bluuebunny/crossref_metadata_2025 huggingface dataset:
Bicarbonate uptake by Southern Ocean phytoplankton
Authors: Cassar Nicolas , Laws Edward A. , Bidigare Robert R. , Popp Brian N.
Published: April 2004

Marine phytoplankton have the potential to significantly buffer future increases in atmospheric carbon dioxide levels. However, in order for CO2 fertilization to have an effect on carbon sequestration to the deep ocean, the increase in dissolved CO2 must stimulate primary productivity; that is, marine phototrophs must be CO2 limited [Riebesell et al., 1993]. Estimation of the extent of bicarbonate (HCO3−) uptake in the oceans is therefore required to determine whether the anthropogenic carbon sources will enhance carbon flux to the deep ocean. Using short‐term 14CO2‐disequilibrium experiments during the Southern Ocean Iron Experiment (SOFeX), we show that HCO3− uptake by Southern Ocean phytoplankton is significant. Since the majority of dissolved inorganic carbon (DIC) in the ocean is in the form of bicarbonate, the biological pump may therefore be insensitive to anthropogenic CO2. Approximately half of the DIC uptake observed was attributable to direct HCO3− uptake, the other half being direct CO2 uptake mediated either by passive diffusion or active uptake mechanisms. The increase in growth rates and decrease in CO2 concentration associated with the iron fertilization did not trigger any noticeable changes in the mode of DIC acquisition, indicating that under most environmental conditions the carbon concentrating mechanism (CCM) is constitutive. A low‐CO2 treatment induced an increase in uptake of CO2, which we attributed to increased extracellular carbonic anhydrase activity, at the expense of direct HCO3− transport across the plasmalemma. Isotopic disequilibrium experimental results are consistent with Southern Ocean carbon stable isotope fractionation data from this and other studies. Although iron fertilization has been shown to significantly enhance phytoplankton growth and may potentially increase carbon flux to the deep ocean, an important source of the inorganic carbon taken up by phytoplankton in this study was HCO3−, whose concentration is negligibly affected by the anthropogenic rise in CO2. We conclude that biological productivity in this region of the world's ocean is unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of a constitutive CCM.
Iron and silicate regulation of new and export production in the equatorial Pacific: A physical‐biological model study
Authors: Jiang Mingshun , Chai Fei
Published: April 2004

Based on a physical‐biological model with monthly mean forcing, numerical experiments are conducted to examine the effects of iron and silicate availability on phytoplankton growth in the equatorial Pacific. The effects of elevated iron are realized in terms of increasing the phytoplankton light acclimation efficiency (α) and decreasing the molar ratio Si/N of diatom uptake. The increase of α (decrease of Si/N) would release the iron stress to phytoplankton growth in the equatorial Pacific thus support higher organic production, which tends to drive the system into nitrate depletion. However, the biogenic silica (BSi) production remains largely unchanged due to the reduction of Si removal for unit production of organic nitrogen by diatoms. As a result, the BSi production is lower than the new production and the Si(OH)4 is higher than the NO3, which is consistent with conditions during glacial periods as inferred from the geological records but opposite to the conditions in the modern equatorial Pacific. The numerical experiments suggest that, if other conditions stay the same, low NO3 concentration during glacial periods may prevent diatoms from out‐growing small phytoplankton in the equatorial Pacific.
Oceanic evidence of climate change in southern Australia over the last three centuries
Authors: Thresher Ronald , Rintoul Stephen R. , Koslow J. Anthony , Weidman Chris , Adkins Jess , Proctor Craig
Published: April 2004

Chemical analysis of deepwater octocorals collected at 1000 m depth off southern Australia indicates long‐term cooling, beginning in the mid‐18th century. This cooling appears to reflect shoaling of isotherms along the continental shelf, that can be related statistically, observationally and by modeling to increasing coastal sea‐surface temperatures, that in turn reflect a poleward extension of the SW Pacific boundary current (the East Australian Current). The oceanographic changes implied by the coral record suggest climate change in temperate Australia starting about the time of European settlement. Correlations between temperate Australian and Antarctic indices suggest these long‐term changes might also be relevant to Antarctic climate.
A decrease in the sink for atmospheric CO2 in the North Atlantic
Authors: Lefèvre Nathalie , Watson Andrew J. , Olsen Are , Ríos Aida F. , Pérez Fiz F. , Johannessen Truls
Published: April 2004

Global ocean carbon models and available syntheses of the oceanic CO2 flux suggest that the North Atlantic subpolar gyre (50°N–70°N, 80°W–10°W) is a region of increasing uptake of CO2 from the atmosphere, with the oceanic partial pressure of CO2 (pCO2) increasing more slowly than the atmospheric CO2 over time. Our analysis of available CO2 data shows that, on the contrary, seawater pCO2 has increased faster than the atmosphere in recent decades, especially in summer, resulting in a decrease in uptake from the atmosphere. A decrease in the biological productivity of the region may be the underlying cause of this trend. From the observed trend we estimated a significant decrease in the annual carbon uptake in this region.
Photodissociation of acetone: Atmospheric implications of temperature‐dependent quantum yields
Authors: Arnold S. R. , Chipperfield M. P. , Blitz M. A. , Heard D. E. , Pilling M. J.
Published: April 2004

