bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in the Western Boundary Undercurrent Outflow since the Last Glacial Maximum, from smectite/illite ratios in deep Labrador Sea sediments
Authors: Fagel Nathalie , Hillaire‐Marcel Claude , Robert Christian
Published: February 2004

High‐resolution mineralogical studies were performed on late glacial and deglacial sediments from two deep piston cores from the Labrador Sea, located at the inlet (SW Greenland Rise) and outlet (Labrador Rise) of the Western Boundary Undercurrent (WBUC) gyre. At the two sites, smectites transported from the eastern Iceland and Irminger basins by the WBUC are observed. Clay mineral changes are used as proxies for the paleointensity reconstruction of the WBUC. On the Greenland Rise, a clay mineral index (smectite/illite (S/I) ratio) is defined. A S/I ratio of ∼1 characterized the Last Glacial Maximum. It increased after ∼17 ka. and reached a maximum value of 4 during the early Holocene. The mineralogical changes are gradual and do not show any reversal during the Younger Dryas. This pattern, which is confirmed by first‐order estimations of smectite and illite fluxes, suggests gradually increasing sedimentary fluxes and WBUC intensity since the Last Glacial Maximum. A peak in the velocity of the WBUC at ∼9 ka, as recorded by clay assemblages, is consistent with other regional studies based on pollen, foraminifera, or grain‐size measurements. A massive dilution of smectites by illite and chlorite (S/I ≈ 3) occurs at ∼8.5 ka. It corresponds to a period of rapid sediment accumulation and reflects an intensified illite‐rich detrital supply by meltwaters from the southern Greenland Ice Margin. On the Labrador Rise, the smectite content varies between 20 and 60% with no obvious trend through time. The mineralogical composition is strongly influenced by ice‐rafted deposition and by the abundance of fast deposit units (cf. Heinrich layers in the North Atlantic) which contain abundant detrital carbonates spilled‐over from the North‐West Atlantic Mid‐Ocean Channel. In such layers, smectites are present but are diluted by the addition of illites, chlorites, and kaolinites. This provides evidence for a discrete and continuous WBUC supply of fine particles from the Irminger and Iceland Basins as far as the southeastern part of the Labrador Basin. Early deglacial smectite‐rich layers (up to 60%) are also observed at this site. They indicate an increase in the outflow of the WBUC at ∼13.5 ka. (Bölling‐Alleröd), as previously reported from grain size or foraminiferal assemblage studies.
High‐resolution stable isotope records from southwest Sweden: The drainage of the Baltic Ice Lake and Younger Dryas Ice Margin Oscillations
Authors: Bodén Per , Fairbanks Richard G. , Wright James D. , Burckle Lloyd H.
Published: February 2004

Benthic foraminifera in two shallow marine sediment cores from southwest Sweden were analyzed for oxygen isotopes. Several deglaciation events, previously recognized in terrestrial and lake sediments throughout the Baltic region, are identified and radiocarbon dated. An initial Baltic Ice Lake (BIL) drainage at the Mount Billingen threshold is inferred from a distinct 2.4‰ δ18O oscillation in one of the investigated cores. The estimated radiocarbon age for this event is approximately 10,900 14C years. The final drainage, which ended BIL history, occurred in two steps during the younger part of the radiocarbon plateau at 10,000 14C years. Biostratigraphy suggests that the final drainage took place in the climatically warm early Preboreal. At approximately 11,150 14C years, the deglacial trend toward lighter isotopic composition was interrupted and slightly reversed and is interpreted to represent the onset of the cold Younger Dryas period. For a few hundred years prior to the Younger Dryas, melting of the Fennoscandian ice sheet appears to have been rapid. Besides the dramatic drainage events and reduced melting during the Younger Dryas, the records display several other salinity variations. These variations may reflect ice margin oscillations during the Younger Dryas, previously identified throughout Scandinavia as ice marginal deposits. The isotopic records suggest approximately 10 climatically induced ice margin recession/readvances of the southeastern Fennoscandian ice sheet during the Younger Dryas.
The history of Martian volatiles
Authors: Jakosky Bruce M. , Jones John H.
Published: February 2004

The behavior of water and other volatiles on Mars is key to understanding the evolution of the climate. The early climate played a fundamental role in producing the observed surface morphology and possibly in enabling the existence of an early biosphere. Geochemical and isotopic data can be used to infer the history of volatiles. On the basis of the isotopic data from the atmosphere and from components of the surface (as measured in meteorites that come from Mars), there appear to be at least two reservoirs of volatiles, one that has undergone exchange with the atmosphere and has been isotopically fractionated, and a second that is unfractionated and may represent juvenile gases. The fractionation of the atmospheric component has occurred primarily through the escape of gas to space. In addition, the atmospheric gases have mixed substantially with crustal reservoirs of volatiles. Such exchange may have occurred in aqueous or hydrothermal environments. The history of escape to space, as driven by the properties of the Sun through time, is consistent with the surface geomorphology. Together, they suggest an early environment that was substantially different from the present one and the evolution through time to a colder, dryer climate.
Composition and cycling of colloids in marine environments
Authors: Guo Laodong , Santschi Peter H.
Published: February 2004

