81P/Wild 2 cometary dust by confocal high resolution synchrotron X-ray fluorescence     

Sylvia Schmitz  Frank E. Brenker  Tom Schoonjans  Bart Vekemans  Geert Silversmit  Laszlo Vincze  Manfred Burghammer  Christian Riekel 《Geochimica et cosmochimica acta》2009,73(18):5483-5492

We detected additional CAI-like material in STARDUST mission samples of comet 81P/Wild 2. Two highly refractory cometary dust fragments were identified in the impact track 110 [C2012, 0, 110, 0, 0] by applying high resolution synchrotron induced confocal and conventional XRF analysis (HR SR-XRF). The use of a polycapillary lens in front of the detector for confocal spectroscopy dramatically improves the fidelity of particle measurements by removing contribution from the surrounding aerogel. The high spatial resolution (300 × 300 nm²; 300 × 1000 nm²) obtained allowed the detailed non-destructive in situ (trapped in aerogel) study of impacted grains at the sub-μm level.For the two largest particles of the track, the terminal particle and a second particle along the impact track, Ca concentration is up to 30 times higher than CI and Ti is enriched by a factor of 2 compared to CI. High resolution (HR) SR-XRF mapping also reveals that the highest concentrations of Ca, Ti, Fe (and Ni) measured within each grain belongs to different areas of the respective maps which indicate that the particles are composed of several chemically diverse mineral phases. This is in agreement with the finding of a complex phase assemblage of highly refractory minerals in the first ever detected Stardust mission CAI grain “Inti” of Track 25.Principle component analysis (PCA) is a powerful tool for extracting the dominant mineral components and was applied to the two grains indicating that regions in the terminal particle and the second particle are consistent with anorthite or grossite and gehlenite, monticellite or Dmitryivanovite (CaAl₂O₄), respectively.Our new findings demonstrate that the HR SR-XRF with confocal geometry and PCA analysis is capable of identifying CAI-like fragments without the need to extract particles from the aerogel matrix which is a time-consuming, complex and destructive process.Furthermore, the detection of new CAI-like fragments in the coma dust of comet 81P/Wild 2 strengthens the observation that strong mixing effects and, therefore, mass transport before or during comet formation must have occurred at least up to the region where Kuiper Belt comets formed (30 AU).  相似文献   

How Many Facets are Needed to Represent the Surface Energy Balance of an Urban Area?     

Aurore Porson  Ian N. Harman  Sylvia I. Bohnenstengel  Stephen E. Belcher 《Boundary-Layer Meteorology》2009,132(1):107-128

We investigate the question of how many facets are needed to represent the energy balance of an urban area by developing simplified 3-, 2- and 1-facet versions of a 4-facet energy balance model of two-dimensional streets and buildings. The 3-facet model simplifies the 4-facet model by averaging over the canyon orientation, which results in similar net shortwave and longwave balances for both wall facets, but maintains the asymmetry in the heat fluxes within the street canyon. For the 2-facet model, on the assumption that the wall and road temperatures are equal, the road and wall facets can be combined mathematically into a single street-canyon facet with effective values of the heat transfer coefficient, albedo, emissivity and thermodynamic properties, without further approximation. The 1-facet model requires the additional assumption that the roof temperature is also equal to the road and wall temperatures. Idealised simulations show that the geometry and material properties of the walls and road lead to a large heat capacity of the combined street canyon, whereas the roof behaves like a flat surface with low heat capacity. This means that the magnitude of the diurnal temperature variation of the street-canyon facets are broadly similar and much smaller than the diurnal temperature variation of the roof facets. Consequently, the approximation that the street-canyon facets have similar temperatures is sound, and the road and walls can be combined into a single facet. The roof behaves very differently and a separate roof facet is required. Consequently, the 2-facet model performs similarly to the 4-facet model, while the 1-facet model does not. The models are compared with previously published observations collected in Mexico City. Although the 3- and 2-facet models perform better than the 1-facet model, the present models are unable to represent the phase of the sensible heat flux. This result is consistent with previous model comparisons, and we argue that this feature of the data cannot be produced by a single column model. We conclude that a 2-facet model is necessary, and for numerical weather prediction sufficient, to model an urban surface, and that this conclusion is robust and therefore applicable to more general geometries.  相似文献   