We have used a photochemical box model to investigate the effect of temperature‐dependent quantum yields of acetone on the chemistry in the upper troposphere. The T‐dependent quantum yields produce a photolysis (J) rate up to factor 3–10 slower than room temperature values. This reduced J rate significantly reduces the contribution of acetone to the mid‐latitude and tropical UT HOx budgets by factors of 4 and 2 respectively, when constrained with location‐specific aircraft observations. The calculated lifetime of acetone in the UT increases from ∼75 to ∼250 days at mid‐latitudes and from ∼10 to ∼20 days in the tropics, for the conditions used here. This will significantly affect studies aimed at balancing the atmospheric acetone budget. Overall, these new quantum yields have large implications for our understanding of the role of acetone in the chemistry of the UT.
Seismological evidence of stress induced by dike injection during the 2001 Mt. Etna eruption
Authors: Musumeci Carla , Cocina Ornella , De Gori Pasquale , Patanè Domenico
Published: April 2004

A selected dataset of 151 events, leading the July 17–August 9, 2001 lateral eruption at Mt. Etna volcano, has been analyzed for three‐dimensional hypocenter locations, focal mechanisms and stress tensor inversions. The seismic pattern provided indications for two main spatial clusters of foci located along and eastwards of the 2001 Mt. Etna eruptive fractures system. The 151 fault plane solutions (mostly strike slip) were inverted for stress tensor parameters, and space variations of seismogenic stress orientations have been identified. The stress inversion results and the axi‐symmetric orientation of P‐axes, in the region surrounding the modeled dike, well support the evidence of a unique stress source in agreement with the ground deformation results.
The anomalous amplification of M2 tide in the Taiwan Strait
Authors: Jan Sen , Chern Ching‐Sheng , Wang Joe , Chao Shenn‐Yu
Published: April 2004

The complex tidal wave propagation pattern in the Taiwan Strait invites parochialism. Along the eastern (Taiwan) boundary of the strait, the anomalous amplification of M2 tide in the middle often led to the parochial view that two tidal waves coming from both ends of the strait collide in the middle, creating wave resonance. Along the western (China) boundary, one sees a southward progressive tidal wave and hence no wave collision. To reconcile, we examine a few solutions of a numerical tidal model below. Both realistic bottom bathymetry and idealized bottom topographies are used to identify dominant mechanism leading to the complex tidal wave propagation. Our process of elimination identifies the wave reflection of southward propagating tidal wave by the deep trench in the southern strait as the true cause responsible for the complex wave propagation pattern.
Comparison of MODIS and AVHRR 16‐day normalized difference vegetation index composite data
Authors: Gallo Kevin , Ji Lei , Reed Brad , Dwyer John , Eidenshink Jeffery
Published: April 2004

Normalized difference vegetation index (NDVI) data derived from visible and near‐infrared data acquired by the MODIS and AVHRR sensors were compared over the same time periods and a variety of land cover classes within the conterminous USA. The relationship between the AVHRR derived NDVI values and those of future sensors is critical to continued long term monitoring of land surface properties. The results indicate that the 16‐day composite values are quite similar over the 23 intervals of 2001 that were analyzed, and a linear relationship exists between the NDVI values from the two sensors. The composite AVHRR NDVI data were associated with over 90% of the variation in the MODIS NDVI values.
A coupled ice‐ocean modeling study of the northwest Atlantic Ocean
Authors: Zhang Sheng , Sheng Jinyu , Greatbatch Richard J.
Published: April 2004

A coupled ice‐ocean modeling system is developed for the northwest Atlantic Ocean based on the second version of the Los Alamos sea ice model and a regional ocean circulation model. The coupled ice‐ocean system differs from other coupled systems for the same region mainly in two ways. First, the semi‐prognostic method suggested by Sheng et al. [2001] is used in the ocean component. This method adjusts the momentum equation of the ocean component to reduce drift of the modeled ocean state, allowing us to carry out a multiyear simulation. Second, the sea ice component uses the elastic‐viscous‐plastic ice rheology developed by Hunke and Dukowicz [1997] and Winton's [2000] three‐layer thermodynamics. The coupled system is forced by climatological monthly mean atmospheric forcing at the atmosphere/ocean, atmosphere/ice interface, and oceanic forcing at the model open boundaries. The system is integrated for 3 years. Model results from the third year compare favorably with the observations in the region. The coupled system reproduces reasonably well the phase and magnitude of the annual cycle of sea ice. We demonstrate the effect of the ice heat capacity, previously unaccounted for in earlier model results of this region, in delaying the springtime sea‐ice melt on the Labrador and Newfoundland Shelves.
Lagrangian turbulence in the Adriatic Sea as computed from drifter data: Effects of inhomogeneity and nonstationarity
Authors: Maurizi A. , Griffa A. , Poulain P.‐M. , Tampieri F.
Published: April 2004