Colloidal (COM) or macromolecular organic matter makes up a significant portion of the bulk dissolved organic matter (DOM) pool in aquatic environments. Because of their high specific surface areas and complexation capacities, marine colloids are of great importance not only in the global carbon cycle but also in the biogeochemical cycling of many particle‐reactive nuclides and trace elements in the ocean. However, the colloidal pool as a whole is still poorly understood and largely uncharacterized. Recently, cross‐flow ultrafiltration and other separation techniques, which have been successfully used to isolate marine colloids, combined with a multitracer approach, have greatly advanced our understanding of the cycling of COM and its associated trace elements in marine environments. In this paper we focus on recent developments on isotopic and elemental composition of colloids which allow organic matter cycling in marine environments to be constrained. Major sections review sampling techniques for aquatic colloids, concentrations and distribution of COM, biochemical and elemental (organic and inorganic) characterization, and stable isotopic (13C and 15N) and radioisotopic (14C and 234Th) characterization of marine colloids. We discuss sources and turnover rates of organic matter in the ocean, importance of benthic boundary layer processes in the cycling of DOM, changes in the paradigms of marine organic matter cycling, and research needs for a better understanding of the biogeochemistry of marine colloids.
Estimating wet delays using numerical weather analyses and predictions
Authors: Bevis Michael , Chiswell Steven , Businger Steven , Herring Thomas A. , Bock Yehuda
Published: February 2004

Zenith wet delay (ZWD) is a key parameter in analyses of microwave propagation delays caused by atmospheric water vapor. This paper explores the possible use of numerical weather analyses and numerical weather forecasts to estimate the ZWD in nearly real time or, more likely, after the fact. The predicted precipitable water (PW) field is transformed into a predicted ZWD field using the relationship ZWD = PW/II, where II is a function of various physical constants and a weighted mean temperature of the atmosphere,Tm, that is also predicted using the numerical weather analysis or forecast. We use the +12‐hour forecasts of the Nested Grid Model to predict ZWD and test these predictions against values derived from independent radiosonde observations. By performing these comparisons at more than 100 North American radiosonde stations over a 6‐month period we find that forecast errors tend to increase as the magnitude of ZWD increases. This provides a simple framework for characterizing the maximum level of error associated with a ZWD estimates obtained by interpolating in time between analytical “nowcasts” and numerical weather predictions. We develop a preliminary approach to estimating the ZWD forecast errors at any time between zero and 12 hours from model initialization.
Multiple polar cap arcs: Akebono (Exos D) observations
Authors: Obara T. , Mukai T. , Hayakawa H. , Tsuruda K. , Matsuoka A. , Nishida A. , Fukunishi H. , Zhu L. , Sojka J. , Crain D.
Published: February 2004

Akebono (Exos D) observations demonstrate that polar cap arcs sometimes have a fine structure, that is, multiple (double or triple) arcs with spacing of a few tens of kilometers. The multiple polar cap arcs are dominantly observed in the nightside polar cap region, suggesting that low background conductance favors the appearance of the structured arcs. A relationship between the spacing and the average energy of the precipitating electrons is investigated. Results show that a higher energy leads to a wider spacing. Akebono observations also show the existence of a downward current region embedded between upward current regions (arcs). Comparison of the observations with results from a coupled magnetosphere‐ionosphere Sun‐aligned arc model is made, which shows good qualitative agreement between the modeling and observational results on the spacing‐energy dependence and the effect of background ionospheric conductance.
Propagation in general media using transmission‐line modeling
Authors: Christopoulos Christos , Trenkic Vladica , Benson Trevor M.
Published: February 2004

Propagation of electromagnetic waves in general media consisting of nonuniform anisotropic materials is studied using the transmission‐line modeling (TLM) method. A general approach to the derivation of the scattering properties of the most general nodes used in TLM simulation is presented together with an assessment of the numerical dispersion of TLM schemes and ways in which it can be reduced.
Calculation of the wave field scattered by an echelette grating in the short‐wave approximation: Transverse electric case
Authors: Zalipaev Victor V.
Published: February 2004

The two‐dimensional problem of the plane wave scattering by a periodic grating, an echelette with a right angle, is considered in the case of high‐frequency approximation (the wavelength is assumed to be small compared with the period of grating) and where only one face of a wedge is illuminated. The ray optical solution to the problem (short‐wave asymptotic result) derived on the basis of the method of summation of multiple diffracted fields, which is well‐known in the geometric theory of diffraction, is described. Numerical results are presented and compared with calculations based on an integral equation approach.
Semi‐inversion method for an accurate analysis of rectangular waveguide H plane angular discontinuities
Authors: Kirilenko Anatoly A. , Rud' Leonid A. , Tkachenko Vladimir I.
Published: February 2004