Interpolation of Fluvial Morphology Using Channel-Oriented Coordinate Transformation: A Case Study from the New Jersey Shelf     

John A. Goff  Sylvia Nordfjord 《Mathematical Geology》2004,36(6):643-658

We present a new methodology for interpolating channel morphology that incorporates a transformation from geographic to channel-based coordinate systems. Interpolation in the transformed space enables enforcement of downstream continuity of morphology and edge delineation through any changes in channel direction. The transformation is guided by a channel center line, which approximately tracks the path of the channel through geographic space; coordinates are given in distance along and across the center line. Accurate interpolation requires a track line density sufficient to unambiguously trace channels from one track line to the next. Channel continuity is ensured by first interpolating along paths defined by the channel thalweg and edges, which must be chosen by the user, and along several interim paths between the edges and thalweg. The completed interpolations for each channel are transformed back into geographic coordinates, and channel confluence is handled through a maximum depth criterion. The method is applied here to shallowly buried channels mapped with high-resolution chirp seismic data on the New Jersey shelf, but should be applicable to a wide range of subaerial and buried fluvial systems.  相似文献   

The WKB approximation and the plasma ring of Jupiter     

Sylvia Novo 《Earth, Moon, and Planets》1991,53(2):183-189

In this note we study the behaviour of hydromagnetic oscillations along the field lines of Jupiter's magnetosphere crossing the Io plasma ring. We compare the shape and period of these oscillations, as found by a direct numerical calculation, with those obtained with the WKB method, in order to show the unaccuracy of this approximation.  相似文献   

Holocene environmental history and climate of Råtåsjøen,a low-alpine lake in south-central Norway     

Gaute?VelleEmail author  Jorunn?Larsen  Wenche?Eide  Sylvia M.?Peglar  H. John. B.?Birks 《Journal of Paleolimnology》2005,33(2):129-153

The Holocene environmental history and climate are reconstructed for Råtåsjøen, a low-alpine lake in south-central Norway. The reconstructions are based on chironomids, diatoms, pollen, plant macrofossils, and sediment characteristics. From plant macrofossil evidence, birch trees (Betula pubescens) immigrated ca. 10,000 cal BP. The chironomid-inferred mean July air temperature was high, but may be unreliable during the early stages of the lakes history due to the high abundance of Chironomus anthracinus type, a taxon that may include several species. From ca. 9000 cal BP the inferred mean July temperature was lower (ca. 9 °C). Temperatures increased towards 8000 cal BP and pine (Pinus sylvestris) reached its upper limit near the lake. July temperature may have become a significant factor controlling long-term pH in the lake, starting shortly after 8000 cal BP. High pH values were associated with periods of warm summers and lower pH values occurred during periods of colder summers. Alkalinity processes within the lake and/or the catchment are possible factors controlling this relationship. A temperature decline at ca. 5400 cal BP separated two 10.6 °C temperature maxima around 6400 and 4500 cal BP. The 1.5 °C decline in July air temperatures from ca. 4400 cal BP was paralleled by a decrease of pH from 7.2 to 6.8. Following the temperature drop, first pine and then birch trees declined and disappeared from the catchment and organic accumulation in the lake increased. The increased organic accumulation rate had a positive effect on diatom production. At ca. 2700 cal BP the temperature reached a minimum (ca. 9.2 °C) and correspondingly a second pH minimum was reached. Temperature decreased again slightly at ca. 400 cal BP during the Little Ice Age, before increasing by about 0.5 °C towards the present. Percentage organic carbon as estimated by loss-on-ignition appears to be better correlated with chironomid-inferred July temperatures than organic accumulation rates, at least for the last 9000 years. Accumulation rates of organic sediments are more coupled with catchment-related processes, such as erosion and major changes in vegetation, than is percentage organic carbon.  相似文献   

On the separation of core and crustal contributions to the geomagnetic field     

A. M. K. Szeto  W. H. Cannon 《Geophysical Journal International》1985,82(2):319-329

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Recent Environmental Change and Atmospheric Contamination on Svalbard as Recorded in Lake Sediments – Modern Limnology, Vegetation, and Pollen Deposition     