The properties of mesoscale Lagrangian turbulence in the Adriatic Sea are studied from a drifter data set spanning 1990–1999, focusing on the role of inhomogeneity and nonstationarity. A preliminary study is performed on the dependence of the turbulent velocity statistics on bin averaging, and a preferential bin scale of 0.25° is chosen. Comparison with independent estimates obtained using an optimized spline technique confirms this choice. Three main regions are identified where the velocity statistics are approximately homogeneous: the two boundary currents, West (East) Adriatic Current, WAC (EAC), and the southern central gyre, CG. The CG region is found to be characterized by symmetric probability density function of velocity, approximately exponential autocorrelations, and well‐defined integral quantities such as diffusivity and timescale. The boundary regions, instead, are significantly asymmetric, with skewness indicating preferential events in the direction of the mean flow. The autocorrelation in the along mean flow direction is characterized by two timescales, with a secondary exponential with slow decay time of ≈11–12 days particularly evident in the EAC region. Seasonal partitioning of the data shows that this secondary scale is especially prominent in the summer‐fall season. Possible sampling issues as well as physical explanations for the secondary scale are discussed. Physical mechanisms include low‐frequency fluctuations of forcings and mean flow curvature inducing fluctuations in the particle trajectories. Consequences of the results for transport modeling in the Adriatic Sea are discussed.
Examining the sediments and soils of Gusev Crater with the Athena science payload
Authors: Sutter Brad , Moore Jeffrey M. , Zent Aaron P.
Published: April 2004

Gusev Crater has been proposed to contain lacustrine and/or fluvial sediments; however, eolian or volcanic sediments, global dust, and locally produced soils may also be present. This work describes the use of the Athena instrument package to evaluate the environment in which sampled materials were deposited in Gusev Crater. Sedimentary or soil deposits can be sampled by examining materials that were (1) excavated by impact events, (2) revealed by wind deflation, or (3) exposed by spinning the rover wheels. Specific observations can be diagnostic of a depositional environment. For instance, Pancam images of thin (e.g., 5 cm) planar layers of phyllosilicates and/or evaporites (as determined by Mini‐TES) would suggest lacustrine sediments. Layered sediments (e.g., trough cross bed) with rounded particles (>4 mm) would suggest fluvial activity. Eolian sediments may have planar or cross‐planar layers of well‐sorted grains (∼200 μm). Parallel bed deposits consisting of glass observable by Microscopic Imager would indicate volcanic ash. Soil bulk chemistry reported by Alpha Particle X‐ray Spectrometer that is similar to the Viking and Mars Pathfinder soils would indicate global dust. Mini‐TES should only detect primary mineralogies in physically weathered soils, whereas chemically weathered soils may contain carbonates, sulfates, and/or phyllosilicates. Although some characteristics are unique to a sediment or soil deposit, different deposit types can have similar characteristics (e.g., planar layering). Determining the depositional history of material in Gusev will require the integration of measurements from several instruments and careful geologic reasoning by the investigators.
The cross‐sectional shape of tidal sandbanks: Modeling and observations
Authors: Roos Pieter C. , Hulscher Suzanne J. M. H. , Knaapen Michiel A. F. , Van Damme Ruud M. J.
Published: April 2004

To improve our understanding of tidal sandbank dynamics, we have developed a nonlinear morphodynamic model. A crucial property of the model is that it fully resolves the dynamics on the fast (tidal) timescale, allowing for asymmetric tidal flow with an M0, M2, and M4 component. This approach, extending earlier research on the formation of tidal sandbanks, leads to equilibrium profiles. Their heights (60–90% of the water depth) and shapes are controlled by the mode of sediment transport and the hydrodynamic conditions. Bed load transport under symmetrical tidal conditions leads to high spiky banks. Several mechanisms tend to lower and smooth these profiles, such as the relaxation of suspended sediment, wind wave stirring, and tidal asymmetry. This last causes the profiles to be asymmetric, as well. The morphodynamic equilibrium expresses a tidally averaged balance between a destabilizing flux due to fluid drag and the downslope transport induced by both tidal flow and wind wave stirring. The modeled profiles are in fair agreement with observations from the North Sea.
Europe's First Space Weather Think Tank
Authors: Belehaki Anna , Lilensten Jean , Clark Toby
Published: April 2004

A new European intergovernmental action devoted to space weather has been recently approved. This paper describes the political and scientific context in which this action takes place, and the goals of this action, called COST 724.
Endangered aquifers: Groundwater management under threats of catastrophic events
Authors: Tsur Yacov , Zemel Amos
Published: April 2004

We study optimal management of groundwater resources under risk of occurrence of undesirable events. The analysis is carried out within a unified framework, accommodating various types of events that differ in the source of uncertainty regarding their occurrence conditions and in the damage they inflict. Characterizing the optimal policy for each type, we find that the presence of event uncertainty has profound effects. In some cases the isolated steady states, characterizing the optimal exploitation policies of many renewable resource problems, become equilibrium intervals. Other situations support isolated equilibria, but the degree of prudence they imply is sensitive to the nature of the event risk.
Estimating accounting prices for common pool natural resources: A distance function approach
Authors: Koundouri Phoebe , Xepapadeas Anastasios
Published: April 2004