The new efficient numerical‐analytical method of electromagnetic interior boundary value problem solving is presented with waveguide H plane angled bends as examples of implementation. The method essence consists in the analytical inversion of the main difference part of the initially derived ill‐posed matrix equation of the first kind. As a result, the problem is reduced to the solution of the second‐kind matrix equation with an invertible operator. The proposed method differs from other methods dealing with so‐called modified Wiener‐Hopf geometry structures. It is oriented to noncoordinate boundary value problems to which the ones of angular waveguide discontinuities are related. Detailed consideration of all parts of the numerical algorithm ensures its high efficiency. As application examples, the data for designing the matched truncated corners in a single‐mode waveguide and the results of comparison of the known high‐frequency heuristic estimations with the exact simulation of waveguide corners having a quasi‐optical mirror are presented.
Synthetic ΔK frequency plans in relation to radio wave object characterization
Authors: Aarholt Eldar
Published: February 2004

A system to generate data channels to enhance the classification of objects by means of selective radar frequency and antenna distribution plans using signal generating and processing methods which can be utilized in conventional multifrequency radar systems. The implementation of specific synthetic ΔK algorithms for the generation of large numbers of data channels derived from the initially selected radar frequency or antenna distribution plans increases the amount of information available for classification after detection. The distance to a reflector can be uniquely resolved across virtually any range ambiguity interval, and at the same time the reflector velocity can be determined over virtually any practical velocity variation.
Angular memory effect of millimeter‐wave scattering from two‐dimensional conducting random rough surfaces
Authors: Chan T. K. , Kuga Y. , Ishimaru A.
Published: February 2004

An experimental study was conducted to investigate the angular memory effect of millimeter‐wave scattering from two‐dimensional conducting random rough surfaces. The surfaces under investigation were machine‐fabricated with known Gaussian roughness statistics, and the copolarized and cross‐polarized angular correlation functions (ACFs) of scattering amplitudes were measured. It was found that for the case of reference antenna positions located bistatically in a backward direction, the measured ACF exhibits broad response when single scattering dominates but two peaks when multiple scatteringdominates. These observations are in good agreement with the second‐order Kirchhoff approximation (KA2). Specifically, the observed broad and peak responses are analytically identified to be due to the first‐order and second‐order (ladder and cyclical) scattering components, respectively, in KA2.
Electric fields and currents induced in organs of the human body when exposed to ELF and VLF electromagnetic fields
Authors: King Ronold W. P. , Sandler Sheldon S.
Published: February 2004

Formulas for the transverse components of the electric and magnetic fields of the traveling‐wave currents of three different types of three‐wire, three‐phase high‐voltage power lines and of a typical VLF transmitter are given. From them, exposure situations for the human body are chosen which permit the analytical determination of the total current induced in that body. With this, the fraction of the total axial current, the axial current density, and the axial electric field in each organ of the body are obtained at any desired cross section. The dimensions and conductivity of these organs must be known. The electric field so obtained is the average macroscopic field in which the cells in each organ are immersed when the whole body is exposed to a known incident field. It corresponds in vivo to the electric field used in vitro to expose cells in tissues.
Diffraction by a wedge with higher‐order boundary conditions
Authors: Osipov Andrey V.
Published: February 2004

This paper presents a powerful analytical procedure that combines the higherorder boundary condition approximation and the modified Malyuzhinets technique to treat electromagnetic scattering from arbitrarily angled wedge‐like composite configurations with penetrable faces. The configuration may be either a perfectly conducting wedge covered with a dielectric/ferrite material or a composite wedge with strongly absorbing faces, so that the field transmitted directly through the configuration can be neglected. With special emphasis on retaining the passivity of the media interfaces, boundary conditions on the wedge faces are approximated using impedance conditions with higher‐order field derivatives. To close the formulation of the diffraction problem, a class of contact conditions is described that ensures the uniqueness and reciprocity. A general solution which is valid for passive boundary conditions of arbitrary odd orders is derived in an explicit form as the Sommerfeld integral involving arbitrary constants. By choosing these constants in such a way as to fit its analytical behavior for kr ≪ 1 to the exact one obtained by directly integrating the Maxwell equations, a specific form of the contact conditions can be deduced that reflects the design features of the configurations near their edges and forces the solution to be an asymptotic one for vanishing thicknesses of the coatings or skin layers. As a result, the unique and reciprocal closed‐form solution associated with third‐order boundary conditions is presented which uniformly describes the diffraction of an H‐ polarized plane electromagnetic wave by a variety of the composite wedge‐like structures with penetrable faces, extending thereby Malyuzhinets' solution for an impedance wedge to more sophisticated boundary conditions.
Inference of fog characteristics from attenuation measurements at millimeter and optical wavelengths
Authors: Vasseur H. , Gibbins C. J.
Published: February 2004