H.J.B. Birks  D.T. Monteith  N.L. Rose  Vivienne J. Jones  Sylvia M. Peglar 《Journal of Paleolimnology》2004,31(4):411-431

Twenty-four lakes on Svalbard were sampled for palaeolimnological studies and are described in terms of their geographical location, catchment characteristics, water chemistry, and flora. No sediment could be retrieved from one of the lakes. There is a close correlation, as detected by redundancy analysis, between lake-water chemistry and catchment variables, particularly bedrock geology and geographical location for 23 lakes. The flora of the lake catchments is statistically related, as shown by canonical correspondence analysis, to bedrock geology, climate (geographical location), and nutrient status (bird impact). Modern pollen assemblages from eleven lakes contain 2–25% far-distance extra-regional pollen. The modern local and regional pollen depositions are dominated by Oxyria digyna, Poaceae, Saxifraga, Salix, and Brassicaceae pollen.  相似文献   

Coupled motions of the inner core and possible geomagnetic implications     

A.M.K. Szeto  D.E. Smylie 《Physics of the Earth and Planetary Interiors》1984,36(1):27-42

As the inner core is a good electrical conductor any ambient magnetic field would diffuse into it on a time scale long compared to several thousand years, and conversely be frozen there on shorter time scales. From the observations that the dipole component of the Earth's magnetic field has been inclined persistently to the spin axis over hundreds of thousands of years, and that the dipole drifts and decays significantly more slowly than the nondipole field, it is suggested that the external dipole is simply a manifestation of a field frozen in an inclined inner core. It is shown that the much neglected gravitational restoring torque can be significant for an inclined inner core, so much so that its motion is in the main determined by gravity, with electromagnetic and inertial coupling effects being of secondary importance. A regular precession of the inner core is shown to be possible where its spin axis drifts westward relative to the mantle with a period of ～ 7000 y. Some preliminary calculations of the possible motions of a gravitationally coupled mantle-inner core system are shown.  相似文献   

Living macromolluscs from a paleo-reef region on the northeastern Venezuelan continental shelf     

Joaquín Buitrago  Juan Capelo  Javier Gutirrez  Martín Rada  Ricardo Hernndez  Sylvia Grune 《Estuarine, Coastal and Shelf Science》2006,66(3-4):634-642

Drowned reefs, fossil reefs or paleo-reefs, are important ecologically as areas of high biodiversity, foraging, shelter environment, and as areas supporting the spawning aggregations of economically important reef fish species. This is particularly significant when the structures are situated in a wide soft-bottom continental shelf. The presence of limestone structures, fossil reefs and pinnacles dating from circa 8 to 9 ka, to the north of the Paria Peninsula in north-eastern Venezuela, has been known to local fishermen for decades. Using echograms obtained during acoustic fisheries evaluations and the scarce previously available information, an improved location map of hard-bottom structures was made. Benthic samples to study macromolluscs were taken at depths between 54 and 93 m using an unmodified 2-m beam trawl. Four trawl samples were located over fossil reef areas while another four were situated in soft-bottom valleys between limestone structures. Fossil reefs in the area showed a highly patchy distribution. A total of 91 species from 43 Bivalvia, Gastropoda and Scaphopoda families were found, Gastropoda being the dominant class with 49 species. Paleo-reef-covered areas showed higher species richness and only 21% of the species found were common to both substrates. Gastropods Tonna maculosa and Polystira albida were the most abundant species and occurred in both substrate types. Bivalve life habits, a mixture of organism–substrate relationships, shell fixation, mobility and feeding type, differed significantly according to bottom type. Six species are recorded for the first time for eastern Venezuelan waters. Bottom heterogeneity plays an important role in marine ecosystems, providing shelter to fish populations and may be significant as breeding and nursery areas. Its presence in a region with biogeographical interest, situated in the confluence of three major provinces and with oceanographic conditions varying seasonally from upwelling dominated to Orinoco River discharges, makes this the area of interest and it should be evaluated as a possible Marine Protected Area.  相似文献