We extend existing methodology for estimating shadow prices for exhaustible natural resources to renewable resources with common pool characteristics, using groundwater in irrigated agriculture as an example. The resource's shadow price is defined in terms of a perfect foresight open loop Nash equilibrium. Furthermore, we introduce a new estimation approach and derive shadow groundwater scarcity rents by estimating a stochastic restricted distance function using duality results between distance and cost functions. This approach is appropriate when price information is not available or when cost, profit, or revenue functions representations are precluded because of violations of the required behavior assumptions. We use our results to study policy implications for groundwater management.
Linking shore with the ocean floor in real time in archeological oceanography
Authors: Ballard Robert D.
Published: April 2004

In ocean exploration, physically transporting human beings in small numbers to remote regions of the world, and then taking them to the bottom of the ocean in very smaller numbers for short periods of time to explore short stretches of the sea floor, is clearly a costly and inefficient way to explore these vast regions of our planet.

Recently, however, during an expedition to the Black Sea, lnternet2 and a high‐bandwidth satellite link made it possible for scientists to work on the ocean floor without ever having to leave the comfort of their university laboratory Internet2 is a consortium of 206 universities working with industry and the government to develop and deploy an advanced Internet network that operates at 10 gigabits per second. Its future goal is to offer 100 megabits per second via its Abilene backbone to its user base (see www.internet2.edu/about).
Recent surface faulting investigated in Kamchatka volcanic arc
Authors: Tibaldi Alessandro
Published: April 2004

Large tectonic earthquakes may trigger other phenomena such as volcanic eruptions and edifice collapse, and such events in coastal areas can generate tsunamis. When closely spaced in time, these events can combine their effects to cause more serious and variable consequences than anticipated from volcanic or seismic hazards forecasts separately. The Kamchatka volcanic arc, because it has been affected recently by such events, provides an invaluable natural laboratory for understanding these phenomena.

An international team recently studied the strong events (Mw >5.5) that have occurred in Kamchatka over the last 10 k.y. The work was organized within the framework of a new NATO‐Russia collaborative project studying major events occurring in volcanic arcs. It follows a joint US‐Russian investigation carried out during 2000–2002 under the umbrella of the National Geographic Society, the U.S. National Science Foundation, and the Russian Foundation for Basic Research, which combined volcanic, geochemical, and geophysical evidence of catastrophic geological events. This work is now being built on by providing the field data necessary to understand the age, magnitude, and recurrence intervals of paleoearthquakes; and the location, geometry, and kinematics of the main active crustal faults. Improvements in the understanding of the age and size of Holocene eruptions and refinement of their ash dispersal patterns are also being made. A multidisciplinary team is involved in this work, as well as similar studies in the Aeolian volcanic arc in Italy thus allowing the results to be combined and compared to arrive at more general perspectives.
Windstorm impact reduction legislation finds support, but presents some concerns
Authors: Showstack Randy
Published: April 2004

Legislation to establish a National Windstorm Impact Reduction Program, provide improved federal interagency coordination, and encourage mitigation measures was approved by the U.S. House of Representatives Science Committee on 29 March.

At a 24 March hearing, held by the House Science Subcommittees on Research and on Environment, Technology, and Standards, witnesses generally were supportive of the legislation's aims and the mitigation components of the proposed program. These components include an improved understanding of windstorms, and windstorm impact assessment and reduction.
Review of French research enterprise underway in light of mass resignations
Authors: Jacobs Judith
Published: April 2004

Mass resignations in March by thousands of high‐level individuals throughout the French research establishment, to express grievances vis‐a‐vis government policies, have led to an ongoing review of all aspects of the way research is conducted in France. The committee undertaking the examination is due to issue a preliminary report this summer.

The number of individuals in high‐level administrative and research positions in scientific institutions across France who have resigned is now over 3,500; and includes directors and laboratory team leaders associated with the geosciences. The mass resignations began on 9 March.
NSF Geosciences Advisory Committee solicits input
Authors: Anonymous
Published: April 2004

The Geosciences Advisory Committee of the U.S. National Science Foundation (NSF) is soliciting the views and concerns of the geosciences community in advance of the committee's spring meeting, scheduled for 28–30 April at NSF headquarters in Arlington, Virginia. At this meeting, the committee will consider current and future geoscience plans and programs, priority areas in cooperation with other NSF directorates, and additional issues of relevance to the community.

The chair and members welcome and solicit the views and concerns of the geosciences community so they may better represent their constituencies at upcoming meetings of the committee.
Anoxic “dead zones” in oceans
Authors: Jacobs Judith
Published: April 2004

The number of oxygen‐starved “dead zones” in the world's oceans and seas is rising, according to a report of the United Nations Environment Programme published 29 March—the result of excessive nutrients, mainly nitrogen, from the use of synthetic fertilizers in agriculture, as well as vehicle and factory air emissions. The issue is raised in UNEP's first ever Global Environment Outlook Year Book, which claims that there are now nearly 150 such zones in coastal areas around the world.