Measurements of fog characteristics are rare, and therefore models for predicting the attenuation caused by fog on the propagation of electromagnetic radiation are scarce. Using experimental transmissions through moderate radiation fogs on a 500‐m path, a method is developed to infer the physical characteristics of fog from concurrent attenuation measurements at millimeter, infrared, and visible wavelengths. It makes it possible to retrieve the fog drop size distribution, as well as its time evolution, during the life cycle of a fog event. It yields realistic values for fog features, such as liquid water content and average drop diameter, which are not easily measured. Furthermore, on an experimental basis, simple relationships between inferred fog characteristics and measured attenuation are proposed. It is found that while both millimetric and infrared attenuations are directly related to liquid water content, visible attenuation strongly depends on the size of the fog droplets. Overall, using these relationships together with statistical distributions of inferred fog characteristics enables predictions of fog attenuation at any wavelength from millimetric to visible.
Vegetation characteristics and underlying topography from interferometric radar
Authors: Treuhaft Robert N. , Madsen Søren N. , Moghaddam Mahta , van Zyl Jakob J.
Published: February 2004

This paper formulates and demonstrates methods for extracting vegetation characteristics and underlying ground surface topography from interferometric synthetic aperture radar (INSAR) data. The electromagnetic scattering and radar processing, which produce the INSAR observations, are modeled, vegetation and topographic parameters are identified for estimation, the parameter errors are assessed in terms of INSAR instrumental performance, and the parameter estimation is demonstrated on INSAR data and compared to ground truth. The fundamental observations from which vegetation and surface topographic parameters are estimated are (1) the cross‐correlation amplitude, (2) the cross‐correlation phase, and (3) the synthetic aperture radar (SAR) backscattered power. A calculation based on scattering from vegetation treated as a random medium, including the effects of refractivity and absorption in the vegetation, yields expressions for the complex cross correlation and backscattered power in terms of vegetation characteristics. These expressions lead to the identification of a minimal set of four parameters describing the vegetation and surface topography: (1) the vegetation layer depth, (2) the vegetation extinction coefficient (power loss per unit length), (3) a parameter involving the product of the average backscattering amplitude and scatterer number density, and (4) the height of the underlying ground surface. The accuracy of vegetation and ground surface parameters, as a function of INSAR observation accuracy, is evaluated for aircraft INSAR, which is characterized by a 2.5‐m baseline, an altitude of about 8 km, and a wavelength of 5.6 cm. It is found that for ≈0.5% accuracy in the INSAR normalized cross‐correlation amplitude and ≈5° accuracy in the interferometric phase, few‐meter vegetation layer depths and ground surface heights can be determined from INSAR for many types of vegetation layers. With the same observational accuracies, extinction coefficients can be estimated at the 0.1‐dB/m level. Because the number of parameters exceeds the number of observations for current INSAR data sets, external extinction coefficient data are used to demonstrate the estimation of the vegetation layer depth and ground surface height from INSAR data taken at the Bonanza Creek Experimental Forest in Alaska. This demonstration shows approximately 5‐m average ground truth agreement for vegetation layer depths and ground‐surface heights, with a clear dependence of error on stand height. These errors suggest refinements in INSAR data acquisition and analysis techniques which will potentially yield few‐meter accuracies. The information in the INSAR parameters is applicable to a variety of ecological modeling issues including the successional modeling of forested ecosystems.
Proximity effects of absorbing boundary conditions on antenna radiation patterns
Authors: Rayner M. , Olver A. D. , Monk A. D.
Published: February 2004

Absorbing boundary conditions (ABCs) are needed to surround an antenna and terminate the finite difference time domain spatial domain. The proximity effects of ABCs which are close to the antenna are investigated, and it is shown that specifying the physical distance between an antenna and the ABCs is preferable to specifying the number of cells. Distances of about 0.5λ have been shown to produce good radiation patterns at all observation angles, but larger distances are necessary for accurate prediction of antenna gain. The use of the Berenger perfectly matched layer (PML) allows the amount of free space surrounding a structure to be reduced but incurs a computational penalty depending on the thickness of the PML.
Simulation of passive microwave wind direction signatures over the ocean using an asymmetric‐wave geometrical optics model
Authors: Kunkee D. B. , Gasiewski A. J.
Published: February 2004

The geophysical nature of polarized azimuthal microwave brightness temperature signatures over the open ocean and their relation to surface wind‐driven anisotropies are investigated. The broadband nature of previously observed wind‐direction signatures suggests the predominance of a frequency‐independent thermal emission mechanism involving scattering and emission from asymmetric ocean gravity waves and ocean foam. To verify this hypothesis, an asymmetric‐wave Monte Carlo ocean surface model and a geometrical optics electromagnetic emission model of the upwelling radiation field were developed. The model incorporates the effects of ocean wave asymmetry, ocean foam, and multiple geometric scattering. Model calculations compare favorably with the results of a previous study that used data from the Special Sensor Microwave Imager (SSM/I) at 19 and 37 GHz and at wind speeds of 7.9 and 12.2 m/s and at 53° incidence. The good agreement between the satellite data and the asymmetric‐wave geometrical optics model supports the hypothesis that azimuthal brightness temperature signatures at wind speeds characteristic of moderate‐to‐heavy wind conditions are to a great extent caused by ocean wave asymmetry and foam.
Bistatic properties of Kirchhoff diffractals
Authors: Ivanova K. , Yordanov O. I.
Published: February 2004

The angular dependence of wave intensity scattered from two types of fractal surfaces is calculated and analyzed within the physical optics approximation. The focus is on surfaces whose dimensions are close to the dimension of a Brownian fractal surface. It is shown that the effect of diversification of the angular patterns in the case of bistatic scattering manifests itself as a transition from the single‐to the double‐peaked form of the scattering indicatrix and takes place when the wavelength is of order of the topothesy of the surface. In addition, the effect of the small‐wavenumber cutoff is quantified.
Theory and sensing applications of a long antenna on layered media
Authors: Dai Renshou , Young Charles T.
Published: February 2004

We solved a Sommerfeld‐type boundary value problem of an infinitely long antenna lying over multilayered media. Rigorous integral representation was obtained for the current on the antenna when excited by a delta function voltage. Comprehensive analyses of the singularities of the integral representation were performed for the most general case of antenna lying over multilayered media. The analyses indicate that the total current on this open waveguide structure consists of, generally, infinite number of discrete modes (contributions from poles) and continuous modes (contributions from the integrals along the branch cuts). Detailed expressions were given for the discrete and continuous spectral representations of the total current. Numerical results were obtained for the input impedance spectra of an infinite antenna lying over various layered media. Experiments were also conducted with a long antenna connected to a time‐domain reflectometer and placed horizontally over tap water. Both the numerical and experimental results suggest that a single long antenna could be used as a sensing tool to detect the profile of a layered medium.
An electromagnetic fractal‐based model for the study of fading
Authors: Franceschetti Giorgio , Migliaccio Maurizio , Riccio Daniele
Published: February 2004

Whenever a coherent electromagnetic field illuminates a natural surface, it is affected by fading, and this is related to the surface roughness. A model of the electromagnetic interaction with the rough surface allows the determination of some of its parameters; accordingly, the characteristics of the radar echo return are of great relevance. In this paper we consider a Kirchhoff‐fractal electromagnetic model which is applicable to the problems of remote sensing, and we systematically and parametrically analyze the first‐order fading characteristics.
Ultra‐wideband electromagnetic pulse propagation in a homogeneous, cold plasma
Authors: Dvorak Steven L. , Ziolkowski Richard W. , Dudley Donald G.
Published: February 2004

In this paper, we investigate the propagation of an ultra‐wideband electromagnetic pulse in a homogeneous, cold plasma which is used to represent a simplified model of the atmosphere. The standard procedure for the computation of the corresponding transient field involves the application of a fast Fourier transform (FFT) to a well‐known, analytical, frequency‐domain solution. However, because of the long tails in both the time and frequency domains, a large number of sample points are required to compute the transient response using this FFT approach. In this paper, we introduce a new asymptotic extraction technique which dramatically reduces the number of sample points required by the FFT. First, we review the recently derived closed‐form expression for a double‐exponential pulse propagating in a homogeneous, collisionless, cold plasma. Since the high‐frequency behavior does not depend on the electron collision frequency, an analytical frequency‐domain expression, which is similar in form to the one encountered for the collisionless, cold plasma and encompasses this high‐frequency behavior, can be subtracted from the exact expression for the plasma with a nonzero collision frequency. The extracted term is evaluated analytically. The remaining expression, which can be transformed to the time domain with a FFT, requires only a modest number of sample points. This dramatically improves the numerical efficiency of the approach. We find that the extracted analytical term provides a very good approximation for the early‐time behavior of the transient pulse.
Effects of finite beam width and wind field divergence on Doppler radar measurements: Simulations
Authors: Palmer R. D. , Larsen M. F. , Vangal S.
Published: February 2004