Some of the earliest recorded dead zones were in Chesapeake Bay and the Gulf of Mexico in the United States, the Baltic Sea, and the northern Adriatic. However, other newer zones have appeared off South America, China, Japan, southeastern Australia, and New Zealand.
2004 Fellows Citations
Authors: Anonymous
Published: April 2004

AGU congratulates the forty‐one distinguished scientists who were selected by a committee of their peers to be AGU Fellows for 2004.This selection was based on the individuals' attainment of acknowledged eminence in a branch of geophysics.

The number of Fellows selected annually is limited to no more than 0.1% of the AGU membership. The names of the new Fellows are listed below, along with citations recognizing their specific achievements.
Winter mesospheric temperatures above South Pole (90°S) and their relationship to the springtime ozone hole size
Authors: Hernandez G.
Published: April 2004

Mesospheric kinetic temperatures measured above South Pole Station (90°S) during the austral polar winter show significant changes, which have been found to be associated with changes in the size of the polar stratospheric clouds temperature area (PSC‐1), vortex area, and ozone hole area later in the austral spring. Linear regression fits show a statistically significant correlation −95% confidence level‐ between the mesospheric temperature and the stratospheric dynamic variables. These results show the sensitivity of the mesosphere to changes in the atmosphere below. Thus, the mesospheric temperatures measured above South Pole provide a quantitative indication of the dynamic scenario upon which the ozone hole chemistry will be played later in the spring. That is, these mesospheric temperatures can be used as a forecasting tool with which some of the stratospheric dynamical variables affecting the area of the ozone hole can be estimated with statistical confidence.
Geothermal evidence of very low glacial temperatures on a rim of the Fennoscandian ice sheet
Authors: Šafanda Jan , Szewczyk Jan , Majorowicz Jacek
Published: April 2004

The mean ground temperature as low as −10°C and the existence of more than 500 m thick permafrost during the last glacial are apparent from the analysis of deep borehole temperature profiles in north‐eastern Poland. The largest thickness of the permafrost and its survival deep into the Holocene was restricted to a region known for its low terrestrial heat flow, where the negative temperature gradient is observed in the uppermost 400 m. We have shown that the profiles are consistent with warming from the glacial mean of −10°C to the Holocene mean of +7°C 14 ka ago and to +8°C in the last 150 years.
Assessing the resolution‐dependent utility of tomograms for geostatistics
Authors: Day‐Lewis F. D. , Lane J. W.
Published: April 2004

Geophysical tomograms are used increasingly as auxiliary data for geostatistical modeling of aquifer and reservoir properties. The correlation between tomographic estimates and hydrogeologic properties is commonly based on laboratory measurements, co‐located measurements at boreholes, or petrophysical models. The inferred correlation is assumed uniform throughout the interwell region; however, tomographic resolution varies spatially due to acquisition geometry, regularization, data error, and the physics underlying the geophysical measurements. Blurring and inversion artifacts are expected in regions traversed by few or only low‐angle raypaths. In the context of radar traveltime tomography, we derive analytical models for (1) the variance of tomographic estimates, (2) the spatially variable correlation with a hydrologic parameter of interest, and (3) the spatial covariance of tomographic estimates. Synthetic examples demonstrate that tomograms of qualitative value may have limited utility for geostatistics; moreover, the imprint of regularization may preclude inference of meaningful spatial statistics from tomograms.
Inefficiencies in water project design and operation in the third world: An economic perspective
Authors: Howe Charles W. , Dixon John A.
Published: April 2004

Water projects in less developed countries (LDCs) frequently are poorly operated and maintained. As a result, project benefits and development impacts fall short of plans. The problems begin in the project identification, design, and construction stages: donor and host country biases lead to inappropriate projects, unsustainable technologies, and shoddy construction. Later operation and maintenance are then difficult or impossible. Causal factors include donor desire to build monuments and sell technology, provision of excessive capital to favored sectors or institutions, and an unwillingness to require a reasonable quid pro quo from the host country. Host country factors include excessive administrative centralization, lack of rewards for good operation and maintenance, and widespread corruption in forms that seriously distort allocative efficiency. Until individual actors on both sides can be motivated to pursue the long‐run good of the LDC, Third World water projects will continue to have low or negative net payoffs.
Scattering of an em wave off a whistler wave in the ionosphere
Authors: Sawhney B. K. , Tripathi V. K. , Singh S. V.
Published: April 2004

An electromagnetic wave transmitted from the ground‐based transmitter onto the ionosphere, where an obliquely propagating whistler exists, suffers Raman scattering. The scattered wave travels upward but can be detected on the ground after it has been reflected back from the turning point. The scattered wave could provide information about the whistler and would be useful for diagnostics. We have examined this problem using fluid approach. The ratio of power of the scattered wave to that of the pump wave is found to increase with increasing plasma frequency, frequency of the whistler, and the angle of the whistler with the magnetic field but is found to decrease with increasing frequency of the pump wave. Analysis shows that in the equatorial F ‐region for a pump wave of 1 MW power, sent at an angle of θe = 10° with the vertical, the scattered wave received on the Earth would have a power of 3 μW and for northern Scandinavia and at Arecibo, Puerto Rico, for θe = 22° and θe = 14° the scattered power would be detected at 38.5 μW and 2.71 mW, respectively.
Regularized inversion of a two‐dimensional integral equation with applications in borehole induction measurements
Authors: Arikan Orhan
Published: April 2004