This paper describes the effects of wind field divergence on Doppler radar measurements. Incompressible fluids inherently have a horizontal divergence component in the presence of vertical gradients in the vertical motion. We present simulation studies which show that the radial velocity, measured from a vertically directed beam, can show a bias due to divergence created by the finite beam width effect. The bias can be of the order of a few centimeters per second, depending on beam width, divergence, and altitude. Furthermore, an indirect effect on the cross spectrum, which is typically used in multiple‐receiver wind profiling algorithms, will be presented. Divergence causes a deformation of the isodops, thus distorting the Doppler sorting process implicit in the interferometric crossspectrum calculation.
Ionospheric structures invisible in satellite radiotomography
Authors: Saksman Eero , Nygrén Tuomo , Markkanen Markku
Published: February 2004

In satellite ray tomography the measurements are samples of Radon transform of the ionospheric electron density. Regardless of the practical numerical method applied, the goal of tomographic inversion is to calculate the inverse of the Radon transform. From mathematical point of view it is clear that provided a function satisfies certain mathematical conditions, it has a unique Radon transform, which also has a unique inverse. In satellite radiotomography, however, the sampling of the Radon transform is seriously restricted by the small number of receivers. No unique inverse can be calculated from such an incomplete set of samples. It turns out that an infinite number of electron density functions can be defined which are completely invisible for a given experimental setup, not only in practice but also in a strict mathematical sense. These functions are not necessarily horizontally stratified, but they may contain strong horizontal gradients, in which case their invisibility is not associated with the lack of horizontal rays. In this paper a method of constructing such functions is presented and examples of them are shown, some of which could be reasonable in a realistic ionosphere. An experimental result and a simulation is also presented, which indicates how a wave‐like traveling ionospheric disturbance may be partly invisible to transmitters on the ground.
First results of tropospheric gravity wave momentum flux measurements over Christmas Island
Authors: Chang J. L. , Avery S. K. , Riddle A. C. , Palo S. E. , Gage K. S.
Published: February 2004

The importance of gravity wave effects on various dynamical processes has been well documented in the upper and middle atmosphere. However, less is known about these waves in the lower atmosphere. Whereas many studies of longer‐period waves have previously been conducted using wind profiler data, this paper is an effort to quantify the short‐term motions of the troposphere. We will present some of the initial tropospheric gravity wave horizontal variance and momentum flux estimates calculated from Christmas Island wind profiler data collected during 1994. A coplanar antenna beam configuration was used for this analysis. For periods shorter than 8 hours, the monthly vertical fluxes of zonal and meridional momentum were estimated to be between ±0.02 kg m−1 s−2 and ±0.01 kg m−1 s−2, respectively. The resulting net accelerations of the mean flow were in the range of ±0.1 m s−1 d−1. In addition to the high‐frequency results, we will discuss some of the difficulties we encountered while processing the data. In order to determine the robustness of applying a coplanar beam analysis method to tropospheric data, we have compared the results of two data sets that were created using two independently developed cleaning algorithms on the same original raw data. These results show that the high‐frequency calculations are sensitive to the quality of the cleaned data used for this analysis. As long as the uncertainties associated with cleaning the data are considered, it is possible to establish the magnitude of the vertical momentum fluxes and the resulting accelerations in the troposphere.
Effects of artificially modified ionospheres on HF propagation: Negative Ion Cation Release Experiment 2 and CRRES Coqui experiments
Authors: Fitzgerald T. Joseph , Argo Paul E. , Carlos Robert C.
Published: February 2004

We report the results of measurements obtained in conjunction with a series of high‐altitude chemical release experiments of effects of artificially modified ionospheres upon high‐frequency, ionospherically reflected radio paths. Computer simulations indicate that under optimum conditions, ionospheric modifications induced by chemical releases could perturb or even disrupt a communication channel; our experiments corroborate this but also indicate that it is very difficult to actualize such disruptions. Our experiments have shown that an ionospheric depletion, in which the electron density hole forms a huge radio frequency lens, generates new modes which, however, do not significantly affect a communications system. Under optimum path geometry a signal strength decrease of 10 dB or more is possible for several tens of minutes. Enhancements, such as those produced by barium releases, act as reflecting mirrors that can create a large shadow zone on the ground and block off significant amounts of energy. We measured signal strength decreases of up to 20 dB.
Application of the finite difference time domain algorithm to quasi‐static field analysis
Authors: De Moerloose Jan , Dawson Trevor W. , Stuchly Maria A.
Published: February 2004

In this paper, the use of the standard finite difference time domain (FDTD) algorithm is extended to quasi‐static electromagnetic field problems. While straightforward application of the standard FDTD algorithm at very low frequencies leads to excessively long simulation times, we show that for linear structures this problem can be circumvented by using a ramp excitation function. The use of appropriate absorbing boundary conditions such as Berenger's perfectly matched layer is also shown to be necessary. By combining two plane waves in opposite directions, a uniform electric or magnetic field can be created so that the electric and magnetic field solutions are decoupled, as required in quasi‐static analysis. Calculations of the induced fields and currents in a human model exposed to power line frequency fields provide a realistic example of an application of this novel FDTD technique.
Energy dissipation rates, eddy diffusivity, and the Prandtl number: An in situ experimental approach and its consequences on radar estimate of turbulent parameters
Authors: Bertin Francois , Barat Jean , Wilson Richard
Published: February 2004

Different methods have been proposed to derive the energy dissipation rate and eddy diffusion coefficients from ST radar measurements. However, their validity is still questionable because they implicitly assume that the Prandtl number is always equal to one, an assumption which is not verified. An experimental approach to this question, using balloon‐borne experiment results, is proposed in this paper in order to test the validity/invalidity of the methods generally used. In situ observations show that the potential temperature gradient is more efficiently (and probably more rapidly) eroded by the turbulent activity than the wind shear. As a consequence of this observational evidence already mentioned by Browning and Watkins [1970], the structure function constant for temperature fluctuations (CT2) is vanishing within fully developed turbulent layers and exibits maxima on their boundaries, while the structure parameter for wind fluctuations (CV2) presents a broad maximum within the same layer and is decreasing at its boundaries. Consequently, the gradient Richardson number Ri strongly varies within fully developed turbulent layers, from Ri close to zero (near their center) up to Ri >1 (at their boundaries). By contrast, the flux Richardson number Rf, which describes the evolution of the ratio between buoyancy flux and turbulent energy production, remains apparently quasi‐constant and close to its critical value during the erosion processes, so that the Prandtl number is not a constant close to unity but might also strongly vary during the turbulent life cycle. These results are in good agreement with laboratory experiments in statistically stable fluids reviewed by Thorpe [1973] and with experimental results obtained in the boundary layer [Businger et al., 1971; Gossard and Frisch, 1987]. ST radar are generally not able to observe regions where the potential temperature gradient is eroded by the turbulent activity but may obtain strong responses on the boundaries of fully developed turbulent layers. This behavior does not affect the radar capability of estimating eddy dissipation rate ϵ and eddy diffusivity Kθ (or KM) when complementary information on temperature profiles and humidity are available. It is shown that the “nonlocal” mean potential temperature gradient, the wind shear, and the flux Richardson number are the pertinent parameters allowing a correct estimate of the eddy dissipation rates and eddy diffusion coefficients, from Cn2 (CT2) and rms turbulent vertical wind, in regions where the turbulent activity is observable by ST radars.
Active microwave signatures of snow covers at 5.3 and 35 GHz
Authors: Strozzi Tazio , Wiesmann Andreas , Mätzler Christian
Published: February 2004

During the last 3 years we performed in situ backscattering measurements on various natural snow covers at 5.3 and 35 GHz (C and Ka band, respectively). Most of the measurements were made at a test site above Davos in the Swiss Alps at 2540 m above sea level (asl), but studies were conducted also at other test sites in Switzerland and Austria at heights between 500 and 2200 m asl. The backscattering coefficient γ was measured at hh, vv, hv and vh polarization, together with physical parameters of the snow cover, like snow height, stratification, temperature, density, and permittivity. On the basis of ground information and on a simple distribution of γ, we identified signatures of object classes. The signatures were used in order to evaluate the capability of active microwave sensors at 5.3 and 35 GHz for the classification of snow covers. In addition, semiempirical algorithms for the retrieval of physical parameters of the snow cover, such as water equivalent, liquid water content, and thickness of the refrozen crust, were defined.
Linear and nonlinear prediction techniques for short‐term forecasting of HF fading signals
Authors: Fridman S. V. , Yeh K. C. , Fridman O. V. , Franke S. J.
Published: February 2004

There exist two major mechanisms which are responsible for the fading phenomenon at HF frequencies. They are the multiple‐mode interference and distortions due to the ionospheric irregularities. Fading time series produced by the first of these mechanisms alone should typically represent a multiple‐periodical process. This kind of signal may also be produced by an autonomous dynamical system. The character of the time series produced by the second mechanism depends on the nature of the ionospheric irregularities. Recently, evidence has been accumulated to show that sometimes the ionospheric turbulence on equatorial and middle latitudes represents a low‐dimensional deterministic chaotic process. These facts suggest that for both mechanisms the fading time series may have a deterministic nature and therefore is predictable. Accordingly we apply the nonlinear predicting technique proposed by Farmer and Sidorovich [1987] to the fading time series obtained by the University of Illinois HF sounder. In its application the prediction technique is modified to take into account specifics of the HF data. For comparison, the conventional linear autoregression prediction technique is also tested. It is found that, in general, the nonlinear prediction and the linear autoregressive forecasting allow prediction on a few correlation times and work with roughly the same success.
Structural elements of the southern Tethyan Himalaya crust from wide‐angle seismic data
Authors: Makovsky Yizhaq , Klemperer Simon L. , Huang Liyan , Lu Deyuan ,   Project INDEPTH Team
Published: February 2004