Well bore measurements of conductivity, gravity, and surface measurements of magnetotelluric fields can be modeled as a two‐dimensional integral equation with additive measurement noise. The governing integral equation has the form of convolution in the first dimension and projection in the second dimension. However, these two operations are not in separable form. In these applications, given a set of measurements, efficient and robust estimation of the underlying physical property is required. For this purpose, a regularized inversion algorithm for the governing integral equation is presented in this paper. Singular value decomposition of the measurement kernels is used to exploit convolution‐projection structure of the integral equation, leading to a form where measurements are related to the physical property by a two‐stage operation: projection followed by convolution. On the other hand, estimation of the physical property can be carried out by a two‐stage inversion algorithm: deconvolution followed by back projection. A regularization method for the required multichannel deconvolution is given. Some important details of the algorithm are addressed in an application to wellbore induction measurements of conductivity.
Edge shape effects and quasi‐closed form expressions for the conductor loss of microstrip lines
Authors: Holloway Christopher L. , Kuester Edward F.
Published: April 2004

Much work has been done in the past to determine the conductor loss of microwave and millimeter‐wave integrated circuits (MIMICs). However, much of this work is limited by constraints on the conductor thickness, skin depth, or the shape of the edge. In this paper, a technique will be presented that results virtually in a quasi‐closed form expression for this loss in many cases. This expression will allow the calculation of the attenuation constant for a planar circuit for an arbitrarily shaped edge and any conductor thickness. The question of how different edge shapes affect the conductor loss will be addressed.
Time delay of transionospheric radio signals in a horizontally inhomogeneous ionosphere
Authors: Tsedilina E. E. , Weitsman O. V. , Soicher H.
Published: April 2004

The influence of a horizontally inhomogeneous ionosphere on time delay of satellite‐emitted transionospheric radio signals is considered. Theoretical and numerical time delay calculations are performed using a single parabolic ionospheric density model along the signal radio path. The comparison of this method of calculation of signal time delay in the ionosphere with an approximate one, which does not consider horizontal density gradients is given. It is shown that during sunrise, in the regions of equatorial anomaly and midlatitude trough, the errors of the approximate method can be of the order 20–40%.
Temperature fluctuations near the mesopause inferred from meteor observations with the middle and upper atmosphere radar
Authors: Tsutsumi Masaki , Tsuda Toshitaka , Nakamura Takuji , Fukao Shoichiro
Published: April 2004

Using meteor echo measurements with the middle and upper atmosphere (MU) radar (35°N, 136°E), operated at 46.5 MHz, we examined time‐height variation of the ambipolar diffusion coefficient D, determined from the decay rate of meteor echoes. The height of a meteor trail was determined with an accuracy of about 1 km, by using an interferometer for determination of the arrival angles of meteor echoes. Profiles of D were obtained about every hour owing to the high rate of usable meteor echoes (3000–5000 per day). The mean profile of D showed an exponential increase with a scale height of 5.7–6.5 km in spring, summer, and autumn, which was generally consistent with the density scale height, while the scale height was as much as 8.1 km in November and December. Using the Boussinesq approximation, we found that the normalized D fluctuations D′ can be related to those of temperature T′ as D′/D0 ∼ 3/2 · T′/T0. The observed D′/D0 involved a wavelike structure with periods shorter than the inertial period, which often showed downward phase propagation. Fluctuations of D′/D0 and the horizontal wind velocity showed phase relations consistent with a linear gravity wave theory. Furthermore, frequency spectra of D′/D0 were generally consistent with those of the horizontal wind velocity for wave components with periods longer than about 8 hours, while for short‐period components the spectral amplitudes of D′/D0 were considerably enhanced.
An asymptotic analysis of a wire mesh parallel‐plate waveguide
Authors: Pasik M. F. , Dudley D. G.
Published: April 2004

The method of steepest descents is applied to a parallel‐plate waveguide where one of the plates is a bonded wire mesh modeled by a sheet impedance boundary condition. The structure is excited by a magnetic line source exterior to the waveguide. The parameters of the wire mesh are adjusted to exploit the contributions from the leaky‐wave poles in the near far‐zone radiation pattern. The use of the asymptotic expressions in this region is justified by comparisons with results obtained by numerical integration. The transient response of the structure is obtained by transforming the frequency domain response to the time domain with an FFT algorithm. An alternate representation for the magnetic field which is computationally efficient for determining the response in early time is derived. Numerical results for a double exponential pulse excitation of the line source are presented.
Revised attribution of the power form infiltration equation
Authors: Swartzendruber D.
Published: April 2004