A deep seismic common midpoint (CMP) profile, shot across the southern margin of the Tethyan Himalaya as the first stage of Project International Deep Profiling of Tibet and the Himalaya (Indepth), imaged the top (Main Himalayan Thrust or MHT) and bottom (Moho) of the Indian continental crust underthrusting the Himalaya. We used portable seismographs to record the CMP‐profile shots at a wide range of offsets, up to 155 km. Our short‐offset data corroborate the CMP‐profile data, while our large‐offset data are dominated by a band of reflectivity. We interpret the bright onset of this reflective band as the basal detachment of the South Tibetan Detachment system and a phase at the end of the reflective band as the MHT. We used the CMP‐profile first‐break data to model the uppermost 2 km in detail. The depth of young extensional basins of the Yadong rift system along the CMP profile was constrained to a maximum of 2 km, yielding a throw of 4.6 km across the eastern flank of the rift and a rough estimate of approximately 1.5% east‐west extension by normal faulting in the Tethyan Himalaya. The wide‐angle data were used to construct a crustal seismic‐velocity model down to the MHT. The South Tibetan Detachment System (STDS) basal detachment reflector is observed dipping 12.5°NNE from a depth of about 6 km beneath the surface under the south end of the CMP profile to a depth of 22 km, then flattening to a dip of only 2.5°NNE. Thus our observations suggest that the STDS basal detachment is a deep‐rooted basement fault. For the MHT we observe a dip of 7.5°NNE from 20 km depth below sea level at the crest of the High Himalaya crystalline sheet to 36 km below sea level (40 km beneath the surface) at a distance of about 70 km south of the Indus‐Yarlung Suture (IYS). From geometrical arguments we suggest that Indian crust underthrusts the surface expression of the IYS within the crust but does not form the lower crust of central and northern Tibet.
Geophysical‐geological transect and tectonic evolution of the Swiss‐Italian Alps
Authors: Schmid S. M. , Pfiffner O. A. , Froitzheim N. , Schönborn G. , Kissling E.
Published: February 2004

A complete Alpine cross section integrates numerous seismic reflection and refraction profiles, across and along strike, with published and new field data. The deepest parts of the profile are constrained by geophysical data only, while structural features at intermediate levels are largely depicted according to the results of three‐dimensional models making use of seismic and field geological data. The geometry of the highest structural levels is constrained by classical along‐strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units. Because the transect is placed close to the western erosional margin of the Austroalpine nappes of the Eastern Alps, it contains all the major tectonic units of the Alps. A model for the tectonic evolution along the transect is proposed in the form of scaled and area‐balanced profile sketches. Shortening within the Austroalpine nappes is testimony of a separate Cretaceous‐age orogenic event. West directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the Meliata‐Hallstatt Ocean further to the east. Considerable amounts of oceanic and continental crustal material were subducted during Tertiary orogeny, which involved some 500 km of N‐S convergence between Europe and Apulia. Consequently, only a very small percentage of this crustal material is preserved within the nappes depicted in the transect. Postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments, both detachments nucleating at the interface between lower and upper crust. Large scale wedging of the Adriatic (or Apulian) lower crust into a gap opening between the subduced European lower crust and the pile of thin upper crustal flakes (Alpine nappes) indicates a relatively strong lower crust and detachment between upper and lower crust.
Zircon U‐Pb geochronology of plutonic rocks from the Antarctic Peninsula: Confirmation of the presence of unexposed Paleozoic crust
Authors: Tangeman Jean A. , Mukasa Samuel B. , Grunow Anne M.
Published: February 2004

The Antarctic Peninsula represents the southernmost segment of a magmatic arc once present along the western margin of Gondwanaland. Zircon U‐Pb geochronology of a suite of calc‐alkaline granitoids from the west coast of Graham Land, northern Antarctic Peninsula, resulted in ages for the rocks ranging from 117.0 ± 0.8 to 73.6 ± 0.4 Ma. Discordant zircon populations from peraluminous granites reveal inheritance characteristics resulting from entrainment of older crustal materials. Upper intercept ages of discordant zircon populations in four samples indicate that the age of assimilated material is approximately late Paleozoic. This inherited component is present in four samples from Bone Bay, Charlotte Bay, and Stonington Island and thus may extend for ∼600 km along the west coast of Graham Land. A possible source for the inherited components is the late Paleozoic(?) sediments of the Trinity Peninsula Group. However, a late Proterozoic upper intercept age for the Charlotte Bay granodiorite in western central Graham Land indicates that the age of the rocks assimilated by this suite of plutons is not uniformly late Paleozoic and might include unexposed basement rocks. Finally, an upper intercept age of 431 ± 12 Ma from a granite clast in metaconglomerate from Horseshoe Island provides further evidence for the presence of mid‐Paleozoic basement in Graham Land. These results confirm the already postulated minimum early Paleozoic age for Antarctic Peninsula basement.