An empirical equation, giving cumulative soil water infiltration as a power function of time, has come to be associated with A. N. Kostiakov. Reconsideration shows, however, that instead of the cumulative infiltration it was the hydraulic conductivity which he postulated to be a power function of time. Furthermore, within the setting he posed, there is no rational way of converting the hydraulic conductivity form to the cumulative infiltration form. It is therefore recommended that the cumulative infiltration form be attributed to M. R. Lewis, who made direct and unequivocal use of the cumulative form and appears to have been the first to do so.
A theory of flood warning systems
Authors: Krzysztofowicz Roman
Published: April 2004

A local flood warning system typically serves a small community situated in a headwater area and exposed to flash floods or rapid riverine floods. Forecasts of such flood events are characterized by short lead times and large uncertainties. A Bayesian theory is formulated for a local flood warning system built of three functional components: monitor, forecaster, and decider. The theory offers a modeling framework and mathematical concepts necessary for (1) developing optimal decision rules for issuing warnings based on imperfect forecasts, (2) evaluating system performance statistically, and (3) computing the ex ante economic benefits from a system.
Channeling and Fickian Dispersion in fractal simulated porous media
Authors: Grindrod Peter , Impey Michael D.
Published: April 2004

We simulate tracer transport in a two‐dimensional porous medium. We provide justification for the use of a fractal scaling law to characterize the spatial variability of rock properties, leading to a fractal rock property model. A methodology based on the fractal model discussed is applied to experimental data to generate a “fractal porous medium” and to solve the associated flow and tracer transport problems. Tracer breakthrough curves are computed for three different breakthrough distances. An advection‐dispersion matrix diffusion model, a channeling model, and a channel‐network model are used to assess objectively the relative contributions of Fickian dispersal processes and channeling within simulated tracer transport through the fractal medium. The results reveal early breakthrough is fast‐channel dominated, while later breakthrough is predominately Fickian.
An efficient Eulerian‐Lagrangian Method for solving solute transport problems in steady and transient flow fields
Authors: Zhang R. , Huang K. , van Genuchten M. T.
Published: April 2004

A computationally efficient, yet relatively simple Eulerian‐Lagrangian method is proposed for solving the one‐dimensional convection‐dispersion solute transport equation assuming a steady or transient velocity field. The method uses a modified single‐step reverse particle tracking (MSRPT) technique to handle steep concentration fronts. The scheme utilizes two weighting factors to control the movement of particles during a backward tracking step. One weighting factor greater than unity is used in the upstream region of the convection front, while another weighting factor less than unity is taken in the downstream region. The two factors were related empirically to the grid Peclet and Courant numbers. The MSRPT technique is carried out only within the concentration plume at each time step. For transient flow fields, the weighting factors were determined using an automatically adjustable procedure based on mass balance errors. The MSRPT method maintains the advantages of the traditional single‐step reverse particle tracking (SRPT) procedure, i.e., producing efficient and oscillation‐free calculations, but circumvents numerical dispersion introduced by SRPT. A large number of tests against analytical solutions for one‐dimensional transport in uniform flow fields indicate that the proposed method can handle the entire range of Peclet numbers from zero to infinity. Numerical tests also show that the MSRPT method is a relatively accurate, efficient and mass‐conservative algorithm for solute transport in transient flow fields. The Courant number at present cannot exceed 1. The MSRPT approach was found especially useful for convection‐dominated problems; in fact, an exact numerical solution may be obtained with MSRPT for pure convection.
The Hurst Effect: The scale of fluctuation approach
Authors: Mesa Oscar J. , Poveda German
Published: April 2004

After more than 40 years the so‐called Hurst effect remains an open problem in stochastic hydrology. Historically, its existence has been explained either by preasymptotic behavior of the rescaled adjusted range
, certain classes of nonstationarity in time series, infinite memory, or erroneous estimation of the Hurst exponent. Various statistical tests to determine whether an observed time series exhibits the Hurst effect are presented. The tests are based on the fact that for the family of processes in the Brownian domain of attraction,
converges in distribution to a nondegenerate random variable with known distribution (functional central limit theorem). The scale of fluctuation θ, defined as the sum of the correlation function, plays a key role. Application of the tests to several geophysical time series seems to indicate that they do not exhibit the Hurst effect, although those series have been used as examples of its existence, and furthermore the traditional pox diagram method to estimate the Hurst exponent gives values larger than 0.5. It turned out that the coefficient in the relation of
versus n, which is directly proportional to the scale of fluctuation, was more important than the exponent. The Hurst effect motivated the popularization of 1/f noises and related ideas of fractals and scaling. This work illustrates how delicate the procedures to deal with infinity must be.
Estimation of local precipitation statistics reflecting climate change
Authors: Matyasovszky Istvan , Bogardi Istvan , Bardossy Andras , Duckstein Lucien
Published: April 2004

A stochastic approach is developed to estimate the probability of wet days, and the mean and standard deviation of local daily nonzero precipitation reflecting global climate change scenarios. The approach is based on the analysis of daily atmospheric circulation patterns (CPs) and the linkage between types of CPs and daily precipitation. Three CP data sets are used for the 500‐hPa pressure field: 40‐year historical, 10‐year 1×CO2, and 10‐year 2×CO2scenarios obtained from the atmospheric general circulation model of the Canadian Climate Centre. CP types obtained by clustering techniques and their frequency distribution are similar for the three data sets. The linkage between CP types and precipitation considers an additional variable, the spatial average pressure height within each CP type. The difference in pressure heights among the three CP data sets makes it possible to estimate the effect of global change on local precipitation statistics. Under the dry continental climate of eastern Nebraska the effect of 2×CO2scenario on local precipitation regime is spatially variable and significant: the number of wet days slightly decreases but both the mean and variance of daily precipitation increase resulting in a more variable precipitation regime.
Field experiments in a fractured clay till: 2. Solute and colloid transport
Authors: McKay Larry D. , Gillham Robert W. , Cherry John A.
Published: April 2004

A field tracer experiment was conducted in a lateral flow field in the weathered and highly fractured upper 6 m of a 40‐m‐thick clay‐rich till plain in southwestern Ontario. In the upper 3 m where fractures are closely spaced (<0.13 m) the advancing front (C/C0 = 0.01) of the nonreactive solute tracers, bromide and 18O, migrated at rates of 0.01 to 0.07 m/d, over distances of 4 and 6 m and under a lateral hydraulic gradient of 0.24. In this same zone, two strains of colloid‐sized bacteriophage tracers migrated at rates of 2 to >5 m/d. Simulations with a discrete fracture/porous matrix flow and transport model, which used the cubic law for flow in fractures, showed that diffusion of the solutes, but not the much larger colloids, into the matrix pore water between fractures is sufficient to cause the observed difference in solute and colloid transport rates. Transport‐derived and hydraulic conductivity‐derived fracture aperture values were similar, within a factor of 3 and falling mainly within a range of 5–40 μm. In the upper 3 m the solute tracers were evenly distributed between pore water in the fractures and the matrix, and as a result, solute transport can be closely approximated with an equivalent porous medium (EPM) approach. Below this depth, fractures are more widely spaced (0.13 to >1 m) with concentration peaks tending to occur near visible fractures, and solute transport cannot be adequately described with an EPM approach.
Chlorofluorocarbons (CCl3F and CCl2F2) as dating tools and hydrologic tracers in shallow groundwater of the Delmarva Peninsula, Atlantic Coastal Plain, United States
Authors: Dunkle S. A. , Plummer L. N. , Busenberg E. , Phillips P. J. , Denver J. M. , Hamilton P. A. , Michel R. L. , Coplen T. B.
Published: April 2004

Concentrations of the Chlorofluorocarbons (CFCs) CFC‐11 and CFC‐12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC‐modeled ages were calculated independently for CFC‐11 and CFC‐12, and agreed to within 2–3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from 9±2°C over most of the peninsula to 14±2°C at the southernmost tip of the peninsula in Virginia. The CFC‐modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC‐modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus 3He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of the study strongly support the use of CFCs for dating shallow, aerobic groundwater.
Monthly temperature and precipitation fields on a storm‐facing mountain front: Statistical structure and empirical parameterization
Authors: Fan Ying , Duffy Christopher J.
Published: April 2004

This paper presents an analysis of monthly temperature (T) and precipitation (P) time series at 28 climatologic stations on the storm‐facing slope of the Wasatch Range, Utah. The goal is to examine the space‐time structure of T and P and to develop an empirical model incorporating both seasonal and elevation effects. Each time series (T or P) is decomposed into the sum of a long‐term mean, a seasonal cycle, and a residual random process. The seasonal cycle is well determined by the amplitude and phase of a few harmonics, and the residual noise is approximated by a power law form of the variance spectrum. Empirical correlations are found relating the temporal moments of altitude, allowing the construction of a parametric T‐P model as a function of altitude and season. The observed correlations are discussed within the context of the region's synoptic weather patterns. When combined with digital elevation data, the model can be used to estimate seasonal temperature and precipitation fields as input to mountain front hydrologic studies.
Predictability of geochemical buffering and runoff acidification in spatially heterogeneous catchments
Authors: Kirchner James W. , Dillon Peter J. , Lazerte Bruce D.
Published: April 2004

Catchments are more spatially variable than catchment models assume them to be. Does spatial heterogeneity undermine the validity of spatially aggregated catchment geochemical models? Here we test whether catchment spatial heterogeneity invalidates a recently proposed technique that uses the bulk chemistry of runoff to predict how key water quality variables (e.g., titratable alkalinity and concentrations of base cations, hydrogen ions, and inorganic aluminum) will respond to changes in stream water in acid anion concentrations. We show that these predictions are valid for hypothetical heterogeneous catchments, created by mathematically “mixing” measured runoff chemistry data from chemically diverse streams. The chemical information in the combined runoff yields reliable acidification predictions, without any direct information concerning either the runoff chemistries of the individual source streams or the geochemical properties of their catchments. These results show that catchment acid buffering can be predicted directly from runoff chemistry in spatially heterogeneous catchments, even if the individual source regions cannot be identified and their geochemical characteristics are unknown.